iii. design of the atst
TRANSCRIPT
III. Design of the ATST Advanced Technology Solar Telescope Construction Phase Proposal
III. Design of the ATST Page 57 of 174
III. DESIGN OF THE ATST
1. SYSTEMS OVERVIEW
The Advanced Technology Solar Telescope (ATST) is an all-reflecting, four-meter, off-axis Gregorian
telescope housed in a rotating enclosure. It can deliver up to a 300-arcsec field of view to either a
Nasmyth or a coudé observing station. Energy outside of this field is rejected from the system by a heat
stop located at prime focus, allowing manageable thermal loading on the optical elements that follow. The
telescope also includes an integrated adaptive optics system designed to provide diffraction-limited
images to the focal-plane instruments at the coudé observing station. Our design meets all science
requirements stated in the Science Requirements Document (SRD, ATST Document #SPEC-0001).
A persistent systems approach is essential to the success of a telescope like ATST. Systems engineering
works with project management, the scientific staff, and the other engineers to accomplish various
activities. In this chapter the emphasis will be on design requirements flow-down, error budgets, and
performance predictions. It will conclude with a top-level description of the telescope design that serves
as an outline and general background material for the subsequent detailed design descriptions. Other
aspects of systems engineering are discussed in Part IV of this proposal, Management of the ATST
Construction, Integration, and Testing (see Chapter 6).
1.1 THE FLOW-DOWN PROCESS
Systems engineering has been responsible for flowing the science requirements – as specified by the
scientific community – down to design requirements on the telescope. For example, the science that
ATST will perform requires a sharp image. Systems engineering must first list all of the telescope and
instrument subsystems that have the potential to cause the image to blur. These will include the quality of
the optical components (mirror figures and polish quality), telescope mount vibrations, and thermal
distortion of the air above the telescope enclosure, to name just a few. While the science requirement is
expressed in terms of the size of a point-source image, this constraint must be converted to a mirror-polish
specification, mount stiffness, and maximum allowed temperature variation on the enclosure skin to be
useful when designing the telescope and specifying the manufacturing tolerances of its components.
The process of flowing science requirements down to design requirements began with the ATST SRD.
That document established the top-level science requirements based on the solar community’s vision and
proposed mission of the telescope. These requirements lead directly to a set of critical science use cases
listed in the SRD, selected because they place the most stringent technical requirements on the telescope
and instrumentation. These use cases lead, in turn, to specific performance requirements placed on the
telescope and instrumentation. All of this has been spelled out in the SRD.
It has been the task of the engineering team to produce a design that meets the top-level telescope and
instrument requirements, and hence the science requirements. The process followed to get to an initial
design was highly iterative, involving a baseline concept that was proposed, tested against the top-level
requirements and error budgets, and modified until cost and performance requirements were met. The
following paragraphs highlight a few aspects of this process.
1.1.1 Science Use Cases
The Science Requirements Document describes 18 science use cases. They establish minimum
performance requirements on a variety of important parameters and operational modes that must be
supported by ATST to meet the science requirements. The detailed justification of each requirement is
included in the SRD. Table 1.1 lists the use cases that demand the highest performance in each area:
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1.1.2 Telescope Requirements
The use cases described above and other scientific considerations lead to several direct top-level
requirements on the telescope and site that are justified in detail in the Science Requirement Document:
Resolution:
ATST must have a minimum four-meter aperture.
Table 1.1. Science Use Cases.
1. Spatial Resolution Half of the science use cases require observations at or near to the diffraction limit at visible wavelengths. These include Interaction of strong and weak magnetic fields Flux emergence and disappearance Dynamics of kilogauss flux tubes Internal structure of flux tubes / irradiance variations Magnetoconvection in sunspots Generation of acoustic oscillations Temperature and velocity of the photosphere and chromosphere Prominence formation and eruption Solar Flares
2. Field of View Most use cases require a field of view of several arcmin. The coronal use cases established the most stringent requirement, desiring 3 to 5 arcmin: Prominence formation and eruption Coronal magnetic fields Coronal plasmoid search Coronal velocity and density in active region loops Coronal intensity fluctuation spectrum
3. Wavelength Coverage Three use cases require observations in the thermal IR (12 m): Turbulent/Weak fields Dynamo processes in deep layers of the convection zone Solar Flares Four require observations at or near to the atmospheric UV cutoff at 300 nm: Dynamics of kilogauss flux tubes Internal structure of flux tubes / Irradiance variations Turbulent/Weak fields Hanle effect diagnostics Many use cases require or strongly desire simultaneous observations over broad wavelength ranges.
4. Spectral Resolution Many use cases require spectral resolution of 1 pm (picometer) or less. The most stringent use case requires 0.42 pm at 500 nm: Dynamics of kilogauss flux tubes
5. Polarimetric Sensitivity and Accuracy
The most stringent use cases require polarimetric sensitivity of 10-5
: Turbulent / Weak fields Hanle Effect Diagnostics
6. Scattered Light The coronal observations are the most demanding in terms of sky and instrumental scattered light near the limb of the sun, requiring excellent coronal sky conditions, and low instrumental scatter: Coronal magnetic fields Coronal plasmoid search Coronal velocity and density in active region loops Coronal intensity fluctuation spectrum On-disk observations of large sunspots also place requirements on in-field scattering: Magnetoconvection in sunspots
7. Observing modes The most demanding use cases involve active-region evolution, which require simultaneous observations with multiple instruments in both the visible and the thermal infrared: Dynamo processes in deep layers of the convection zone Solar flares
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ATST shall include high order adaptive optics capable of deriving information from solar
granulation and other solar structure.
Photon flux and sensitivity:
ATST shall provide a minimum collecting area of 12 m2
Polarization sensitivity and accuracy:
The polarization sensitivity must be 10-5
with polarization accuracy of 510-4
Scattered light:
The scattered light from the telescope and instrumentation from angles greater than 10 arcsec
shall be 1% or less
The total instrumental scatter due to dust and mirror microroughness must be less than 2510-6
at
1.1 solar radii (1.6 arcmin from the limb of the sun)
Field of view:
The ATST shall provide a minimum field of view of 2 arcmin square at coudé, and 5 arcmin at
Nasmyth.
Wavelength Coverage:
The ATST shall cover the wavelength range from 0.30 to 28 m
Flexibility:
The ATST must accommodate simultaneous multi-wavelength observations at visible and IR
wavelengths.
It must be possible to carry out simultaneous observations with different instruments.
Image rotation introduced by the telescope must be counteracted by de-rotation, preferably
without additional reflections.
Maximum scientific productivity requires easy and fast (less than 30 minute) switching between
facility instruments.
Lifetime:
The ATST is expected to be the major solar ground based facility for a minimum of two decades.
The useful lifetime of ATST is expected to exceed 40 years.
Adaptability:
The ATST shall be designed with a minimum of limitations for future use and in a way that
allows future upgrades and the addition of new instruments.
Availability:
Scheduled engineering and maintenance should not exceed 10-15%.
Of the remaining time, ATST should set a goal of achieving telescope reliability that allows
observing during 97-98% of the available clear time (similar to the best nighttime telescopes). Location:
ATST should be located at the best affordable site in terms of seeing, sky clarity and sunshine
hours. This will maximize the telescope performance and minimize the cost of adaptive optics.
The science use cases also place several important derived requirements on the telescope:
Pointing and Tracking:
Absolute (blind) pointing shall be accurate to better than 5 arcsec.
Offset pointing shall be accurate to better than 0.5 arcsec.
Open-loop tracking stability must be better than 0.5 arcsec for one hour.
Active Optics:
Active control of the primary mirror figure will be required to achieve the necessary resolution.
The active-optics system must be run in an open-loop mode during coronal observations when
real-time wavefront information is unavailable.
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1.1.3 Design Requirements
Many design requirements were derived directly from the science requirements; others were determined
from error allocations within several systems error budgets (see Section 1.1.4). The job of flowing
science requirements down to design requirements is often made easier because existing telescopes have
met similar requirements, and the engineering specifications of those systems are known and available.
Table 1.2 lists important subsystems that are constrained by a specific telescope requirement. Note that
the demand for high-resolution observations yields the greatest number of design requirements. The error
budget discussion that follows helps to show how some of these constraints were derived in detail. The
table also shows the current “compliance” status, noting briefly what features of our design allow us to
meet the more challenging requirements.
Table 1.2. Flow-down to Subsystems, and Compliance Status
Telescope Requirement
Subsystem Flow Down Design Compliance and Strategy
Resolution Constrains telescope mount drives, control, and thermal systems; coudé rotator and drive systems; pier design; M1 aperture size, figure, support system and thermal control; active optics performance; heat stop thermal control; M2 figure, mount, and thermal control; feed optics figures and thermal control; adaptive optics performance; guiding systems; tip-tilt performance; optical alignment; polarimetry optics; Nasmyth and coudé optics; instrument lab thermal control; science instrumentation; telescope control software; enclosure thermal system (both skin temperature requirements and ventilation requirements); support facility location and construction methods.
In Compliance – Challenging
4-m aperture
Diffraction-limited optical design
Active thermal control of components
Heat rejection at prime focus
Rigorous error budgeting applied to many subsystems
Photon flux and sensitivity
Constrains M1 aperture area, M1 ancillary equipment (mirror washing), and all mirror coating specifications, telescope and instrument optical designs (number of reflections).
In Compliance – Straightforward
4-m aperture
Polarization sensitivity and accuracy
Constrains mirror coatings, polarimetry analysis and calibration performance, and science instrumentation.
In Compliance – Challenging
Pre-Gregorian Modulation
Pre-Gregorian Calibration
Optical designs
Charge-caching cameras
Scattered light Optical design, mirror polishing specifications, M1 mirror cover design, M1 ancillary equipment (mirror cleaning and washing), occulting system, mirror coatings, baffles and stops, enclosure thermal system (specifically the interior ventilation system and its relation to dust control), and coating and cleaning facilities.
In Compliance – Challenging
Off-axis configuration
Nasmyth station
In-situ cleaning/washing
Active ventilation filtration. Prime-focus occulting
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There are some technical risks associated with the more challenging requirements. These and the
project’s plans for risk mitigation are outlined in the management discussion (see Part IV of this proposal,
Chapter 8, Risk Assessment and Management).
Table 1.2. Flow-down to Subsystems, and Compliance Status (continued)
Field of view Constrains heat stop dimensions, thermal systems, occulting system, feed mirror dimensions, mirror cell dimensions, acquisition system, and instrument designs.
In Compliance – Straightforward
Optical design
Mechanical design
Wavelength coverage
Constrains mirror coatings, imaging system optical materials, science instrumentation, and restricts the general use of transmissive windows for thermal control of the light path.
In Compliance – Straightforward
All-reflecting design
Laminar-air coudé station isolation
Flexibility Constrains coudé rotator and drives, polarimetry analysis and calibration, coudé stations, instrument control system, science instruments, telescope control system, data handling system, and the observatory control system.
In Compliance
Software design
Modular components
Facility instrument concepts
Coudé station layout
Lifetime The ATST lifetime requirement affects all designs, requiring a high level of robustness or replacement and reconfiguration strategies.
In Compliance
Coudé station design
Modular component design
Adaptability The adaptability requirement drives many of the features of the feed optics, the Nasmyth and coudé observing stations, the science instruments, the instrument control system, telescope control system, and observatory control system.
In Compliance
Software designs
Modular component designs
Facility instrument concepts
Coudé station design
Availability The Availability requirement affects many systems that could impede observing efficiency. In particular, it drives the of mirror cleaning and washing facilities, acquisition and guiding, polarimetry analysis and calibration, Nasmyth and coudé platform configurability, optical enclosure ventilation system, mirror cleaning and coating facility.
In Compliance
In-situ mirror cleaning and washing
Control software
Facility instrument concepts
Coudé layout
Location The location of ATST constrains the details of the pier design, in-situ M1 mirror cleaning and washing (depending on local dust levels), mirror thermal control systems, enclosure thermal control (based on wind and temperature extremes), coudé room thermal control, site infrastructure, buildings, facility equipment, and coating and cleaning facilities (and their broader availability).
In Compliance
Design meets requirements at our selected site and alternate site
Site specific details have been rolled into the design since selecting Haleakala
Pointing and tracking
Constrains telescope mount, drive system, telescope control software.
In Compliance – Straightforward
Mount Design
Pointing Kernel selection
Active Optics Constrains M1 Mirror, M1 support structure, M1 controller, wavefront sensor, telescope control system, and AO (adaptive optics) system.
In Compliance – Challenging at high zenith distances
Thin meniscus mirror
120 axial supports
24 lateral supports
Dedicated wavefront sensor
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1.1.4 Error Budgets
Error budgeting is fundamentally a systems-level issue. A given error budget will typically be distributed
across many disparate subsystems. These have been designed by different engineers and will be
fabricated by different vendors (Systems Error Budget Plan, ATST Document #SPEC-0009). Error
budgeting is a useful tool at all levels of design since it represents a means to negotiate design trades in
the broadest possible context. This process is central to the mission of systems engineering.
The highest priority error budgets developed to support the flow-down process involved delivered image
quality. Three different image quality error budgets were derived from science use cases:
Diffraction limited observations at 500 and 630 nm
The ATST shall provide diffraction-limited observations (at the detector plane) with high Strehl
(S>0.6 required, S>0.7 goal) at 630 nm and above during excellent seeing conditions (r0 (630 nm) >
20 cm) and S > 0.3 at 500 nm and above during good seeing (r0 (500 nm) = 7 cm).
Seeing limited on-disk observations at 1.6 m
[For] excellent seeing conditions, (r0 at 1.6 micron 100 cm)… Minimum requirement: 50%
Encircled Energy Diameter < 0. 15 arcsec.
This requirement is derived from science use cases performing on-disk observations, but it presumes
excellent seeing conditions when good images can be obtained over a wider field of view than can
typically be obtained with single-conjugate AO (adaptive optics). Hence, the aO (active optics
controlling M1 figure) and tip-tilt loops are closed, but the high-order AO system is not in use.
Seeing limited coronal observations at 1.0 m
Off-pointing up to 1.5 solar radii, wavelength 1 micron, excellent seeing conditions: r0 (1 micron)
50 cm, FWHM seeing limited PSF 0.4 arcsec. The minimum resolution required for coronal
magnetometry is 2 arcsec. The Telescope shall deliver the following image quality:
50% Encircled Energy Diameter < 0 .7 arcsec
85% Encircled Energy Diameter < 2 arcsec
This requirement applies to most coronal observations. “Off-pointing” implies that there is no
granulation present within the 5-arcmin field of view, so no wavefront information is available. The
AO loop (including tip/tilt) is open. Similarly, the aO loop is open, and the primary mirror figure
and telescope alignment must be corrected based on constant or repeatable errors via a look-up table
or function fit.
These three cases are of particular interest because they span the range of possibilities for wavefront
correction, as shown in Table 1.3. Complete error budgets with Monte Carlo simulations show that
ATST meets the science requirements for each of these cases. An example is shown in Section 1.1.5.
Table 1.3. Wavefront correction cases.
Active Optics Loop Tip-Tilt Loop High-order AO loop
Diffraction limited observations
Closed Closed Closed
Seeing limited on-disk observations
Closed Closed Open
Seeing limited coronal observations
Open (Look-up table)
Open Open
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The fundamental difference between these three observing modes is the range of spatial frequencies that
can be corrected in each case. It is useful to look at the problem in terms of power spectral density (PSD)
of spatial frequency errors in the wavefront delivered to the focal plane. For example, if optical surface
polishing errors are analyzed in this way, it is found that they obey a power law distribution over a very
broad range (five orders of magnitude) of spatial frequencies extending from dimensions near to the full
aperture all the way down into the realm of surface microroughness. Throughout this range the slope of
the PSD on a log-log plot is roughly –2. This is shown schematically in Figure 1.1.
When the active optics loop is closed allowing figure errors to be compensated, the power at the lowest
spatial frequencies is reduced considerably. Switching on the adaptive optics system causes further
improvement at higher frequencies. This analysis suggests that it is useful to allocate errors separately
within each of four frequency regimes. Table 1.4 defines the four frequency regimes.
With these definitions in place, it is possible to use the same error tree for all three observing cases, and
modify the error allocation according to the active controls available.
Table 1.4. Frequency regimes.
Definition Spatial Period (mm) Description
Low 4000 800 Active Optics Influence Range
(10 actuators across the 4-m primary)
Intermediate 1 800 200 Adaptive Optics Influence Range (100 mm sub aperture on primary)
Intermediate 2 200 4 Uncorrectable figure errors
High 4 down Microroughness
Figure 1.1. Periods and spatial frequencies are relative to the four-meter entrance pupil.
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Telescope Delivered Image Quality w site stats Haleakala
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.8 1.6 2.4 3.2
50% Encircled Energy (arc sec)
Pro
ba
bilit
y
Site Seeing
Bottom Up
Telescope & Instrument
Figure 1.2. Telescope delivered image quality with site statistics.
A specific example of how these conventions are used to flow down to design requirements is shown in
Table 1.5.
Table 1.5. Flow down example.
M2 Errors apportioned by frequency band
Low Int. 1 Int. 2 High Total Correction applied
140 30 8 2 144 None (Manufacturing Spec.)
70 30 8 2 77 aO loop open (with look-up table)
2 30 8 2 31 aO loop closed
0 2 8 2 8 AO loop closed
Taken in combination, the three delivered image quality error budgets place limiting constraints on many
telescope subsystems, and hence form the basis of their design requirements.
1.1.5 Performance Predictions
The error budgets maintained for ATST are used in two different modes. The first mode represents
snapshots in time, assuming specific observing conditions. The image-quality science requirements, for
example, specify seeing conditions that are good or excellent. We make assumptions about other free
parameters, like ambient temperature, wind speed, and zenith distance, and these values are entered as
constants. Normally we must adopt “worst case” values when the science requirement does not include
these details. Details are contained in the System Error Budget Plan, ATST Document #SPEC-0009.
The second mode used in the error budgets brings additional information into the error calculation. This
includes distributions of expected parameter values that affect the image quality. For example, wind-
speed statistics are available for the Haleakalā site. With these distributions in place, Monte Carlo
simulations can be performed to randomly select wind speeds weighted by the probability functions
(histograms). By looking at thousands of system manifestations, it is possible to predict the fraction of
the time that the telescope system will deliver images of a given quality.
Wind speed is a particularly interesting case to study because of the diverse ways in which wind affects
the final image. As wind speed increases the flushing it provides to the enclosure and mirrors will
improve seeing by sweeping away the warm, turbulent boundary layer. The vents in the enclosure will
allow some fraction of this flow to flush the telescope mirrors and mount structure, again improving self-
induced seeing. In all of these cases more wind is better. Wind can also degrade performance, however.
The pressure of the wind on the thin M1 mirror will cause it to deform, resulting in poorer images. The
wind also excites vibrations in the telescope mount assembly. For these cases the higher the wind-speed,
the worse the performance.
The Monte Carlo simulations built into
the error budget spreadsheets allow all of
these effects to be analyzed with
underlying wind-speed probability
distributions, and any other error
parameters for which probability
distributions exist or can be estimated.
Figure 1.2 shows the results of such an
analysis for the seeing-limited error
budget using both seeing and wind-
velocity probability distributions for
Haleakalā. The green curve shows that
ATST will meet the 0.15 arcsec
requirement most of the time. The blue
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curve includes seeing statistics for the site, and the red includes telescope effects. These give a more
accurate prediction of how often we will meet the requirements. Similar positive results are produced for
the adaptive optics and coronal cases (ATST Document #SPEC-0009).
1.2 DESIGN OVERVIEW
The design for ATST that has emerged from the requirements flow down is described in detail in the
chapters that follow. We describe each subsystem in some detail starting with the requirements place
upon it, followed by a general description of the design. The top-level organization of these subsystems
is as follows:
Telescope Assembly Enclosure
Wavefront Correction Systems Support Facilities and Buildings
Instrument Systems Remote Operations Building
High Level Controls and Software
While most aspects of these subsystems can be discussed in isolation, several important features of the
ATST design span several subsystems: the optical design and its overall performance, thermal control,
and special features of the design that facilitate coronal observations. Each of these will benefit from a
brief systems-level description. The individual components are discussed in more detail later.
1.2.1 Optical System Design
The ATST optical design has two features that distinguish it from nighttime telescopes with similar
aperture size: it is off axis and Gregorian. Both of these features are included in the design in direct
response to the science requirements, and have many practical advantages as well.
The basic idea of the off-axis configuration is
shown in Figure 1.3. M1, the primary mirror,
is a four-meter section of a 12-meter parent
parabola. The parent is shown as a wire-frame
structure across the bottom of the figure, but
only the solid shaded part on the left is used in
ATST. Similarly M2, the secondary mirror, is
a 0.62-meter section of a two-meter parent
ellipsoid. Only the solid shaded section on the
top right is used. The red beam filling the
primary represents a point source at zenith.
The underlying Gregorian design has the
feature that an image is formed in front of M2,
offering an opportunity to reject most of the
energy in the concentrated beam before
introducing it onto M2 and the optics that
follow. There is an 80-mm diameter image of
the sun formed at prime focus. Only a 13.5-
mm diameter circular section of this image (five arcmin unvignetted) is passed through the heat stop and
on to the Gregorian focus. The rest of the energy is reflected away or absorbed by a liquid-cooled heat
stop. As a result, the irradiance incident upon M2 is roughly the same as that on M1 (i.e., the same as any
object lying in direct sunlight). For an aluminum reflective coating that is 88% efficient only about 40
watts of power is absorbed by M2, which is manageable.
M1
M2
Gregorian
Focus M1
M2
M1
M2
Gregorian
Focus
Figure 1.3. Basic off-axis configuration.
Prime Focus
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The stray-light performance of such a telescope is improved by the lack of diffraction around M2 or a
spider structure supporting it. This is critical to coronal observations close to the limb of the sun (see
Section 1.2.3 below). Also, the beam is unobstructed by M2, so the full four-meter aperture is available.
Both the heat stop and the secondary mirror are outside of the beam incident on M1, reducing the effect of
any residual heat plume on seeing performance. The off-axis design also simplifies the delivery of cooling
and other utilities to the heat stop, limb occulter, and M2 since these services can be provided without
crossing the beam. Studies carried out by the ATST project and commercial vendors during the design
and development phase have shown that the off-axis optical elements do not represent any significant
technical challenge or unmanageable risk.
The ATST needs a total of sixteen mirrors to deliver the beam to the Nasmyth and coudé instrument
stations. They are organized into five different groups based on their function and relationship to the
telescope’s mechanical components.
The OSS Gregorian Optics include M1 and M2 which form an image at the f /13 Gregorian focus. They
are attached to the Optical Support Structure (OSS), which is the element of the telescope mount that
moves to track the sun as its altitude changes (see section 2.1.2). The Gregorian focal plane is not
intended for focal-plane science instrumentation, but is instead used to inject calibration sources and
targets. These mirrors will be coated with aluminum to optimize UV performance.
The OSS Nasmyth Transfer Optics include M3N, M4N and M5N which relay the Gregorian focus to the
Nasmyth observing station (Figure 1.4). They are attached to the optical support structure. M3N is a flat.
M4N and M5N are off-axis paraboloids. The Nasmyth focal plane is the first science observing station.
It is a zero-magnification transfer of the f /13 Gregorian image to a location on the altitude axis of the
optical support structure. This station will be used primarily for infrared coronal spectro-polarimetry and
other observations requiring minimum telescope polarization and maximum throughput. While the
Nasmyth image quality is not diffraction limited over the full field of view, it meets the science
requirements for the observations to be performed there. Observing at the Nasmyth station precludes use
of the coudé observing station since some of the coudé mirrors need to be moved so as not to interfere
Figure 1.4. The Nasmyth Transfer Optics.
Altitude Axis
M3N
Gregorian Focus
M5N
M4N
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with the Nasmyth beam. These mirrors are also coated with aluminum to optimize UV performance.
The remaining eleven mirrors are used to transfer the Gregorian focus across the altitude and azimuth
axes and down to the coudé observing station (Figure 1.5). At noted previously, it is not possible to
observe simultaneously at the Nasmyth and coudé observing stations. All of the remaining mirrors are
coated with protected silver to optimize the visible and near-IR throughput at coudé.
The OSS Coudé Transfer Optics include M3 and M4. They serve to place the beam over the altitude axis
and, like the Nasmyth transfer optics, they are attached to the optical support structure. M3 is a folding
flat, and M4 is an off-axis ellipsoid that transfers the Gregorian focus to an unused focal plane above the
coudé room. M4 also re-images the telescope’s entrance pupil onto M5, the fast steering mirror. M3,
because of its location close to a focus, is also part of the quasi-static alignment system (see section 3).
The Mount Base Assembly Coudé Transfer Optics include M5 and M6. Their primary function is to
transfer the beam over the azimuth axis. M5 also serves as the fast steering mirror for the wavefront
correction subsystem (see section 3). While folding the beam vertically down the azimuth axis could be
Figure 1.5. The ATST coudé optical train (left) with exploded views of the OSS and mount base
transfer optics (top right) and the coudé feed optics (lower right).
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accomplished with one mirror at a 45 angle, this significantly increases the size and hence the mass of
the mirror which would degrade its ability to make fast tip-tilt corrections to the wavefront. M6 is close
enough to the pupil image that it is used as a compensator in the quasi-static alignment system. Both
mirrors are attached to the mount base assembly, which caries the optical support structure, and rotates to
track the sun in azimuth.
The Coudé Rotator Optics include M7 through M13. They have several important functions, including
beam folding, wavefront conditioning (to remove aberrations inherent in the off-axis Gregorian
telescope), and beam scaling to form a correctly sized pupil image on M9 (the wavefront correction
subsystem’s deformable mirror described in more detail in section 3). These optics rotate with the coudé
observing station.
The output of this set of optics is a collimated optical beam rather than an image. This is desirable due to
the varying focal-ratio and beam-size requirements of the coudé focal-plane instrumentation. Each
instrument will form its own, optimized image. The collimated beam is also ideally suited for beam
splitting and bandpass selection when multiple instruments are in use simultaneously since beam splitters
and filters inserted into a collimated beam do not introduce significant optical aberrations as they would
in a converging beam. The beam diameter emerging from M13 has a diameter of about 320 mm.
The characteristics and functions of the sixteen mirrors that make up the ATST optical system are
summarized below in Table 1.6.
Table 1.6. Summary of the functions of characteristics of the ATST mirrors
Mirror Function Diameter Optical Characteristics
OSS Gregorian Optics
M1 Primary mirror 4240 mm Off-axis concave parabola
M2 Transfer f /2 prime focus to f /13 Gregorian,
compensate for misalignment, and provide fast
steering during coronal observations.
635 mm Off-axis concave ellipsoid
OSS Nasmyth Transfer Optics
M3N Folding mirror 220 mm Flat
M4N Nasmyth transfer mirror 1, f /13 to f /13 390 mm Off-axis concave parabola
M5N Nasmyth transfer mirror 2, f /13 to f /13 370 mm Off-axis concave parabola
OSS Coudé Transfer Optics
M3 Folding mirror, quasi-static alignment
compensator
160 x 114 mm Flat
M4 Transfer f /13 Gregorian to f /50 intermediate
focus
450 mm Off-axis concave ellipsoid
Mount Base Assembly Coudé Transfer Optics
M5 Fast steering mirror for coudé observations 220 mm Flat
M6 Quasi-static alignment compensator 275 mm Flat
Coudé Rotator Transfer Optics
M7 Wavefront conditioning mirror 540 mm Off-axis concave
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hyperboloid
M8 Folding mirror 450 mm Flat
M9 High-order deformable mirror 220 mm Nominally flat
M10 First corrector mirror 275 mm Off-axis concave ellipsoid
M11 Second corrector mirror 175 mm Off-axis convex ellipsoid
M12 Folding mirror and AO beam splitter 275 mm Partially silvered flat
M13 Folding mirror 450 mm Flat
1.2.2 Thermal Control
The ATST design addresses the need to control so-called “self induced seeing,” which is seeing that
results from the presence of the telescope and telescope enclosure. This component of seeing tends to
contain higher spatial frequencies than atmospheric wavefront distortions, so the adaptive optics system is
less effective at correcting it when present. We have paid careful attention to this problem, particularly
because observations will be carried out during daylight hours when the sun can heat exposed
components to well above ambient air temperatures.
One of the common methods of controlling seeing at existing solar telescopes involves evacuating the
beam column within the optical system. This scheme is not possible with ATST because of the
requirement to observe simultaneously over a broad range of wavelengths, thus precluding use of an
entrance window at the top of the column. Instead ATST will use the approach of actively controlling the
temperature of insolated components such as telescope mirrors and the enclosure’s outer skin. Our
various studies and experiments have shown that if the temperature of these components can be
maintained close to or slightly below the ambient air temperature, self-induced seeing can be controlled
within the error-budget allocations. The details of this process are discussed item by item in the following
chapters.
Another aspect of the general thermal-control strategy involves active or passive flushing of surfaces
within or near to the optical beam. This is provided either by natural winds that enter the enclosure
through passive ventilation gates, or forced ventilation provided by fans that can be operated during
periods of little or no wind. These systems will be discussed in more detail below in the enclosure
description (Section 6).
1.2.3 Coronal Capabilities
The case for including coronal capabilities in the ATST design has already been made in the scientific
justification for the facility. In summary, while space-based instruments can generally deliver very sharp
images of the sun, it is the four-meter aperture of ATST that will make it a unique and powerful tool for
coronal science. An aperture of this size will allow scientists to do high-resolution coronal spectroscopy
– and hence polarimetry – to probe the magnetic properties of off-limb coronal features.
As we noted in the error budget discussion in Section 1.1.4 above, the requirement on image quality for
coronal use cases is relaxed considerably relative to the diffraction-limited observations. The diffraction
limit for a four-meter telescope operating at 1 μm is 0.06 arcsec, while the SRD specifies 0.7 arcsec (both
in terms of 50% encircled energy diameter) for coronal use cases. The emphasis here shifts to observing
relatively faint features that are very close (as close as 5 arcsec) to the bright limb of the sun. In addition
to the requirement to build ATST at a site with low levels of atmospheric scatter, several features of the
design presented in the following chapters are included specifically to address coronal science needs:
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A Nasmyth platform is included, providing a faster beam with a minimum number of reflections
(described in Section 4.1.2) and minimum telescope polarization.
The coronal Near IR Spectro-polarimeter is housed in a cryostat to reduce thermal background
levels (described in Section 4.2.3).
An active occulting system is provided to block light from the photosphere while observing the
corona (described in Section 2.3.2).
Many aspects of the optical and mechanical design of the telescope (including the off-axis
configuration discussed above) are driven by the desire to keep stray light to a minimum.
This last bullet point addressing stray-light control is critical to the success of ATST. The science
requirements for coronal observations at the Nasmyth focus place strict and challenging constraints on the
stray-light performance. Detailed stray-light analysis on the ATST design confirms what experience has
shown with existing coronal instruments operated at excellent sites: when observing close to the limb of
the sun, instrumental scattered light is dominated by scatter off the telescope’s mirror. When mirror
surfaces are clean, the underlying microroughness imparted by the polishing process limits the
performance. By specifying an RMS microroughness of 2 nm, the science requirements can be
comfortably met. This level of polish is routinely achieved using modern polishing techniques, so this
requirement presents no manufacturing challenge.
It is found, however, that close attention must be paid to keeping the surfaces clean during coronal
observations. If only 0.01% of the mirror surface is covered by dust, the dust contribution to instrumental
scatter overtakes mirror microroughness to become the dominant factor. The length of time necessary to
reach or exceed the 2510-6
at 1.1 solar radii requirement will vary considerably depending on weather
conditions, but even under average conditions this contamination level can be reached in just a few days
of uncontrolled exposure. The ATST has adopted several strategies to mitigate and control dust
contamination and its effects:
Occulting of the sun’s disk at prime focus. The ATST design includes a reflecting occulter that rejects the
sun’s photosphere at the prime-focus image, passing only the corona. (See “Active Occulter Insert,” in
Section 2.3.2 below.) For the coronal instruments operating at the Nasmyth focus, this eliminates dust on
M2 and subsequent mirrors as significant contributors since the bright on-disk radiation never reaches
those mirrors. It also reduces stray light due to diffraction around the Lyot stop located just down stream
of M2, again because no direct power from the sun’s photosphere ever reaches that point in the optical
system.
In-situ cleaning and washing system. This system – part of the M1 mirror mount assembly – incorporates
a CO2 snow cleaning capability that is efficient and convenient for use as often as required during coronal
observations, and a wet washing station that can be used without removing the mirror from its cell. (See
“Cleaning and Washing System” in Section 2.2.2 below.)
Closed vent gates. As noted above, coronal observations have much-relaxed image-quality requirements
compared with on-disk diffraction-limited observations. Hence, the open vent gates and active flushing
that must take place during high-resolution observations is unnecessary, allowing better short-term
control of dust infiltration.
Block Scheduling. Whenever possible, coronal observations will be block scheduled to take maximum
advantage of freshly coated or recently cleaned optical surfaces and good observing conditions (dark sky
conditions and low dust levels).
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Figure 2.1. Telescope Mount Assembly
2. TELESCOPE ASSEMBLY
The Telescope Assembly is comprised of (1) the telescope mount assembly; (2) the M1 assembly; (3) the
heat stop assembly; (4) the M2 assembly; (5) the feed optics; (6) system alignment; and (7) the
acquisition system. These are described in detail in this chapter.
Wherever possible, the designs of the Telescope Assembly and its subcomponents were based on
previous successful large telescope systems, including the structural layout, the servo and control systems,
the pier concept, and many of the mechanical subsystems. Where it was impossible to emulate existing
telescope designs, we have verified the ATST Telescope Assembly design by a variety of proven
methods. For example, full static and dynamic finite element (FE) studies have been performed on the
overall structure and pier. Transient thermal analyses, computational fluid dynamics (CFD) studies, and
various other calculations have been performed as well to support the design.
In addition to these analyses, a variety of potential telescope fabricators and vendors were involved in
design evaluation studies from early in the project. The purpose of these evaluations was to review the
subassembly designs, suggest technical improvements, and provide fabrication cost estimates. The results
helped refine the overall system design, and concentrated the project’s efforts on reducing technical risk
and improving overall telescope performance. Industry involvement of this type is especially useful in
addressing manufacturing concerns and logistics early in the design process.
2.1 TELESCOPE MOUNT ASSEMBLY
The Telescope Mount Assembly (TMA) appears in Figure 2.1. It provides structural support for the
major optics and instruments of the ATST observatory. It includes a variety of mechanical subassemblies,
bearings, controllers, drives, and equipment that are
used to point, track, and slew these optics and
instruments during science observations. The TMA is
comprised of six major components: (1) the Mount
and Drive System; (2) the Nasmyth rotator and drive
system; (3) the Coudé Rotator and Drive System; (4)
the Mount Control System; (5) the Pier, and (7)
Ancillary Mechanical Systems. These six items are
described in detail, below.
2.1.1 Telescope Mount Assembly Design Requirements
The Telescope Mount Assembly serves a number of
important roles and functions during science
observations. Of these, five are considered to be top-
level, or most important to the performance of ATST.
These top-level functional requirements are as
follows:
Optics Mounting: The TMA provides precise and
stiff mounting interfaces for the M1 through M13
mirror assemblies, heat stop, occulter assembly,
polarimetry optics, the acquisition system, and the
telescope alignment system. The nominal positions
and allowable deflections for all of these mounted
assemblies are derived from the Static and Dynamic
Optical Alignment specifications, which are derived
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from the delivered image quality error budgets. The worst-case (i.e., most stringent) error budget terms
were then used to design the structural members and mounting points of the Telescope Mount Assembly.
A series of detailed FE analyses was performed to optimize and validate the design and mounting
interfaces.
Nasmyth Instrument Interface: The TMA provides a precise and stiff mounting interface for the
Nasmyth instrument rotator. The specifications of this interface were derived from the requirements of the
Nasmyth instrumentation, including such factors as geometric size, overall mass, required mounting
stiffnesses, thermal considerations, stray light requirements, and a variety of handling and operational
concerns.
Coudé Lab Instrument Interface: The TMA provides a precise and stiff mounting platform for all the
coudé-lab instruments. Structural designs and stiffnesses were flowed down from the respective error
budget provisions into the design. The overall layout and detailed design of the Coudé Rotator was
validated in terms of these requirements by way of FE analyses.
Pointing, Tracking, and Slewing: The TMA provides for accurate and repeatable pointing, tracking and
slewing of the ATST optics and instruments over their required full ranges of travel. The specifications
for pointing, tracking, and slewing are based on a combination of direct flow-down from the SRD and
from derivations of the delivered image quality error budgets (e.g., drive jitter).
Throughput, Thermal, Stray Light: The TMA provides for an unobstructed optical path from the sun to
the Nasmyth instrument station and to the coudé instrument station. It does this without imparting excess
thermal input into the beam (i.e., degrading seeing) or adding deleterious stray light into the science light
paths. A number of thermal and stray-light analyses were performed to verify that the TMA layout met
these requirements.
In addition to these top-level requirements, there are a number of second-level functions that the TMA
provides. For example, the TMA is designed to allow for periodic removal of the major optics for
servicing operations (e.g., M1 stripping and recoating). The TMA also provides a variety of features and
safety systems designed to protect personnel and the telescope from damage (e.g., failsafe M1 cover; GIS
interface, etc.). The complete specifications and design for the Telescope Mount Assembly, including all
the top-level and second-level requirements are outlined in the TMA Specifications Document (ATST
Document #SPEC-0011).
2.1.2 Telescope Mount Assembly Design Description
Mount and Drive System: The mount is
comprised of two major structural elements:
the Optics Support Structure (OSS), which
rotates about the altitude axis, and the Mount
Base, which rotates about the azimuth axis.
These are shown in Figure 2.2. The OSS
provides mounting interfaces for the M1
Assembly, the heat stop, the M2 Assembly,
feed optics M3 and M4, and the Nasmyth
rotator and instruments. The mount base
provides interfaces for the M5 and M6 coudé
transfer optics.
The structural design of the Mount employs
large steel weldments that have been stress
Figure 2.2. Telescope Mount Assembly, showing the OSS,
access platform, Mount Base, Azimuth Track and Bearings.
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relieved prior to final machining. This technique has been very successful on many other recent telescope
projects, such as Gemini, SOAR, and WIYN. Detailed FE analyses were used to verify the basic design.
The Mount structure is configured with minimal bolted joints. This is critical to minimizing non-
repeatable errors that can affect telescope performance. The overall Mount layout provides high stiffness
(i.e., minimize static and dynamic flexure), is resistant to vibrations (e.g., wind-induced resonances), and
allows for direct load paths from the supported optic assemblies down into the structure, the bearings, and
ultimately the concrete pier and the ground. The major subcomponents of the Mount are described as
follows.
Optics Support Structure: The OSS is a performance-based design, configured to accommodate the
large bending loads of the off-axis optical layout. The M1 Assembly alone weighs more than 12,000 kg,
with its center of gravity cantilevered four meters horizontally from the altitude axis. This structural
challenge is met by an optimized two-piece layout of the OSS.
The bottom portion of the OSS carries the large off-axis loads via an arrangement of large square and
rectangular steel tubes that are optimized for bending loads. This system maximizes stiffness by making
the best use of the relatively stiff section properties (i.e., large area-moment of inertias) of the rectilinear
shapes. The upper portion of the OSS, in contrast, utilizes round steel tubing members to reduce weight. It
also results in minimal thermal mass, and reduces airflow obstructions on the upper portion, which
improves thermal flushing and minimizes the cross-sectional areas and coefficients of drag of the upper
OSS.
The upper portion of the OSS is joined to the lower via non-slip bolted joints. The two-piece
configuration eases manufacturing and helps facilitate transportation from the fabricator to the ATST
observatory site. When bolted together, the complete assembly is partially self-compensating for relative
M1-to-M2-to-altitude axis displacements as the OSS rotates from zenith to horizon. It also is extremely
stiff, allowing only minimal deflections and rotations of the heat stop assembly, M1-M2 optic assemblies,
and the Nasmyth instruments. The M1 Assembly is installed into the OSS from underneath via a
specialized handling/lifting cart.
Mount Base: The mount base assembly is a large machined weldment constructed of plate steel of
moderate thicknesses. All welds are full penetration-type welds. To reduce the possibility of structural
creep and/or hysteresis, all components are thermally stress-relieved after welding and prior to final
machining.
The mount base coudé transfer assemblies (M5 and M6) are supported on a reinforced vertical column
that rises from the top of the plinth up to the altitude axis. The static and dynamic structural performance
of the Mount Base has been verified with FE analyses.
Bearings and Azimuth Track: The mount bearing system is comprised of three major parts: i) the
azimuth bearings; ii) the altitude bearings; and iii) the hydraulic supply system that is located in the
support and operations building (S&O building). The azimuth bearings are comprised of two major
subassemblies: a) the axial bearing shoes; and b) the radial guide pads. The axial bearing shoes are
mounted to the underside of the mount base and bear vertically downward against the top surface of the
azimuth track. There are four of these shoes, each located in a stiff corner of the mount base. The altitude
bearing system is comprised of large bore rolling element bearings that are sized to stiffly carry the OSS
radial and axial loads, while minimizing start-up and running friction. The bearings are designed to be
replaced in-situ and fully lubricated by periodic preventative maintenance operations.
The azimuth track is a large diameter welded and machined steel structure that serves a number of
purposes. First and foremost, it provides a stiff and smooth surface upon which the azimuth axial
hydrostatic bearings ride. It also provides mounting surfaces for the azimuth drive ring gear assembly (on
the O.D. of the track) and the azimuth brake rotor (on the I.D. of the track). The track itself is constructed
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of steel plate and is built in segmented sections that allow into relatively easy transportation and handling.
These sections are designed in such a manner as to facilitate reassembly at the observatory site and future
replacement of track segments in the event of damage. The azimuth track is designed to carry the axial
loads of the mount structure above it, via the azimuth axial bearings, transferring this load directly into
the pier. The azimuth track also serves as a large collection trough for the oil spent by the azimuth axial
hydrostatic bearings.
Mount Drive System: The mount drive system is comprised of the drive assemblies, encoders, fiducials,
brakes, and over-travel stops that allow slewing, pointing, and tracking of the mount structure. The
altitude and azimuth axes of the mount are driven by way of a gear-type drive system. To help minimize
the number of spare parts required, the same drive motors, amplifiers, and tachometers are used for both
axes of the mount. Drive motors are brushless DC-type. The design of the mount drive system is dictated
primarily by the pointing, tracking, and slewing requirements of ATST. The top-level specifications are
as follows:
Blind pointing < 5 arcsec
Offset-pointing < 0.5 arcsec
Tracking stability < 0.5 arcsec/hr
Tracking rate = solar rate
Slew speed = +3º/sec.
On the azimuth axis, a large precision helical-cut ring gear is mounted to the outside diameter of the
azimuth track assembly. This gear is built in replaceable sections that help facilitate shipping and
assembly on site. The gear segments are keyed together to ensure that there is no cogging or other drive
error when the pinion gears pass over the joint and so that assembly/reassembly can be readily facilitated.
The altitude drive system is comprised of two pairs of drive motors and gear heads. One pair is located at
the bottom of the inner surface of the +X-side mount column. The other pair is in a similar location on the
-X-side mount column. Large ring gear segments are bolted to the outer radius of the altitude disks.
Mount Thermal Control: The telescope mount has a large amount of surface area, much of it above the
level of the primary mirror. It is therefore
important that the mount temperature track
the ambient air temperature closely to avoid
self-induced seeing. The mount temperature
is controlled primarily by shading it from
direct solar radiation with the enclosure. In
the absence of solar heat loads, the main
source of temperature differences is the
thermal inertia of the mount. As the ambient
temperature rises in the morning and falls in
the late afternoon, the mount temperature
will lag the ambient air temperature by an
amount that depends upon the wind speed.
Wind passing over the mount helps to
reduce the lag time. In addition, provision is
made for drawing ambient air through the
interior of structural members to reduce the
thermal equilibration time.
Figure 2.3. The Nasmyth Rotator Structure.
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Nasmyth Rotator and Drive System: The Nasmyth Rotator Structure is comprised of two major
components: the inboard support assembly and the outboard support assembly (Figure 2.3). These two
components provide support and rotation of the Nasmyth instrument. The range of travel of the Rotator is
± 270 degrees. The Nasmyth rotator drive system is comprised of the drive unit, encoder, fiducials, and
brakes that allow slewing, pointing, and tracking of the Nasmyth instrument. The drive system for the
Nasmyth structure is comprised of a direct-drive brushless DC-type motor (e.g., Kollmorgen) built into
the rotator structure inboard support assembly. SOAR has used a similar system with good results on its
Nasmyth rotator, as also have the SOLIS
telescope project on its mount drives.
Amplifiers are fully-programmable
devices from the same manufacturer as
the motor.
Coudé Rotator and Rotator Drive
System: The coudé rotator structure is
comprised of the coudé frame, the
support tower, the coudé bearings, the
coudé floor system, and the instrument
support beam assemblies (Figure 2.4).
The coudé frame is octagonal is shape
when viewed in plan. It is constructed of
large wide-flange and box beam
structural elements. These items are
joined together in large weldment
assemblies that are then bolted together
with high-strength bolts and precision dowels. The final configuration of this combination of weldments
and mechanical fasteners is intended to minimize the number of bolted joints yet still allow for relatively
easy and straightforward reassembly at the Site within the confines of the complete pier structure.
To accommodate the requirement for maximum configurability of the coudé lab (i.e., the positions and
orientations of the instruments located on the rotator), instrument support beams (ISB) and stand-offs are
used. The ISBs are supported on the flanges of the horizontal beams of the coudé frame. These ISBs can
be moved (slid) into the correct position and then locked down via a flange clamp at each end. The ISBs
are sized to fit flush within the coudé frame, while still maximizing their stiffness.
Coudé Rotator Drive System: Because the sizes and inertias of mount structure and the coudé rotator
structure are very similar, identical drive system components are used wherever possible. This simplifies
the final design effort, and also minimizes the number of spares that are required during operations.
Requirements were derived primarily from the worst-case pointing, tracking, and slewing requirements:
Blind pointing < 5 arcsec;
Offset-pointing < 0.5 arcsec;
Tracking stability < 0.5 arcsec/hr;
Tracking rate = nominally solar rate (dependent upon site);
Slew speed = +3º/sec.
Pier: The telescope pier is a large, steel-reinforced monolithic concrete structure that supports the mount
structure and transfers its loads into the soil. The telescope pier also supports coudé rotator in a similar
manner. The pier includes the foundations and interface to the soil. The pier also includes the stationary
floors, stairs, ladders, man-lift, crane, and other equipment attached directly to it.
Figure 2.4. Coudé Rotator.
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Mount Control System: The Mount Control System (MCS) provides control software for the telescope
mount assembly. It is an integral part of the ATST telescope mount. The MCS operates all associated sub-
assemblies, including azimuth and altitude drives, coudé rotator, Nasmyth rotator, and thermal
management. It is controlled by the Telescope Control System (TCS) for all operations except low-level
engineering activities and safety interlock situations. The MCS is directly connected to the GIS to
perform safety operations.
Ancillary Mechanical Systems:
M1 Cover Assembly: The M1 Cover provides three key functions on the TMA:
1. Impact damage: The M1 Cover protects the primary mirror from damage whenever the telescope is
not observing the sun. Dropped tools and other types of impact damage are a significant danger over
the 40-year lifespan of an observatory the size and complexity of ATST. The M1 Cover is designed to
survive an impact load of a falling 2.5 kg weight released from a height of 15 m, and to carry the
weight of up to three workers simultaneously walking on top of it.
2. Contamination control: The M1 Cover keeps dust and other airborne particulates from collecting on
the mirror at night and during non-operational periods. A slight overpressure of dry air is maintained
underneath the cover, to reduce the infiltration of contaminants.
3. Thermal safety system: The M1 Cover serves as an integral part of the ATST thermal safety system.
The Cover is designed to rapidly close in an emergency event. This closure results in an interruption
of the light path from the sun to M1, and thereby causes a safe removal of focused light from reaching
the Heat Stop.
The M1 Cover design, shown in Figure 2.5,
is based on a traditional folding panel
system common to many modern large
telescopes. Aluminum honeycomb panels,
joined with full-length hinges, ride on
guide rails. The M1 Cover is opened via an
electric drive, and then held in that position
via an electromagnetic clutch. In the event
of a power failure, the clutch disengages,
and large fail-safe springs passively close
the cover in approximately 15 seconds.
Rotary dampers are used to smooth and
control the spring closure of the cover.
The M1 Cover is designed to operate in
any orientations of the OSS. When open,
the cover folds back into a low-profile
package inboard of the mirror, to minimize
air flow obstructions.
Cable Wraps: Powered cable wraps are employed on all three axes of the TMA (Mount Altitude, Mount
Azimuth, and Coudé Rotator Azimuth; see Figure 2.6) to manage the system utilities throughout their
ranges of travel. These utilities include AC and DC power lines, copper and fiber signal and data lines,
coolant supply and return hoses, compressed air and nitrogen lines, and various communication links.
Powered wraps minimize non-repeatable torque inputs that can affect the pointing and tracking
performance telescope by isolating utility cable stiction and slip from being input into the telescope
structure. The wraps are mechanically isolated from the telescope, and they are continuously slaved to
follow the respective TMA axis rotation.
Figure 2.5. M1 Cover
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Sensor Arrays: The Telescope Mount Assembly is outfitted with an array of functional, safety, and
diagnostic sensors that are distributed on and around the system. These sensors are used to continuously
monitor the performance and health of the TMA, and to provide feedback to the Telescope Control
System (TCS), Observatory Control System (OCS), and Global Interlock System (GIS) systems. The
sensor arrays include (1) Thermal Sensors; (2) Vibration Sensors; (3) Wind Velocity Sensors; and (4)
Bearing Health Sensors.
2.2 M1 ASSEMBLY
The M1 Assembly (Figure 2.7) is the heart of the
ATST telescope; it contains the primary four-meter
diameter off-axis mirror (M1) that is the first element
in a chain of optics that collects and focuses the solar
energy into high-resolution images. The assembly
consists of the four-meter diameter M1, the axial and
lateral support system for M1, the M1 cell, the M1
thermal control system, M1 cleaning and washing
system and M1 control system. The M1 assembly
defines the position of M1 and maintains its optical
figure under the operating conditions of changing
gravity load, thermal conditions and wind loading.
The M1 support system also has active optics
capability, to slowly adjust the figure of M1 to
compensate for a wide range of effects, including
changes in zenith angle and thermal conditions.
2.2.1 M1 Design Requirements
The SRD defines the aperture of the telescope as four meters. The M1 is consequently sized at 4.24
meters diameter to yield a 4.0-meter clear aperture after the necessary baffling of the outer edge and
accommodation of the inclination angle of M1 with respect to the incoming solar beam. The optical
quality of M1 is critical to maintaining the required solar image quality; a surface figure of 32 nm rms
must be maintained over the operational limits of 0 to 80 zenith angle (changing gravity vector),
thermal conditions and wind loading. A comprehensive M1 Assembly Specification has been developed
to address these requirements (ATST Document #SPEC-0007).
Figure 2.7. M1 Assembly.
Figure 2.6. Azimuth (left) and Altitude (right) cable wraps.
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120 AXIAL
SUPPORTS
24 LATERAL
SUPPORTS
Figure 2.9. Axial and Lateral Support Point Locations
2.2.2 M1 Design Description
M1 Mirror: The M1 Mirror is a 4.24-meter diameter constant thickness meniscus, approximately 100
mm in thickness. The physical configuration is an off-axis paraboloid with a focal length of 8000 mm,
effectively part of a 12.1-meter diameter f/0.67 parent paraboloid as shown in Figure 2.8.
The material for M1 is ultra-low expansion fused silica or glass-ceramic; this choice was driven by the
large temperature gradients from the front to the back of M1 due to solar loading and thermal control.
Two materials are available at the four-meter scale for an M1 blank: ULE from Corning, Inc., New York
and Zerodur from Schott Glasswerke of Germany. Both materials have a long and well-established
history of use in astronomical and solar telescopes.
M1 Support System: The function of the M1 Support System is to support the weight of the M1 and
maintain its nominal surface figure to within 80 nm RMS over the operational zenith angles and thermal
environments of the telescope; it also defines the position and orientation of M1. In addition, the support
system makes small changes to the surface figure of M1 by applying active optics correction forces
through the axial support actuators.
The support system is composed of an array of axial
supports and an array of lateral supports. The axial
support system consists of 120 discrete support
actuators arranged in five concentric rings on the
back of the mirror. The body of each axial support
actuator is attached to the M1 cell and a support rod
from each actuator is attached to a load spreader
bonded to the back of M1. The lateral support system
consists of 24 discrete support actuators arranged
around the periphery of M1. The body of each lateral
actuator is attached to a bracket at the outside edge of
the M1 cell and a support link from each lateral
actuator is attached to an Invar pad bonded on the
outer edge of M1. Figure 2.9 shows the arrangement
of axial and lateral support points and arrows that
represent the force vector applied by each support.
Ø12100 PARENT PARABOLOID
4000
(4100)
100
ALL DIMENSIONS IN MM.
Ø4237
SURFACE A
SURFACE B
ATST PRIMARY MIRROR
PARENT
PARABOLOID
GEOMETRICAL AXIS OF
ATST PRIMARY MIRROR BLANK
AXIS A,
GEOMETRICAL AXIS OF
PARENT PARABOLOID
A A
SECTION A-A
Figure 2.8. M1 Mirror.
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Finite element analysis was used to optimize the axial and lateral support systems, a process that has been
used for the last two decades for the design of modern astronomical telescopes. However, whereas
nighttime telescopes optimize for best performance at zenith and allow slow performance degradation
with increasing zenith angle, ATST requires a high level of performance at all zenith angles, with
optimum performance at zenith angles of 65° to 80° to match the best atmospheric seeing profiles
measured at the chosen Haleakala site. First, the radial location and force value of the five rings of axial
support actuators was determined. The goal in this process was to reduce the deflections of the optical
surface of M1 caused by gravity at a zenith pointing position to an acceptable level that meets the error
budget requirements for image quality. During the optimization, it was assumed that the force applied by
all the axial actuators in any given ring is equal (this clearly follows from the rotational symmetry of M1).
Also, the nominal force values for each of the five rings were limited to a range to allow one actuator to
service all rings.
Since the back of the meniscus mirror is
smooth and continuous, there were no
constraints on the radial locations of the
five rings. The optimization was successful
in yielding a surface figure accuracy of 18
nm rms with a force value of 180 N for
ring 1 and 320 N for rings 2 through 5.
These force values are for the passive
support of M1; small changes up or down
from the nominal value will be made for
active optics correction. Figure 2.10 shows
a plot of the optical surface deformation of
M1 at zenith pointing on the 120 axial
supports.
A similar optimization routine was carried
out for the lateral support system. In this case M1 is at the position for a telescope zenith angle of 80º,
oriented in a near vertical, slightly over-hanging configuration. The 24 lateral support points on the
perimeter of M1 are equally spaced 15º apart and symmetrical about the vertical axis as shown in Figure
2.9. This arrangement was chosen to allow adequate clearance between actuators. During the optimization
process, the force level and vector orientation of each support was varied. Figure 2.11 shows a plot of the
optical surface deformation of M1 at a zenith angle of 80° with active optics correction applied.
50 nm
-80 nm
50 nm
-80 nm
Figure 2.10. M1 Optical Surface Deformation for a Zenith
angle of 0°
Figure 2-11. M1 Optical Surface Deformation for a Zenith angle of 80°
Figure 2.12. M1 Cell Configuration
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M1 Cell: The M1 cell is a steel structure, with a structural plate on the rear and a honeycomb rib
structure attached to the rear plate (see Figure 2.12). The cell will support the axial and lateral support
mechanisms and the air distribution system for the thermal control of M1, and interface to the OSS. The
axial support mechanisms extend through holes in the rear plate of the cell and are serviceable from the
rear of the cell. The overall design of the M1 cell is proven, having been used on many existing nighttime
telescopes such as ESO NTT, WIYN and Gemini.
Thermal Control System: The M1 receives the largest amount of solar heat load of any optic and also
contributes the most to localized “seeing” that can degrade the quality of the solar image. The purpose of
the M1 thermal control system is to remove the solar energy that is absorbed by M1 and to maintain the
M1 optical surface temperature as close to ambient as possible. Both analysis and empirical observation
has shown that maintaining the optical surface of M1 at or slightly below ambient temperature, combined
with wind flushing of the surface will minimize local seeing to acceptable levels.
The M1 thermal control system is an array of air
jets, or tubes, located behind the rear surface of
M1 that direct conditioned air against the rear
surface. Several hundred air jets are fed by a
network of larger distribution tubes; the array of
air jets and distribution tubes are divided into six
zones with each zone fed by its own fan and
liquid/air heat exchanger (see Figure 2.13).
Considerable analysis has been performed to
determine the effectiveness of the thermal control
system under different conditions. There is a
significant time lag between when a change in
cooling air temperature is applied to the backside
of the mirror and when this change is seen on the
front optical surface due to the thickness and
relatively poor thermal conductivity of the M1
substrate. However, the diurnal change in
temperature over the observation period is
predictable and daily temperature models will be developed and used as base curves to drive the mirror
temperature. This approach, combined with wind flushing across the optical surface of M1, allows the
temperature of the optical surface to be maintained within 1 to 3 ºC below the ambient air temperature.
Cleaning and Washing System: The M1 Assembly will have the capability of cleaning the M1 on a
daily basis and in-situ washing of M1 on a periodic basis. A CO2 dispersal device will be attached to the
M1 cover for cleaning of M1 at the beginning of each day (Figure 2.14). This will be done with the
telescope in a near horizon pointing position; particulates and other matter removed from the surface of
M1 during the cleaning operation will be collected by a vacuum trough and removed from the area.
During in-situ washing, the telescope is moved to a near horizon pointing position and a collection trough
is positioned at the lower edge of the mirror to collect cleaning effluent (Figure 2.15). The optical surface
of M1 is washed, rinsed and subsequently dried with an air knife that is mounted on the telescope mount
next to the M1 cell.
Figure 2.13. M1 Air Jet Distribution System
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M1 Control System: The primary tasks of the M1 Control System are to control the application of active
forces to M1 and to control the M1 thermal management system. The system accepts input mirror figure
information at up to 10 Hz and blends and averages this figure information at up to 0.1 Hz. It also controls
the temperature of the front side of M1 and the aperture stop to within a pre-determined range around
ambient temperature. The system will also store and apply a 24-hour thermal profile estimation to be used
in the thermal control of M1. The M1 Control System also provides status information at up to 10 Hz and
interfaces to the TCS, GIS and OCS.
2.3 HEAT STOP ASSEMBLY
The main purpose for arranging the ATST optics
in a Gregorian optical configuration is to reject
energy at prime focus before the concentrated
beam is directed onto M2. The full 32-arcmin
solar image is formed at prime focus, but only a
five arcmin part of that image is allowed to
proceed onto subsequent elements of the optical
system (Figure 2.16). The rest is reflected,
trapped, and the heat is pumped away.
In many observing scenarios (on-disk and off-
disk coronal), the heat stop assembly is simply
required to block the occulted field (OF) and pass
the field of view (FOV). Observations very near
the solar limb, however, require the prime focus
occulter to quickly and actively track the solar
limb.
2.3.1 Heat Stop Design Requirements
The heat stop assembly provides five top-level functions. They are as follows:
Block OF: The heat stop assembly blocks (reflects) solar disk light at prime focus over an area
sufficient to allow off-pointing as much as 2.5 solar radii (SRD requirement, approximately 82
arcmin).
Pass FOV: The heat stop assembly allows a 5-arcmin FOV to pass to M2.
Figure 2.14. CO2 dispersal as M1 cover opens.
Figure 2.15. Edge seal and collection trough for M1
washing.
Figure 2.16. Cross sectional view of the heat stop
reflecting cone and trap when the telescope is pointed at the center of the sun.
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Track Solar Limb: During near-limb coronal observations the heat stop assembly occults limb
light while actively compensating for telescope shake and atmospheric seeing. It must permit
coronal observations as close as 5 arcsec of the solar limb.
Remove Irradiance Load: The heat stop assembly removes the prime focus irradiance load of up
to 2.5 MW/m2 from the optical path.
Minimize Self-Induced Seeing: The heat stop assembly introduces no more seeing than the error
budgets allow. Experiments and scaling laws for small hot objects near M2 indicate insensitivity
for seeing-limited observations (e.g., Beckers and Melnick, 1994, and Zago, 1997). A reasonable
bottom line requirement is that surface temperature must be kept within some 10 ˚C of the
ambient air temperature.
In addition to the five top-level requirements, there are a number of second-level requirements such as
easy periodic removal of major subsystems for servicing and replacement, and safety systems to protect
personnel and the telescope from damage.
The complete specifications and design for the heat stop assembly are outlined in the Heat Stop Design
Requirements Document (ATST Document #SPEC-0003).
2.3.2 Heat Stop Design Description
The heat stop assembly consists of a reflector assembly, an active occulter insert, a coolant loop, a plume
control system, a beam dump, and control and interlock systems.
Reflector Assembly: The Reflector Assembly is the heart of the heat stop assembly (Figure 2.17). The
assembly is designed to remove high heat flux with minimum temperature rise in a compact package. The
reflector cone is the first component encountered after M1 and sees the solar image at prime focus, nearly
2.5 MW/m2 irradiance at noon. The reflector is made of a highly conductive, high strength alloy of
copper, coated with AlMgF2 to provide high reflectivity. The reflector is cooled from behind by an array
of liquid jets.
Outside the reflector assembly lies the safety shield, a ceramic ring mounted on the M1 side of the heat
stop assembly to provide passive irradiance protection in the event of a tracking or power failure.
Figure 2.17.Three-D view of the reflector
assembly and heat trap.
Figure 2.18. The Active Occulter Insert
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Figure 2.19. M2 Assembly.
Active Occulter Insert: The Active Occulter Insert (AOI) is a small conical device that fits into the
conical interior of the Reflector Assembly (Figure 2.18). The AOI provides a solar limb shaped occulting
edge in the center of the FOV for observations very near the limb (as close as 5 arcsec). The AOI rotates
to follow the solar limb and senses and tracks the solar limb at rates of several tens of Hz. The poor
quality of the prime focus image away from the center of the field of view will require that the radius of
the occulting edge be somewhat greater than that of the sun’s image.
Plume Control System: The Plume Control System (PCS) creates a flow of air across the Reflector
Cone that sweeps away buoyant flows. The PCS consists of a blower, a getter, supply fans, and ductwork.
Coolant Loop: The Coolant Loop supplies the Reflector Assembly with coolant of the proper
temperature, pressure, and flow rate. The Coolant Loop consists of a heat exchanger that transfers energy
to the primary coolant supplied by System Services, a secondary coolant that is both highly effective at
transferring heat and compatible with optics and mirror coatings, pumps that circulate the coolant, an
accumulator that stores a sufficient volume of coolant to supply the HSA during emergency conditions,
pressure relief valves, and an array of functional, safety, and diagnostic sensors.
Beam Dump: The Beam Dump absorbs the irradiance reflected
from the Reflector Plate. The irradiance at the Beam Dump is
spread out over a large area cooled by liquid from System
Services.
Control and Interlock Systems: The Heat Stop Control System
(HSCS) is responsible for the control and coordination of the
HSA, including the pump speed, system pressure, coolant
temperature, and the sundry sensors. The HSCS provides all
software interfaces for these components to the TCS, OCS, and
GIS. The HSA Local Interlock Controller provides an
independent safety override.
2.4 M2 ASSEMBLY
The M2 Assembly (Figure 2.19) contains the 635-mm diameter off-axis mirror (M2) that is the second
element in a chain of optics that collects and focuses the solar energy into high-resolution images. The
assembly consists of M2, the M2 positioning system composed of a hexapod and fast tip-tilt mechanism,
the M2 thermal control system and the M2 control system. The M2 positioning system defines the
position of M2 and maintains its position under the operating conditions of changing gravity load, thermal
conditions and wind loading. The M2 positioning system also provides fast tip-tilt motion to compensate
for some aspects of atmospheric seeing.
2.4.1 M2 Design Requirements
The M2 is sized at 635-mm diameter to yield a 5-arcmin unvignetted field after the necessary increase in
diameter to eliminate outer edge effects. The optical quality of M2 is critical to maintaining the required
solar image quality; a surface figure of 32 nm rms must be maintained over the operational limits of 0 to
80 zenith angle (changing gravity vector), thermal conditions and wind loading. A comprehensive
Design Requirements Document (ATST Document #SPEC-0008) has been developed to address these
requirements.
Interface requirements for the M2 Assembly include interfaces to the OSS, the M2 lifter used to install
and remove M2 from the assembly, the TCS and the required utility services.
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Figure 2.20. M2 Configuration
2.4.2 M2 Design Description
The M2 is a 635-mm diameter structured mirror, approximately
75-mm in thickness at the center. It consists of a continuous
facesheet with a triangular rib pattern on the backside (Figure
2.20). Bosses are provided at three areas to allow attachment of
the mounting flexures. The optical configuration is a concave
off-axis ellipsoid with radius of curvature of 2081 mm.
The baseline material for M2 is silicon carbide. This choice was
driven by the requirement for extremely low mass and high
stiffness to achieve the desired fast tip-tilt motion; in addition,
the excellent thermal conductivity of silicon carbide minimizes
optical surface deformations under the solar load. Finite element
thermal and structural analysis was employed to evaluate
several potential materials and it was shown that silicon carbide
has the best optical and thermal performance under the ATST
operating conditions. Figure 2.21 shows a global figure change
of only 40 nm P-V for a silicon carbide M2 during the peak
solar load.
There are many different processes for fabricating silicon carbide mirror substrates, but special attention
has been given to CVD (chemical vapor deposition), reaction bonding and sintering since each of these
processes has a demonstrated capability of producing a blank of the required size. ATST personnel have
been in contact with major silicon carbide manufacturers during the design and development phase to
determine the most feasible, lowest risk and cost effective methods of M2 blank fabrication.
Positioning System: The function of the M2 positioning system is to support the weight of M2 and
define its position and orientation over the operational zenith angles and thermal environments of the
telescope. In addition, the positioning system provides fast tip-tilt motion of M2.
Figure 2.21. Global figure change for SiC substrate during peak solar load - 40 nm P-V
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The M2 positioning system is composed of a commercial off-the-shelf hexapod, a fast tip-tilt mechanism
and a three-point flexure support that connects M2 to the fast tip-tilt mechanism. The hexapod provides
six degree of freedom movement to allow x-y positioning, focus, tip-tilt and rotational orientation of M2
with respect to M1. The fast tip-tilt mechanism provides rigid body motion of M2 at a rate of up to 10 Hz
and an amplitude of 5 arcsec to counteract certain types of atmospheric seeing.
Thermal Control System: Similar to M1, M2 receives a significant amount of solar heat load and also
contributes to the localized “seeing” that can degrade the quality of the solar image. The purpose of the
M2 thermal control system is to remove this solar energy that is absorbed by the optical surface of M2
and to maintain the M2 optical surface temperature as close to ambient as possible.
The M2 thermal control system is an array of air
jets, or tubes located behind the rear of M2 that
direct conditioned air into each triangular pocket
of the structured mirror (Figure 2.22).
Approximately 140 air jets are utilized, fed by a
manifold system that provides conditioned air
from a fan and liquid/air heat exchanger.
Ancillary Equipment: The ancillary equipment
consists of the necessary utilities services for the
M2 assembly. This includes the conditioned
coolant for the M2 thermal control system.
Control System: The M2 Control System
monitors and controls the M2 positioning
system, the M2 thermal system and the M2 fast
tip-tilt system. Control of the positioning system involves taking wavefront correction input and making
the necessary changes in the position of M2 by moving the appropriate actuators on the hexapod. Look-up
tables will also be utilized to compensate for slow and predictable changes in the position of M2 with
respect to M1 due to deflections in the optical support structure over changing zenith angles.
2.5 FEED OPTICS
A train of smaller reflective optical components, both flat and powered, are used to transfer the solar
beam from the Gregorian focus of the moving telescope assembly to the stationary observing floor, then
down to the coudé observing rooms. Their specific functions were discussed above in Section 1.2.1, and
illustrated in Figure 1.5. Their optical properties were summarized in Table 1.6.
As with M1 and M2, all of these mirrors receive significant solar heat loads, and active cooling and
thermal control will be necessary at various levels to maintain their optical surface temperatures at or near
ambient air temperatures.
Most of the feed optics in the ATST are lightweighted blanks made from high conductivity material such
as silicon carbide that are impingement cooled from the rear. The material properties combined with thin
stiffening ribs and facesheets allow the required heat transfer with only small gradients through the optic
and a much faster response time which allows working fluid temperatures closer to ambient.
Finally, there are a few optics where the absorbed flux is so high that only water cooling will provide
sufficient heat transfer coefficients to maintain optical surface temperatures within their limits. These
optics are expected to be fabricated of silicon carbide or high conductivity copper using either pin-post or
channel flow heat exchanges behind the optical surface. Many examples of this type of design can be
found in synchrotron optics, where significantly greater heat transfer efficiency is required.
Figure 2.22. Thermal control air jet nozzles directed
toward back of M2.
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2.6 SYSTEM ALIGNMENT
The liberal use of off-axis optics within the ATST design presents special challenges for both initial
installation alignment and maintaining alignment of the optics within supporting structures that deflect
due to gravitational and temperature changes. These challenges are dealt with by a combination of initial
alignment procedures that ensure the optics are positioned accurately and by an active system that
maintains bore-sight and wavefront quality during operation.
2.6.1 Initial Alignment
System alignment components are provided to facilitate initial rough alignment of the telescope,
realignment after optical elements are removed and reinstalled as part of operational maintenance
procedures, and realignment at other times as required. These would include theodolites, targets, laser
positioning systems, and portable wavefront sensors.
Initial alignment of the optics must be done relative to the mechanical constraints of the azimuth and
altitude axes of the telescope. Therefore, the procedure for populating the observatory with optics starts
with the mirrors that define that portion of the beam path. These would include the OSS and mount base
assembly transfer optics for both the coudé and Nasmyth optical systems (see table 1.6 in section1.2.1 for
an overview of the optical components).
Next M1 and M2 are installed and adjusted in position relative to the transfer optics. This is first done
using mechanical datums referenced to optical metrology testing of the mirror figure, and positioning
them with accuracy better than 100 microns. After this, progressive nighttime optical testing at the prime
focus, Gregorian focus and then intermediate (coudé) or Nasmyth focus will be possible to confirm or
adjust alignment of the optics. Gross M1 deformations and alignment errors producing coma and other
low-order aberrations apparent in the point-spread function will be removed based on guidance provided
by the optical alignment sensitivity analysis at various field points. Wavefront analysis will then be used
while observing bright stars – again depending on the sensitivity analysis for guidance – to complete
alignment of the M1 through M6/MN5, and initial calibration of the active optics system.
Installation of the remaining coudé mirrors will than be done first using mechanical datums relative to the
intermediate focus and telescope azimuth rotation and optics testing at night will continue until the entire
optical train is populated, prior to working with extended sources and the thermal issues that come with
working during the day.
2.6.2 Active Alignment
Once initial alignment is achieved it must be maintained. Finite element analysis of the telescope’s
optical and mechanical structures has given us guidance about what sort of flexure to expect as the
telescope tracks the sun. It is clear that while our mount will be stiffer than many modern nighttime
telescopes, it is still insufficient to maintain correct positioning and alignment of the optics as the
temperature and gravity vector changes. We have demonstrated that wavefront errors can be
compensated by repositioning M2, and bore-sight can be maintained with two flat mirrors in the coudé
transfer optics. The information necessary to do this will be provided by one on-axis wavefront sensor,
and a minimum of three off-axis wavefront sensors.
What complicates this quasi-static alignment algorithm is the simultaneous presence of M1 figure errors
with similar spatial and temporal frequencies to those caused by telescope misalignment. These two
sources of error must be separated from each other in the data from the suite of low-order wavefront
sensors observing multiple field points. We have modeled the telescope optics to allow realistic
perturbations to both mirror figure and system alignment, allowing us to evaluate correction algorithms.
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The work performed to date has shown no difficulty maintaining the required alignment error levels using
realistic deformations and compensators.
2.7 ACQUISITION
The Acquisition System provides a full-disk image of the sun that can be used by the observer to select
and acquire solar features. It is a small auxiliary telescope that will yield one-arcsec resolution images
through a relatively narrow-band H-alpha filter. Other alternative filters (such as one centered on the
calcium K-line) may also be provided.
In addition to the acquisition function, this system may furnish the most basic level of guiding feedback
during observations (limb guiding) to the tracking system to allow limb guiding during off-disk coronal
observations.
3. WAVEFRONT CORRECTION
The Wavefront Correction system includes the Adaptive Optics system and the wavefront-sensing
elements of the active optics system. The individual subsystems include:
A high-order adaptive optics (HOAO) system. This subsystem corrects atmospheric seeing at > 2
kHz rates. The baseline design has a 1369-actuator Deformable Mirror (DM) and a fast tip/tilt
mirror. The wave-front sensor is a correlating Shack Hartmann sensor with 1280 subapertures.
The approach builds on the very successful AO systems deployed at the Dunn Solar Telescope.
Correlation Trackers. Both the Nasmyth and coudé stations will be equipped with tip/tilt sensors
that can be used to provide image motion compensation at a fast rate. Either driving the
secondary mirror for the Nasmyth focal station or the tip/tilt mirror of the coudé transfer optics
accomplishes this task.
Active optics (aO) systems. The main task is to correct slowly changing aberrations that may
arise from gravitational and thermal deformations of the telescope structure. One of the main
objectives of the system is to keep the figure of the primary mirror within the allowed tolerances.
The secondary mirror will also be used as an active element, for example, to correct focus terms.
Alignment. The ATST’s off-axis optical system alignment requires wavefront measurements at
several points within the extended field of view. These multiple field wavefront sensors will be
available at both Nasmyth and coudé stations.
Blending. Information from different wavefront sensors (e.g., AO and aO) will be conditioned
and combined by the Wavefront Correction Control System (WCCS), which then drives the
appropriate corrector elements.
The ATST has several correctors and sensors for wavefront correction including: Quasi-static alignment
(QSA) for keeping the entire optical path– most importantly M1 and M2– aligned in closed-loop. The
active optics (aO) system’s main function is to keep the figure of M1 within spec compensating for
deformation due to gravitational and thermal distortions. Tip/tilt devices are provided for image
stabilization. High order adaptive optics (HOAO) correct atmospheric and internal seeing and residual
optical aberrations.
The sensors are:
Nasmyth active optics wavefront sensor for both QSA and M1 figure.
Nasmyth correlation tracker sensor.
Coudé aO wavefront sensor for QSA and M1 figure.
Coudé LOAO wavefront sensor for a correlation tracker image stabilizer.
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Figure 3.1. Wavefront correction block diagram
Coudé HOAO wavefront sensor for high order image corrections.
The correctors are:
M1 has 120 axial actuators that support the mirror and are able to correct M1 figure errors up to
Zernike spherical mode.
M2 is mounted on a hexapod and thus has 6 degrees of freedom for correcting telescope
alignment and focus. It is also used for image stabilization when the telescope is in Nasmyth
mode.
M3N has slow tip/tilt motion for telescope alignment. M3N is used when the light is fed to the
Nasmyth station.
M3 has tip/tilt motion for telescope alignment. M3 and after are used when the light is feed to the
coudé lab.
M5 is a fast tip/tilt mirror for image stabilization correcting both motion from the atmosphere and
telescope shake at coudé.
M6 has slow tip/tilt motion for telescope alignment.
M9 is a deformable mirror used to correct high frequency high order seeing affects.
The mount will accept offset commands to keep the image stabilization mirrors centered in their
travel.
Each of the wavefront sensors has a computer that processes the wavefront information in real time and
outputs information to one or more correctors. This is done either directly where high bandwidth is
required (e.g., DM) or through the Wavefront Correction Control System (WCCS).
The WCCS is a supervisory computer that coordinates all the wavefront correction systems. It accepts
commands from the Telescope Control System and passes them on to the appropriate system. It
determines which of the sensor systems controls which of the correctors. In some cases it blends
information from different sensors. An example is QSA, where input from the HOAO wavefront sensors
and the aO wavefront sensors is combined and passed on to a reconstructor that drives the appropriate
corrector elements. A solution to the inverse problem of building the QSA reconstructor is described
elsewhere in this conference. Figure 3.1 shows a functional block diagram of the ATST Wavefront
correction system.
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3.1 ADAPTIVE OPTICS
Adaptive optics (AO) is a critical technology that is essential in achieving the science goals of the ATST.
AO will enable diffraction-limited imaging to resolve the fundamental scales in spectroscopic and
polarimetric observations of solar fine structure, which generally require long exposures. Compared to
nighttime AO, solar AO faces a number of different challenges, and solar AO systems are in some aspects
technically more challenging than nighttime AO. The main challenges are the inferior daytime seeing, the
fact that solar astronomers mostly observe at visible wavelengths – although infrared observations are
becoming increasingly important – and the solar wavefront sensor that has to work on low-contrast,
extended, time-varying objects such as solar granulation.
3.1.1 Adaptive Optics Design Requirements
The requirements for diffraction-limited observations are discussed in detail in the SRD, and listed in
above in Section 1.1.4. In summary the requirements are as follows:
The ATST shall provide diffraction-limited observations (at the detector plane) with high Strehl
(S>0.6 required, S>0.7 goal) at 630 nm and above during excellent seeing conditions (r0 (630 nm) >
20 cm) and S > 0.3 at 500 nm and above during good seeing (r0 (500 nm) = 7 cm).
1. The wavefront sensor must be able to lock on granulation and other solar structure, such as pores
and umbral and penumbral structure.
2. Time sequences of consistent image quality are required for achieving many of the science goals.
Spectral or spatial scans often suffer from varying image quality during the scan. The AO system
shall provide consistent image quality during varying seeing conditions (time scales of seconds)
often encountered during the day-time.
3. The AO system shall correct residual (not corrected by active optics) optical aberrations and self
induced and atmospheric seeing to the performance levels specified in the SRD. Mirror seeing or
internal seeing in general must be avoided and any “residual” local seeing components must be
correctable by adaptive optics.
4. The AO system shall be robust enough to perform during transparency fluctuations typically
encountered in thin cirrus clouds.
The detailed AO systems parameters and specifications that flow from these requirements depend heavily
on the site characteristics, such as median r0, range of temporal fluctuations of r0, Greenwood frequency,
and isoplanatic patch size. Now that Haleakala has been selected as the site for ATST, good estimates of
these parameters are now in hand.
For the purpose of defining the baseline AO system we originally used the average of the median r0
values at two sites, measured at the telescope height. The average r0 is about 10 cm. The bandwidth
requirements for the AO system were derived from subaperture tilt spectra measured at the Big Bear Solar
Observatory (BBSO) site and the Sac Peak site using a wavefront sensor (WFS) with 10-cm subaperture.
The power spectra were used to estimate the Greenwood frequencies over a range of seeing conditions. A
detailed performance error budget analysis was performed to define the baseline AO system using the
average median r0 of 10 cm and the average Greenwood frequency (~32 Hz). These design parameters are
being adjusted now that Haleakalā has been selected and the telescope height has been set.
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3.1.2 High-Order Adaptive Optics Design Description
The system design is modeled very closely after the successful high-order AO system operated at the
Dunn Solar Telescope (DST). The system is based on a correlating Shack Hartmann wavefront sensor and
uses a parallel processing approach using commercially available digital signal processors (DSPs).
The ATST HOAO system will be located in the coudé observing station. It will run at approximately a
2000 Hz frame rate with a resulting –3dB bandwidth of at least 200 Hz. It will consist of a Shack-
Hartman wavefront sensor with a 40×40 square grid lenslet array, an 800×800 pixel camera, a 64 DSP
processing system and a deformable mirror with a 41×41 square grid of actuators. It measures the
wavefront by cross correlating an array of subaperture images with a reference image.
Wavefront Sensor: The WFS is a correlating Shack-Hartmann WFS similar to the one successfully used
for the DST and BBSO AO systems. The principle of the WFS is shown in Figure 3.2. The telescope
aperture is sampled by an array of lenslets, which forms an array of images of the object (e.g.,
granulation, sunspots). Cross-correlations between subaperture-images and a selected subaperture-image,
which serves as reference, are computed. These cross correlations are shown in Figure 3.2, upper right.
By locating the maximum of the cross-correlation we determine the displacement of the images with
respect to the reference, thereby measuring the local wavefront tilts.
The pixel resolution in the SH-WFS images and the 2-D cross-correlation, respectively, is typically 0.5
arcsec. The FOV is about 1010 arcsec. Image displacements are computed to subpixel precision by
fitting a parabola to the correlation peak using and interpolating between pixels. A tilt map is shown in
the lower right corner of Figure 3.2. We use a modal reconstructor to derive the actuator drive signals.
Figure 3.2: Principle of correlating Shack-Hartmann wavefront sensor
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The optical portion of the wavefront sensor will consist of a field stop, lenses, filters and a camera.
Following a beam splitter that brings a small percentage of the telescope light to the HOAO system is a
lens that brings to a focus the surface of the sun on to a square field stop that is approximately 10 arcsec
square. A second lens puts a pupil image on the lenslet array. The area of the pupil image on each of the
lenslet array lenses corresponds to a 10 cm by 10 cm area on M1. Following the lenslet array is another
lens that collimates the light onto a zoom lens and the camera. The images from each of these
subapertures will fill 20×20 pixels on the camera.
Wavefront sensor camera: A custom development is needed to produce the approximately 800×800
pixel, > 2 kHz frame rate wavefront sensor camera. The output from the camera will be 32 digital
channels each running at 40 Mpixel per second.
Wavefront Sensor Prossessing Unit: The wavefront sensor/reconstructor processing unit is also well
within the capabilities of existing technology. Utilizing improved DSP technology the ATST processing
unit can be built with a very moderate increase in size and complexity (Figure 3.3). However, we believe
that a parallel processing approach will be essential for the ATST AO not only to achieve the processing
rates required but also the high data throughput needed to achieve the > 2 kHz update rate that will be
needed for this system.
The camera channels will feed into a custom interface that will sort the pixels into subaperture order and
output them on 16 channels directly to 16 blocks of 4 DSPs (baseline: Analog Devices TigerSharc). Each
of the 64 DSPs will cross correlate a reference image with 20 of the 1280 subaperture images. The
multiplication of the resulting vector of image shifts with the pre-computed reconstruction matrix will be
performed by the DSPs as well. A servo algorithm will be applied to drive the 1369 DM actuators in
closed loop. Tip and tilt are separated out and corrected by the fast tip/tilt mirror M5. The HOAO system
will also provide time-averaged low order Zernike terms, which will be off-loaded to aO and possibly
QSA through the WCCS. The WCCS provides these averaged Zernike terms to the TCS for M1 figure
Host Computer/HOAO Control System
DSP
To
Mirror
Inter-
face
SMART
INTERFACE
Camera
To
DSPs
Sorts
Pixels
Into
Subaperture
Images
CAMERA
800x800
32 ports
40 MHz
2000 fps
Fast
Tip/Tilt
Mirror
Deformable
Mirror
FTT Out
Wavefront
Correction
Control
System
64 DSPs
Switch
From Correlation
Tracker
Figure 3.3. The AO real-time processors
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correction, M2 for telescope focus correction and the WCCS for blending with information from the aO
system for QSA.
Deformable Mirror: The deformable mirror (M9) will be a continuous phase sheet DM with 1369
actuators. The mirror is 221 mm in diameter and the actuators are on a 4040 square grid. Each actuator
will have a stroke of at least 5 m. The actuators are located in the corners of the subapertures (Fried
geometry) yielding a total of 1280 useful subapertures. One of the leading DM manufacturers (Xinetics
Inc.) considers the ATST DM a straightforward extension of their 941-actuator off-the-shelf DM system.
Thermal control — The solar irradiance imposed on the front surface of the DM is approximately 10
kW/m2. To maintain the DM front surface at acceptably cool temperatures (<5˚C above ambient
temperature), the stock DM structure will be modified to allow convective cooling of the faceplate.
Coolant (air or helium) enters the rear of the DM and impinges normally on the faceplate backside before
exiting the DM radially. In addition, the faceplate will be sealed to prevent coolant leaks, and a broadband
high reflectivity coating is used on the faceplate. External to the DM, the coolant is recirculated through a
heat exchanger that transfers thermal energy to System Services facility coolant (ethylene glycol or
similar). We are currently working with Xinetics Inc. on developing detailed thermal models and
designing the modifications necessary to the DM to include the thermal control system.
Fast-Steering Mirror: The fast-steering mirror (M5) will have a range ± 1 mrad in both tip and tilt. The
mirror will be made from light-weighted material (SiC). The mirror is 220 mm in diameter. The controller
will have position feed back from the mirror and a closed-loop bandwidth of at least 100 Hz.
Thermal control: The peak solar irradiance imposed on the front surface of the tip-tilt mirror is
approximately 9 kW/m2. To maintain the front surface of M5 at acceptably cool temperatures
(approximately 3 ˚C above ambient temperature), an air jet array cools the rear of the SiC substrate. A
flexible boot encloses the air jet array and prevents the cooling air from passing into the optical beam. In
addition, M5 is coated with a high reflectivity coating to reduce the absorbed solar heat flux. The M5
cooling air is recirculated through a heat exchanger that transfers thermal energy to System Interconnect
facility coolant (ethylene glycol or similar).
3.2 ACTIVE OPTICS
The Active Optics (aO) system includes a dedicated wavefront sensor, wavefront-sensor camera, and the
aO controller. It provides feedback to the M1 axial support system that allows the optical figure of M1 to
be maintained to the desired level. This subsystem is also responsible for maintaining the quasi-static
alignment (QSA) of the telescope using the six degrees of freedom provided by the M2 hexapod and bore
site via slow tip tilt (- correction) of M3 and M6.
3.2.1 Active Optics Design Requirements
As noted in the M1 design requirements (Section 2.2.1 above) a surface figure of 80 nm rms must be
maintained over the operational limits of 0 to 80 zenith angle (changing gravity vector), thermal
conditions and wind loading. Error signals must also be generated by this system that will maintain
quasi-static alignment of the telescope to within 70 nm rms wavefront distortion.
3.2.2 Active Optics Design Description
The ATST will have two identical, multiple field active optics wavefront sensors - one at coudé lab and
one at Nasmyth. The aO wavefront sensors will measure wavefront errors averaged over many
atmospheric realizations. This will provide information about slowly varying (quasi static) aberrations
due to optical misalignment and/or M1 figure errors. FEA analysis and optical modeling show that a low
order wavefront sensor that reliably measures about 11 Zernikes is sufficient for this task. For QSA,
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measuring the wavefront at multiple positions in the field of view is necessary to have enough
information to determine which optical elements need to be adjusted to keep the telescope in alignment.
The main objective is to distinguish between M1 figure errors and error due optical misalignment (e.g.
M2 de-center). The basic idea is to use wavefront sensor measurements from different field points
distributed over a large field of view. M1 figure errors cause wavefront errors that are constant across the
field while optical misalignment in general produces field dependent aberrations. Simulations indicate
that three positions are adequate, the center of the field and two adjacent corners. Although, it would be
straightforward to add additional field points if it turns out to be of advantage, e.g., provides better S/N.
The update rate of the aO will be about 30 seconds.
Wavefront Sensor: The optical design of the aO wavefront sensor is very similar to the AO system with
the exception that a large field of view will be imaged by each lenslet to enable wavefront sensing at
multiple field points. There are some similarities between the aO wavefront sensor approach and the solar
MCAO wavefront sensor approach. The field stop will be about 2-arcmin square pending the results of an
ongoing analysis of sensitivity as a function of separation of the field points. This means that the 2-
arcmin by 2-arcmin subaperture images will be sampled with 192×192 pixels and a camera that has at
least 1546×1344 pixels is required. Figure 3.4 shows the subaperture images as they will be imaged onto
the camera. The small grey squares are the 32×32 pixel subfield areas within each subaperture image
where the three wavefronts will be sensed.
Wavefront Sensor Camera: Wavefront measurements are required once every 30 seconds. About 300
individual wavefront measurements will be co-added in order to average out the seeing. This means that
aO wavefront sensor camera and processing unit will have to be run at about 10 frames per second. 2k×2k
Cameras that run at tens of Hz frame rate are commercially available. The processing power needed for
the aO task is also available off-the-shelf. A Zernike decomposition of the seeing averaged wavefront
measured at different field points will be output to the Wavefront Correction Control System. The WCCS
will pass the values on to the Telescope Control System for M1 figure, the M2 for focus and M1, M2, M3
and M6 for telescope alignment and bore-sight. If the light is at Nasmyth, the alignment will use M3N
instead of M3 and M6.
2’ =
192
pixels
20” =
32 pixels
Lenslet array with pupil outline Subaperture images with field points
Figure 3.4. Active Optics wavefront sensor.
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3.3 WAVEFRONT CORRECTION CONTROL SYSTEM
To coordinate the various components of the wavefront correction system, a Wavefront Correction
Control System (WCCS) will be used. It will enable the Nasmyth or coudé systems depending on where
the light is to be used. It will control which sensing system will control which corrector.
The major task of the WCCS is blending information from the operating wavefront correction systems to
determine corrections for M1 figure and adjustments to the telescope optics for alignment. For instance,
when both the aO and HOAO systems are running at coudé, the aO system is measuring the wavefront at
the center and two corners of the field. However the HOAO is correcting the center of the field so the
HOAO provides the correction it is making to the center of the field to the WCCS which it then combines
with the information from the aO system to determine corrections for M1 figure and quasi-static
alignment.
When the light is at Nasmyth and the telescope is on disk, the WCCS will enable the Nasmyth correlation
tracker to sense image motion. The correction values will be sent to M2. The WCCS will also enable the
Nasmyth aO system to measure the average wavefront in three field positions. This information will be
sent to the WCCS which will provide quasi-static alignment correction values for M2 and M3N and M1
figure correction values.
When the light is at coudé and the telescope is on disk, the WCCS will enable the coudé aO system. If
the telescope is in diffraction limited observing mode the AO system will also be enabled for HOAO and
either the correlation tracker or the HOAO system for image stabilization. The WCCS will blend
information from both the aO and AO systems for quasi-static alignment and M1 Figure.
4. INSTRUMENTATION
The description of ATST instrumentation is divided into two parts: the instrument lab facility and the
focal-plane instrumentation.
4.1 INSTRUMENT LAB FACILITY
The Instrument Lab Facility is a set of common components that directly support science instruments and
observers. The dominant requirements that affect the design of the instrument lab facility are derived
from the telescope requirements associated with resolution, polarization sensitivity and accuracy,
flexibility, adaptability, and availability. These were discussed in Section 1.1.2.
4.1.1 Polarimetry Analysis and Calibration
The Polarimetry Analysis and Calibration system is used both to modulate the beam for determining the
polarization state of solar features, and to calibrate out polarization introduced by the telescope. Because
polarimetry is nearly ubiquitous in observational solar physics, ATST provides polarimetry analysis and
calibration at the facility level, rather than making it part of the requirement for each instrument. The
science requirement for sensitivity (10-5
relative to intensity) and accuracy (510-4
relative to intensity)
place strong constraints on this system, and ultimately dictate the methods and strategies that will be used
to do polarimetry.
Polarimetry at the Nasmyth Station: Ideally the polarization introduced by the telescope and focal-
plane instruments should be kept under 1%. This goal is only approached at the Nasmyth focal plane.
This will be the observing station for the coronal module of the Near-IR Spectro-polarimeter (see
description below in Section 4.2.3), and other instruments for which the spatial and spectral resolution
requirements are relatively low.
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Figure 4.1. Block diagram of a DID.
Time-multiplexed polarization modulation and analysis will be used at all observing stations because it is
versatile, and the issues are well understood. The initial ATST facility-level modulators will include
piezo-elastic modulators (PEMs), ferroelectric liquid crystal (FeLC) modulators, and rotating retarders.
These modulators, as well as calibration polarizers, will be mounted in the Gregorian Optical Station
(GOS), which is located near (and just above) the Gregorian focal plane. It consists of two large rotating
wheels with up to five positions each. The One of the wheels is in the Gregorian focal plane; the other
precedes it.
Telescope polarization at the Nasmyth station will be calculated to the required accuracy using a detailed
Mueller-matrix model of the telescope that includes the properties of aluminum coatings and oxide
overcoatings. This model will be tested and improved by actual measurements. Polarization introduced
by the focal-plane instrument will be measured using calibration polarizers in the turret assembly.
Seeing and tracking errors introduce small changes to the sun’s image that, when sampled too slowly, can
be misinterpreted as polarization of the solar source. Several strategies will be implemented to reduce
this effect. The first is dual beam, or spatial modulation. This will be implemented by using a polarizing
beam splitter as a polarization analyzer, spatially separating the s and p polarization states. Both beams
can be analyzed independently but simultaneously to derive the four polarization states in a way that is
less sensitive to seeing effects. It has the added advantage of utilizing all of the light introduced onto the
polarization analyzer.
Spatial modulation alone still has shortcomings. The two beams will generally not pass through the
instrument along exactly the same optical path. Differential aberrations may then become important.
Furthermore, spatial modulation requires that different detectors or detector areas sample the two beams,
which makes the measurements susceptible to differential-gain effects. Whenever possible the ATST
polarization strategy will also include rapid polarization modulation (>1 kHz) and charge-caching camera
systems. These specialized cameras, dubbed DIDs for Demodulating Imaging Detectors, will be
optimized for highly sensitive and precise differential
imaging. Chopping between the four linearly
independent polarization states can be performed at
speeds in the kHz domain to provide virtually
simultaneous images without the need to read out the
array at kHz frame rates (Figure 4.1). All independent
image planes are observed with the same physical
pixel on the detector, which renders normalized
differences between image planes insensitive to the
gain of individual pixels. DIDs will have a 100%
geometrical fill factor and quantum efficiencies
approaching unity. The technology can be applied to
silicon to cover the 200 to 1100 nm wavelength range,
and to infrared-sensitive materials such as HgCdTe for
the 1000 to over 10,000 nm wavelength range.
Rockwell has expressed an interest in providing these hybrid detectors to ATST. Another possible
alternative is AIM, an independent entity of AEG Infrarot-Module, GmbH. They were part of a proposal
to the EU to develop a DID for landmine detection. These detectors are considered to be a modest and
achievable extension of the detector technology built into the most recent upgrade to the successful
Zurich imaging polarimeters (ZIMPOL II).
Polarimetry at the Coudé Station: To meet the science requirements for polarimetry of solar features
on the sun’s disk ATST must provide large instruments, slow beams, diffraction-limited imaging, and
adaptive optics to correct the wavefront. Instruments that support these observations will reside at the
coudé stations. The uncompensated oblique angles of the transfer optics below the Gregorian focus will
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introduce considerably more telescope polarization than is present at the Nasmyth station. This dictates
that modulation and analysis must be performed near the Gregorian image prior to these strongly
polarizing telescope mirrors. This allows the same GOS polarimetry components to be used for both
Nasmyth and coudé observations.
It will not be practical to pass two orthogonally polarized beams through the adaptive optics system.
Thus, the coudé instruments can perform spatial modulation only after transitioning through the
telescope’s feed optics. The DID strategy described above will also be available at coudé. Based on
experience with existing strongly polarizing telescopes, we expect that ATST will meet polarization
science requirements at coudé as well.
The individual components provided as part of the ATST polarimetry analysis and calibration system are
summarized in Table 4.1.
Table 4.1. Polarimetry Analysis and Calibration System Components.
Component Location
Achromatic Linear Polarizer Calibration Wheel, Turret Assembly
Achromatic Retarder Calibration Wheel, Turret Assembly
UV Linear Polarizer Calibration Wheel, Turret Assembly
UV Retarder Calibration Wheel, Turret Assembly
UV/Vis PEM Modulator Wheel, Turret Assembly
Rotating Achromatic Retarder Modulator Wheel, Turret Assembly
UV Rotating Retarder Modulator Wheel, Turret Assembly
Visible FeLC Modulator Wheel, Turret Assembly
IR FeLC Modulator Wheel, Turret Assembly
Vis/IR LCVR Modulator Wheel, Turret Assembly
Achromatic analyzer Gregorian Focus Wheel, Turret Assembly
8 Polarizing Beam Splitters Nasmyth or coudé stations
8 Linear Polarizers Nasmyth or coudé stations
5 FeLCs Nasmyth or coudé stations
4.1.2 Nasmyth Station
The Nasmyth Station includes transfer and re-imaging optics, mounting fixtures, and connections to
utilities provided via System Interconnects.
This observing station is mounted to the
Nasmyth Rotator Structure, which is part of
the Telescope Mount Assembly (Figure
4.2), and provides a plate scale of 3.95
arcsec/mm at f/13. Hence, the required 3-
arcmin field of view has a diameter of 46
mm, and the goal five-arcmin field has a
diameter of 76 mm. This station is suitable
for instrumentation that is relatively
compact, has relaxed spatial and spectral
resolution requirements, and can tolerate a
changing gravity vector. The Nasmyth
station has the advantage of providing an
image with the minimum number of
reflections. Thus, the Nasmyth station
provides the best throughput and the lowest
levels of scattered light and telescope
polarization. It has the disadvantage
Figure 4.2. The Nasmyth Observing Station.
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(compared with the coudé station) of no high-order wavefront correction, images that are not diffraction
limited over the full field of view, a changing gravity vector while tracking the sun, and more restrictive
instrument size and weight limits.
4.1.3 Coudé Station
The Coudé Station includes the optical tables, imaging optics, standardized mounting fixtures, camera
systems, and a connection to utilities provided via Interconnects and Services. The majority of facility
instrumentation and visitor instruments will be operated at this station. It has the advantage of full
wavefront correction provided by the upstream high-order AO system, diffraction-limited images, and a
large horizontal platform that can accommodate multiple, large instruments in a constant-gravity location.
Six facility instruments will eventually be permanently installed at the coudé station. These are the
Visible Broadband Imager, Visible Spectro-polarimeter, the coudé module of the Near IR Spectro-
polarimeter, the Visible Tunable Filter, the Near IR Tunable Filter and the Thermal-IR Polarimeter and
Spectrometer. These are discussed in Section 4.2.
To accommodate the science requirement for a flexible, adaptable facility that can be quickly configured
for a variety of diverse experiments, and to accomplish this in a cost-effective manner, ATST has adopted
a strategy that calls for a high level of standardization in our approach to instrumentation wherever
possible. This will be particularly true for observations performed at the coudé station. For example, all
facility scientific instruments being fed by a particular beam will conform to a prescribed optical height
above the horizontal optical tables. This will allow common mounts with focus and decenter motions for
cameras, filters, and other incidental optics that will be provided as part of the laboratory instrument
facility. Visitor instruments will be strongly encouraged to adopt the same standards, thus giving them
access to facility components whenever possible.
Solar observations generally utilize multiple cameras in the course of a single observation. This need is
derived from the wavelength diversity requirements placed on the instrumentation, and the need to make
efficient use of the available light. The necessary cameras will be available “off the shelf” as part of the
ATST instrument lab facility. This will allow scientist to take maximum advantage of observational
targets of opportunity. This strategy of using uniform camera and controller systems will also minimize
the cost of developing multiple camera systems and the software that runs them.
The camera systems that will be provided as part of the initial instrument lab facility are listed in Table
4.2. These same systems are available for use at the Nasmyth focus as well.
Table 4.2. Initial Camera Systems.
Camera Type Format Readout Count For use with
Fast CCD 1k1k 100 Hz 5 Vis Spectro-polarimeter, Vis Tunable Filter
Large CCD 4k4k 5 Hz 7 Vis Spectro-polarimeter, Vis Tunable Filter, Broadband Imager, Slit-jaw viewer
Large Hybrid IR 2k2k 5 Hz 5 Vis Spectro-polarimeter, NIR Spectro-polarimeter, IR Tunable Filter
Visible Light DID 2k2k 25 Hz 5 Vis Spectro-polarimeter, Vis Tunable Filter
Near IR DID 2k2k 25 Hz 4 Vis Spectro-polarimeter, NIR Spectro-polarimeter, IR Tunable Filter
4.1.4 Instrument Lab Thermal Control
The Instrument Lab Facility Thermal Control system maintains air and component temperatures within
the bounds necessary to meet self-induced seeing error budget. The coudé lab will be held at a constant,
uniform temperature while the telescope assembly above it will track the ambient outside air temperature.
Over much of the year there will be a large volume of warm air beneath the much cooler air within the
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telescope enclosure, producing a thermal instability. Many of our science use cases require simultaneous
observations spanning wavelength ranges from the visible to beyond the glass cutoff wavelength in the
near infrared. This precludes using a window to separate the two environments. Instead, the coudé
stations will be isolated via a laminar “air curtain.”
4.1.5 Instrument Control System
The strategy outlined for modular instrumentation components places a large burden on the instrument
control software. This is solved using the concept of a virtual instrument which is “assembled” both in a
hardware and software sense as part of the observing setup procedure. The details of the virtual
instrument and other important aspects of the instrument control system are discussed in the high-level
controls chapter of this document (see Section 5.2.5).
4.2 SCIENCE INSTRUMENTS
The science instruments envisioned for ATST and their relative priorities (from the SRD) are listed in
Table 4.3. The first four instruments will be built as part of the construction phase. The priority five and
six instruments – and perhaps others – will be built early in the operations phase.
Table 4.3. Science Instruments.
Priority Instrument Fore-Optics Dispersing System Detector System
1 Visible Broadband Imager Phase Diversity Interference Filters Visible
2 Visible Spectro-polarimeter Visible Polarization Analyzer
Medium Dispersion Spectrograph
Visible or special
3 Near-IR Spectro-polarimeter (Disk and Corona)
Near-IR Polarization Analyzer
Medium Dispersion Spectrograph
Near-IR
4 Visible Tunable Filter Polarization Analyzer Visible Tunable Filter Visible
5 Near-IR Tunable Filter Polarization Analyzer Near-IR Tunable Filter
Near-IR
6 Thermal-IR Polarimeter & Spectrometer
Polarization Analyzer Medium resolution, cold grating
Thermal-IR
7 Visible/Near-IR High-Dispersion Spectrograph
Visible/near-IR high-dispersion spectrograph
Visible and near-IR
Swiss Contribution
UV-Polarimeter Polarization modulation system
Visible spectrograph or narrow-band filter
ZIMPOL Detector System
4.2.1 Visible Broadband Imager
Design Requirements: The primary science requirement of the Visible Broadband Filter instrument
(VBI) is to obtain the highest possible spatial and temporal resolution image sequences from the ATST.
The study of small-scale magnetoconvective processes both inside and outside of sunspots requires spatial
resolutions on the order of 0.01 arcsec and temporal cadence values of 5 seconds or less. The spatial
resolution requirement is near the projected ATST diffraction limit, thus the VBI cannot significantly
degrade the image quality delivered by the telescope. The VBI must also be capable of making broadband
filter images in a range of scientifically important visible spectral bands on a rapid cadence. This drives
the design to use simple, high optical-fidelity, thin-film interference filters for spectral selection. Table
4.4 lists the baseline science requirements of the VBI derived primarily from the SRD and secondarily
from optomechanical design considerations.
Instrument Description: The baseline VBI consists of an optical relay unit (collimating and camera lens
systems), one or more rotating filter wheel mechanisms containing the broadband interference filters, and
a focal plane camera mounting system. Each of these systems will be designed to be removable,
replaceable, and/or reconfigurable within a horizontally mounted enclosure.
Table 4.4. VBI Instrument Requirements.
A. Optical Requirements
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Value Goal Priority Source Notes
Spectral Range 380-800 nm 330-1100 nm 1 SRD CN bandhead 388 nm
Field-of-View 3 arcmin 5 arcmin 1 SRD Unvignetted circular diameter
Spatial Resolution 0.02 arcsec 0.01 arcsec 1 SRD
Spectral Resolution 0.01 nm 0.01 nm 1 SRD Short exposure imaging
Beam Speed F/20-F/45 Variable 2 SRD Multiple plate scales
Scattered Light 10-2
I0 10-3
I0 2 SRD Sunspot umbral imaging
Instrumental Polarization
10-2
I0 10-2
I0 3 SRD No polarimetric capabilities required
Mounting Horizontal Multi-config 2 Coudé or Nasmyth focus
B. Interference Filter Requirements
Value Goal Priority Source Notes
Optical Quality /4 Φ25 mm /4 Φ100mm 1 A.3
Bandpass 0.01-0.1 nm 1 A.1 Varies with spectral region
Transmission 40% 60-70% 1 SRD Temporal resolution
Out-band Blocking 10-4
UV/IR 10-5
UV/IR 1 No active thermal control
Operating Temp. 17±3 ˚C 17±10 ˚C 1 Telescope ambient
Thermal Stability <0.01nm 1 No active thermal control
Mounting Parallelism
3—5 arcmin < 2 arcmin 2 A.3 Fringe avoidance
C. Mechanism Requirements
Value Goal Priority Source Notes
Filter wheel Speed 2 sec 1 sec 1 SRD Including settle time
Focus Lens Speed 2 sec 1 sec 1 SRD Temporal cadence < 5 sec
Camera Exposure <100 msec 10 msec 1 A.3 Freeze atmospheric seeing
The optical relay unit consists of a field lens and stop at the nominal telescope focal plane followed by a
collimating lens unit (CLU) which collimates the light prior to the interference filter stages. Following the
filters, the camera lens unit focuses the image plane onto the camera system. For a given camera pixel
size, no single focal ratio will provide a Nyquist sampled detector plane for all wavelengths in the VBI
spectral range. Therefore the VBI optical relay system will provide a range of focal ratios from F/20 to
F/45 in order to optimize a given detector’s sampling. The current concept uses a varifocal zoom lens to
provide a continuously variable focal ratio that will accommodate any camera system and pixel size.
Table 4.5. Nominal VBI Interference Filter Specifications
Filter
Central
Wavelength
nm
Bandpass
nm
CN molecular band 388.3 0.1
Ca II H & K lines 393.3 & 396.8 0.01
CH molecular G-band 430.5 0.1
Blue continuum 450.4 0.05
Green continuum 555.0 0.05
H-alpha 656.3 0.01
Red continuum 668.4 0.05
TiO sunspot bands 705.7 0.1
Ca II chromospheric magnetic 854.2 0.01
The VBI filter wheel unit will consist of up to three rotating mechanisms containing four broadband
interference filters each. The filter wheel mechanisms will be brushless DC motor driven with optical
encoders for positional readout. Each filter will be a three- cavity thin-film interference filter with a
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nominal diameter of 10 cm. The large diameter is required to ensure a 3-arcmin field of view at the
nominal ATST focal length. A list of VBI spectral ranges, specified in the SRD, is shown in Table 4.5.
The availability of large-format (8k8k), high quantum efficiency (QE ~90%), fast-readout (~1 sec)
detectors is currently extremely limited. Thus it is not possible to design the VBI for a particular camera
that will meet the ATST spatial resolution, temporal cadence, and field-of-view science requirements. We
circumvent this by incorporating a flexible detector mounting using a three-axis linear-motion stage with
micron accuracy. Any given detector can thus be placed anywhere in the 8 to 10 cm diameter focal plane
to cover the full field. In addition, the VBI camera system incorporates two separate detector stages in a
split beam configuration. This allows two cameras to be independently focused and defocused for “Phase
Diversity” (PD) imaging, thus enabling instrumental aberration measurement as well as image restoration
to the diffraction limit.
For the FLI subsystem, we envision the VBI camera stages using one or two standard, readily available
4k4k CCD cameras, and a single filter wheel (or in the simplest cases, a single filter mounted directly to
the camera system input beam). As the commissioning phase of the telescope progresses, the FLI system
can be built into the larger VBI system.
4.2.2 Visible Spectro-polarimeter
Design Requirements: The Visible Spectro-Polarimeter (ViSP) is the instrument responsible for the
spectral analysis of the visible solar light and its polarization state, recording the wavelength dependence
of the full Stokes vector (I, Q, U, V) at each spatial point in the field of view.
In order to meet the science requirements, the ViSP must be able to:
Observe the small-scale magnetic elements (flux-tubes) in the solar photosphere with an angular
resolution of at least 0.05 arcsec, or about 40 km.
Cover a large field of view of at least 3 arcmin.
Routinely attain a polarimetric precision of 10-4
times the continuum intensity. In addition to this
requirement, it would be highly desirable to reach the 10-5
level at least in particular
configurations.
Minimize seeing-induced cross talk. It should be small compared to the polarimetric precision
quoted above.
Fully resolve spectral features, including those arising from hyperfine structure or magneto-
optical effects. The spectral resolution should be at least 3.5 pm at a wavelength of 600 nm.
Observe at least three different spectral ranges in the visible simultaneously (wavelength
diversity), in the range from 380 nm to 900 nm.
The ViSP should be able to operate simultaneously with the infrared spectro-polarimeter
(NIRSP). A compensator for atmospheric differential refraction is needed in order to ensure that
both instruments are observing the same field.
Instrument Description: The basic optical layout of the ViSP appears in Figure 4.3. The instrument
concept is based on modern spectro-polarimeters, with a slit that scans the field of view and a
spectrograph that images the slit spectrum on a 2D detector at each scanning step. However, it
incorporates significant technological advances coupled with an innovative design that are necessary to
fulfill its stringent requirements, far beyond those of any other present instrument of its kind.
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The light beam first passes through a retarder with a time-dependent retardance (modulator) and later
through a polarization analyzer, which can be a linear polarizer or a polarizing beam-splitter depending on
the operation mode (see below). Reflections of the light beam along its path in the telescope introduce
significant instrumental polarization. In order to attain the polarimetric precision set forth by the science
requirements, the modulator and the calibration optics must be placed near the Gregorian focus. At this
location, the light beam has only undergone two reflections and the instrumental polarization can be
controlled at the level required.
Unfortunately, it is not possible to fulfill the wavelength diversity requirement and the 10-5
precision goal
simultaneously. In order to avoid the seeing-induced cross-talk, it is necessary to either run the modulator
at nearly kHz frequencies or to split the beam just before the detector into two beams with opposite
polarizations. In the first case (single-beam scheme), both the modulation and the analysis are done very
rapidly at the Gregorian focus. Subsequent reflections in the optical path to the detector will not affect the
measurements, since the beam has been analyzed already. This scheme, which permits highly accurate
polarization measurements, involves the use of FeLC. These devices are not achromatic and need to be
tuned to a specific wavelength. Therefore, it is not possible to meet the wavelength diversity requirement
with this setup. A dual-beam scheme, on the other hand, does not need such a fast modulation. The
relatively slow (tens of Hz) time modulation of two simultaneous images with opposite polarization is
used to correct the undesirable effects of seeing, at least to first order. The modulator and analyzer (a
polarizing beam-splitter) can be made achromatic over a broad range of wavelengths, which permits the
simultaneous observation of several spectral domains. The dual-beam setup has the analyzer at the end of
the optical path, right before the detector in the coudé focus. Multiple inclined reflections exist between
the modulator and the analyzer, introducing spurious polarization. Imperfections in the calibration to
correct such instrumental polarization and small residual cross talk from the seeing may compromise the
measurements at a level of 10-4
.
The ViSP will have three different operation modes: The single and dual beam modes described above
(for polarimetry at 10-5
and wavelength diversity, respectively), and a “hybrid” mode that combines
advantages from both schemes.
1. High Precision Polarimeter (HPP): The modulator is a FeLC and the analyzer is a linear polarizer,
both at the Gregorian focus. This mode allows for 10-5
polarimetry. The ViSP needs to be tuned to a
specific wavelength and cannot operate in combination with the NIRSP. HPP requires a charge-
caching device as a detector.
2. Fast Achromatic Polarimeter (FAP): This is the hybrid mode. It uses a fast (~1 kHz) rotating
wave plate as an achromatic modulator at the Gregorian focus. The analyzer is made of a FeLC
Figure 4.3. ViSP basic optical layout.
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combined with a linear polarizer, and it is located at the coudé focus immediately before the detector
(one analyzer is required for each detector). Seeing-induced cross-talk is prevented by the fast
modulation, but multiple reflections between the modulator and analyzer complicate the calibration.
The FAP meets the wavelength diversity and 10-4
polarimetric precision requirements. FAP requires
charge-caching detectors, and it works with the NIRSP.
3. Slow Achromatic Polarimeter (SAP): The modulator is an achromatic rotating wave plate at a
frequency of ~10 Hz, located at the Gregorian focus. The analyzer is a polarizing beam-splitter before
the detector. Residual seeing-induced cross talk and calibration errors limit the polarimetric precision
to a few times 10-4
. This mode is capable of wavelength diversity. Conventional CCDs can be used
for detectors, and it works with the NIRSP.
The ViSP design is contained in one plane, allowing easy access to the components for upgrades and
adjustments. The slit width is adjustable to allow for various trade-offs between resolution and photon
flux. A turntable contains several gratings that can be selected to meet the needs of the observing
program.
4.2.3 Near-IR Spectro-polarimeter
Design Requirements: The Near-IR Spectro-Polarimeter (NIRSP) is the instrument responsible for the
spectral analysis of the near infrared solar light and its polarization state. As with the ViSP, this
instrument will record the wavelength dependence of the full Stokes vector (I, Q, U, V) at each spatial
point in the field of view. Infrared spectroscopy requirements are diverse because of the broad flux and
spectral resolution conditions inherent to photospheric and coronal physical conditions. In order to meet
these requirements we describe a modular NIRSP system that satisfies both the low flux, and lower
angular and spectral resolution requirements of the corona, and the higher resolution (spectral and spatial)
needs for observing the photosphere.
Our philosophy in designing the NIRSP has been to achieve ATST spectroscopic infrared science
observing requirements with multi-use optical components (and designs) wherever possible. For example,
coronal spectroscopy will be obtained almost exclusively from the Nasmyth focus, ahead of the many
reflections that bring light into the coudé instrument room. In many cases the spectral resolution for
coronal observing is dictated by the few-million degree temperature coronal line profiles. In practice the
necessary resolution is somewhat higher than what coronal line-widths dictate because we often need
sufficient spectral resolution to separate K and F coronal and scattered-light photospheric spectral features
(for example for calibration). This is achieved with our resolution 4104 coronal spectrometer. Spatial
resolution of even an arc second will provide revolutionary new information about the faint corona's
magnetic field. Disk observations must be obtained after the image is corrected by ATST adaptive optics
at the coudé focus. Thus we require resolution of at least 3105 with diffraction limited spatial resolution
here. As we illustrate below the full requirements can be achieved with common optical components, but
with distinct F/6.6 Nasmyth and F/40 coudé systems.
Instrument Description: Infrared detector technology is a significant driver for the NIRSP. While the
final instrument design will be decided in a year or more, we feel that the most prudent NIRSP concept
now should be based on the stable Rockwell detector HgCdTe technology. This proposal assumes
Hawaii-2-style 2k2k format detectors. Coronal science requirements also dictate the need to observe into
the thermal IR in order, for example, to reach the important 3.9 µm SiIX emission line for magnetic
diagnostics. With common camera elements we describe below a 1-5 µm range coronal and photospheric
NIRSP system. This also allows photospheric thermal IR spectroscopy without significant budget or
technical performance impact.
The NIRSP-C and NIRSP-G (coudé and Nasmyth) optical configurations are scaled versions of a
common reflecting Littrow design. These optics are described below in Table 4.6 and Figure 4.4. NIRSP-
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G is illustrated here with the ATST-supplied focal relay that yields an F/6.6 input beam. The largest optic
is the collimator and camera 35 cm parabolic mirror. The grating is a standard R2 87 line/mm echelle, the
same grating used for NIRSP-C. Table 4.6 lists the optical specifications for both configurations. Pixel
binning is used to properly sample the larger NIRSP-G slit.
With slit choices described in Table 4.6 the diffractive NIRSP grating illumination is comparable to the
geometric illumination. NIRSP-G pixels will be binned 6×6 to yield 0.85-arcsec spatial and 242 mÅ
spectral resolution for coronal observing. This Littrow configuration minimizes off-axis angles from the
parabolic mirror so that the geometrical performance is close to diffraction limited.
Table 4.6. NIRSP Coudé and Nasmyth Optical Specifications
NIRSP-G NIRSP-C
Grating R2: 87 line/mm R2: 87 line/mm
Grating size 400x200mm 400x100mm
Collimator focal length 1.2m 3m
Focal Ratio 6.6 40
Plate scale 0.127 mm/arcsec 0.776 mm/arcsec
Slit Width 108µm 36µm
Pixel scale (at 1 μm) 242 mÅ 32 mÅ
Pixel scale (arcsec) 0.142" 0.046"
Design/Diffraction limited FOV 290" 290"
Hawaii-2 FOV 290" 47"
Cold blocking filter BW 1% 1%
System QE 5% 5%
Grating emissivity 50% 50%
While the optical configurations are similar, the mechanical structures are quite different. This results
from the vastly different background conditions for coronal and disk observing, i.e. NIRSP-C is not
intended for coronal observing conditions. The dashed lines in Figure 4.5 show the expected solar signal
and background versus wavelength assuming an optical table-mounted warm spectrograph, but with a 1%
bandwidth cold order-sorting and blocking filter in the IR camera Dewar module. The figure shows the
Figure 4.4. Optical layout for the NIRSP-G configuration. NIRSP-C is identical but scaled in length by a factor of approximately 2.5.
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mean photospheric flux versus the purely
thermal background contribution. The
background is much lower than the solar
signal blueward of about 5.5 µm at which
point the background dominates the
photospheric flux. This shows that a
warm NIRSP-C will satisfy most
requirements for high resolution
photospheric observations.
Coronal observations with a warm
NIRSP-G configuration would be
dominated by thermal background
photons at all wavelengths longward of
about 1.9 µm. By cooling the grating and
spectrograph optics to approximately 85K
the NIRSP-G will allow coronal
spectroscopy up to a wavelength of at
least 4.5 µm. This is illustrated in Figure
4.5, which shows the coronal signal and
background for a warm and cooled
NIRSP-G configuration as described in
Table 4.6.
The lower spatial and spectral resolution
required for coronal observations also
permits a compact cooled spectrograph design. Our working concept is illustrated in Figure 4.6. The IfA
group recently designed, built, and commissioned an even larger cooled IR echelle spectrograph for the
AEOS telescope which is the basis for some of the schedule and cost estimates for the ATST version.
This NIRSP-G design uses (not shown) six stepper-based mechanisms utilizing vacuum cryogenic feed-
throughs. Additional manual external mechanical adjustments for M1 collimation and grating are also
provided. The design includes provisions for mounting a polarizing beam splitter behind the cold slit.
The slit viewer and integral slit-viewer filter wheel and the final science detector can be removed from the
NIRSP-G without warming the Dewar or breaking vacuum. The entire cylindrical volume of the NIRSP-
G is supported at both ends by ATST-supplied circular bearings, which provide for instrument rotation as
needed for the alt-az telescope configuration.
The NIRSP-C configuration will be constructed on a conventional optical table in the ATST coudé space.
The optical layout is identical to Figure 4.6 (except for scale). Infrared slit-viewer and final science
cameras for NIRSP-C are identical to the NIRSP-G cameras. Of course all mechanism controls and their
hardware and software interfaces will be common for all NIRSP components. NIRSP-C relies on the
ATST coudé room rotation and does not use separate instrument rotation control.
Figure 4.5. Signal and background flux calculations for NIRSP-C
(solid lines) and NIRSP-G (dashed lines). Assuming the optical configuration of Table 1 the expected signal and background per NIRSP resolution element is plotted versus wavelength. A warm spectrograph is adequate for disk observations at wavelengths shortward of about 5.5 µm (dashed lines) while a cold NIRSP-G configuration allows coronal observations shortward of about 4.5
µm. A warm NIRSP-G spectrograph does not satisfy coronal observing requirements.
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4.2.4 Visible Tunable Filter
Design Requirements: The Visible Tunable Filter will be used to obtain narrow spectral bandwidth
observations over an extended area of the sun. This capability will provide us with rapid 3D-imaging
spectrometry, Stokes spectro-polarimetry, and accurate surface photometry. It will also deliver
spectroheliograms to measure Doppler velocity, transverse flows, provide a feature tracking capability,
and generally permit the study of evolutionary changes of solar activity. Investigators will use it in
conjunction with the AO system for high spatio-spectral imaging. The filter will operate at four spectral
bandwidths. These four modes and their associated requirements are given in Table 4.7.
Table 4.7. Visible Tunable Filter Modes and Requirements.
Filter Mode Observations Passband FWHM (pm)
Field of View (arcmin)
Typical Spectral Lines (nm)
Desired Peak Transmission
A. Narrow Passband
(Dual/Triple Etalon Configuration)
Spectro-polarimetry using I,Q,U,V Stokes fractional parameters 3D Spectrometry & 3D Tomography & Flow Geometry
2.0 1 FeI: 524.70, 525.02, 525.06, 630.15, 630.28 629.87, 868.8 CaII: 863.5 FeI: 569.1,557.6, 684.27 CI:538.03
>50%
B. Medium Passband (Single or Dual Etalon Configuration)
Filter Vector Magnetograms Filtergrams
12.0 3 FeI: 525.02, 525.06, 630.15, 630.28 CaII: 863.5 CaII: 863.5 MgI : 517.2
>60%
C. Intermediate Passband (Single Etalon Configuration)
Dopplergrams High-Speed Imagery & Flares
20-30 3 HI: 656.3 FeI: 543.45, 557.6, 630.15 HI: 656.3
>70%
D. Broad Passband (Interference Blocking Filters only)
Advective Flows-Transverse Flows Movies & Active Region Evolution
100-1000 3 CN: 430.5 CaI: 399.3 CN: 430.5 Continuum 450.8
>80%
Figure 4.6. Mechanical concept illustrating NIRSP-G. This "transparent aluminum" schematic diagram illustrates
the critical components of NIRSP-G. The overall Dewar length is 1.2 m and its mass is approximately 900 kg. Light enters from a slit wheel assembly on the right. Removable HgCdTe cameras are indicated on the top
surface. Radiation shielding, cold straps and most mechanisms are not indicated.
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It will meet the minimum system requirements shown in Table 4.8.
Table 4.8. Minimum System Requirements
Minimum Aperture 200 mm
Spectral Range 450 to 750 nm
Spectral Resolution 200,000
Minimum Peak Transmission 50% with blocking filters
Maximum Ghost Transmission 10-4
Maximum Stray Light 10-3
Drift Stability < 0.1 pm per hr
Design Description: Our design for the Visible Tunable Filter is a triple etalon system based, in part, on
the successful German TESOS system, shown in Figure 4.7. We have selected a multiple Fabry-Perot
(FP) spectral filter for the following reasons:
1. It can provide the required spectral resolution
for high-resolution spectral imaging, Stokes
profile analysis and filter magnetograms
(spectral resolution ~250,000 at 500 nm);
2. It has the high etendue (light throughput) to
obtain a sufficient number of spectral samples
within appropriate solar oscillation periods, and
the required magnetic sensitivity on the
timescale that solar features change;
3. It is mechanically and optically simpler in
design than a Lyot filter;
4. It provides the rapid tuning between
wavelengths that is required for finding the line
center and adjusting the wavelength setting for
Doppler-induced shifts;
5. It is a single system capable of simple
spectroscopy, Stokes line profiles, and filter
magnetograms. The etalons can accommodate
large aperture (~200 mm) filter systems
allowing extended field-of-views without
spectral degradation.
A triple etalon system offers several additional advantages, including superior spectral purity and out-of-
band rejection, excellent throughput, and wider bandpass blocking filters that will remain stable over
timescales of years. It is also a conservative choice in terms of the finesse requirements on the individual
etalons.
Table 4.9 shows the relevant parameters of the triple etalon system. FSR = Free spectral range, FWHM =
full width at half maximum, F = finesse, R = reflectance, D = gap distance, and M = order. This
configuration has the same gap ratios as the TESOS system, however the FWHM of our system is 2 pm
compared to TESOS 3 pm FWHM. TESOS has finesses of 30-40, while ours are just above 50.
Figure 4.7. The TESOS instrument at the German Vacuum
Tower Telescope (Tenerife, Spain). The design of the ATST Visible Tunable Filter will use the experience gained from the
development of this successful instrument.
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Table 4.9. Triple etalon system.
Etalon System FSR (nm) FWHM (pm) F R D (μm) M
1 0.106 2.02 52.6 .94 1300 4952
2 0.172 3.27 52.6 .94 802 3055
3 0.242 4.59 52.6 .94 571 2175
Papers by Gary, Balasubramaniam, and Sigwarth (2003), and Gary and Balasubramaniam (2003)
summarize the triple-etalon Fabry-Perot filter of choice for the ATST visible narrow-band filter and
consider the overall instrument requirements. The ATST filter optical design employing a triple Fabry-
Perot etalon system requires that each etalon have a flatness of ~/200 before coating, with the optical
finesse 10-50.
The heritage for the use of etalons and multiple etalon systems in solar physics comes from observatories
in both the United States and Europe, including the NSO/Sacramento Peak, NSO/Kitt Peak, the German
Vacuum Tower Telescope, Big Bear Solar Observatory, and the High Altitude Observatory. The design
of the ATST multiple etalon system relies on the existing experience and expertise from this and other
experience.
4.2.5 Additional Operations Phase Instrumentation
Infrared Tunable Filter: The top-level design requirements for the Infrared Tunable Filter call for a
wide passband range (at least including HeI10830Å, FeI15648.5Å, FeI15652.0 Å, etc), large field of
view, and narrow passband for measurements of solar magnetic field at deeper layers in solar atmosphere.
This filter system can be operated as a spectropolarimeter, a filter-imaging magnetograph, or a high-
resolution imager. The Design Requirements for the IR Tunable Filter are shown in Table 4.10.
The preliminary design combines an interference pre-filter, a tunable Lyot filter and a single FabryPerot
etalon.
Thermal Infrared Polarimeter and Spectrometer: The Thermal Infrared Polarimeter and Spectrometer
(TIPS) will perform vector polarimetry and infrared spectroscopy of the sun’s atmosphere. It will cover
the entire thermal-infrared from 5m out to the long-wave telluric cut-off at 28 m. The TIPS will study
the magnetic structure, dynamics, chemistry, and physical state of sunspots, plages, flares, prominences,
and quiet regions. In the polarimetry mode, vector magnetic fields will be measured using the Mg I
emission lines at 12.3 m. With these lines, the most magnetically sensitive in the solar spectrum, TIPS
will measure the strength and three-dimensional configuration of fields in the upper photosphere. In the
spectroscopy mode TIPS will record spectra at high spectral and spatial resolution.
The primary candidate instrument for the TIPS is a cryogenic grating spectrometer. The instrument will
be placed at one of the coudé stations. A large echelle grating will provide the required high spectral
Table 4.10. IR Tunable Filter Design Requirements.
Properties Requirements Comments
Spectral Coverage 1.0~1.7µm (scanning)
Resolving Power > 150,000
FOV 1~3 arcmin
Bandpass 0.1Å@15650 Å
Spatial Resolution < 0.1 arcsec AO – 0.09 arcsec at 1.6µm
Operation Mode Narrow/Medium/Broad band Consideration of flexibility to serve different observation purposes. This shall be realized by taking in/out individual filter from the system.
Aperture > 36 mm (Lyot), > 150 mm (FPI) Compared to the currently designed similar filter system at BBSO, ATST shall have better performance.
Throughput > 40% (Lyot), > 80% (FPI) Instrumental Principles regarding filters. Polarizer will be a major drag in this design.
Scattered Light < 10-3
Stability ~0.05 Å/hour (FPI & Lyot) Due to the properties of components of Lyot and FPI.
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resolution. One spatial dimension will be imaged along an input slit and the second spatial dimension will
be mapped by stepping the slit across the field. The spectrometer will use a large format (10241024)
As:Si detector array with operation optimized for rapid cadence.
5. HIGH LEVEL CONTROLS & SOFTWARE
The ATST software provides the means to control and coordinate observations performed with the
telescope and instruments. Numerous types of software will be in use on ATST, ranging from the lowest
level servo or logic controller to the highest-level queue and scheduling processes. Each of these software
components fulfills some part of the science requirements for the ATST mission.
The ATST software system is designed to operate the ATST through all stages of observational detail,
from science program submission into observation scheduling, on to instrument configuration and data
collection, through data reduction and archiving, and finally to data retrieval. This chapter describes how
the software performs each of these tasks and how the requirements for each task drive the design in a
particular direction.
5.1 SOFTWARE DESIGN REQUIREMENTS
The software requirements have been derived from several sources. First, the SRD defines a number of
functional and performance requirements that may be traced through the whole ATST software design.
Chief among these are the requirements for flexibility, adaptability, and availability. Second, additional
requirements have been discovered at ATST workshops or design reviews. And third, the technical
engineering requirements further constrain the software design in behavior.
The software design is based upon the scientific and operational requirements. These requirements may
be categorized into four principal areas.
Instruments: The software system must operate all instruments through their complete range of
functionality. Instruments must coordinate observations with other instruments and with other
components of the ATST. The instrument configuration must be flexible and dynamic to support a variety
of experiment setups. Future instruments should not be constrained in their design by ATST software.
Telescope: The telescope must be capable of software control for acquiring, tracking, guiding, and
offsetting on and around the sun. The required accuracies for each function should be met by both the
mechanical and software system. The telescope software must provide the science image to the requested
location with the required image quality.
Observations: Observers must be able to operate the telescope and instruments in a variety of ways.
Observations may be taken at the telescope or remotely, they may be performed in real-time or scheduled,
and they may be synchronized with other local or remote observations.
Data Handling: The instruments may generate a large volume of data. The data must be transferred from
the instruments to a permanent storage facility. The data must contain pertinent state information about
the observation.
5.2 SOFTWARE DESIGN DESCRIPTION
The four categories above are the major systems responsible for each aspect of operation of the software.
The Observatory Control System (OCS) coordinates programs, experiments, and observations. The Data
Handling System (DHS) manages the flow, storage, and processing of image data. The Telescope Control
System (TCS) operates the mechanical and optical structures. The Instrument Control System (ICS)
coordinates the instruments and their associated calibration systems. Together the principal systems
provide a lifecycle structure within which an observing program may reside.
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Communications: The principal systems need to communicate with each other to coordinate telescope,
instrument, data, and observer activities. The communication channels are well defined and simple, since
the majority of activities occur within each system and not between them. There are two types of
communications activities, commands and events. Commands are synchronized activities between a
client, who requests an operation, and a server, who performs the operation. An example of a command is
a request to configure an instrument's mechanical assemblies. The requesting client, usually the OCS's
instrument user interface, issues a configuration command to the instrument's server process. The
command is parsed for accuracy and completeness, the readiness of the instrument is checked, and the
result of the configuration operation is returned to the client. Note that the returned result indicates only
that the configuration was accepted and the required movements or reconfigurations were begun.
Command completion is returned though the event channel. By performing actions in this three-step
process—called command-action-response—the server remains available to execute other commands
while the last command is underway. This is extremely useful if the next command is an abort.
Observations: An observing program is the basis for all that goes on in the control system. The origin of
the observing program is in the science proposal, where an observer selects one of the many possible
operational scenarios. For instance, the observer may choose to use the ATST in its diffraction-limited
mode. The observer further selects the instruments and targets. Finally, the operational strategy, or
sequence of operations, is determined. All of this information is incorporated in an observing program.
Sometimes an observing program may be a richly complex series of instructions and system interactions,
possibly describing a synoptic, or regularly scheduled, observation. Sometimes it may be a simple set of
operations to release telescope and instrument control to the observer for setup or serendipitous
observations. Regardless of the complexity or lack thereof, the observing program encapsulates
everything necessary to execute the planned observations.
5.2.1 Common Services
The Common Services architecture provides the infrastructure used by all ATST software, from the
lowest level communications protocols to the control mechanisms between components. By using a
unified infrastructure, components can take advantage of both design and development of other
components and access to common utilities (such as events, logging, and databases). The control flow of
the ATST software is enforced through the Common Services architecture, allowing new components to
be easily integrated into the software framework.
Services: The Common Services are responsible for the fundamental services provided to all ATST
software. These services are best described by examining the information flows supported by these
services.
Command-action-response directives: Direct control of one system component by another component
is accomplished using the command-action-response model pioneered by Gemini. In ATST, commands
are implemented as state-change directives. To effect a change in behavior of a target component the
controlling component describes the conditions necessary to accomplish the state-change by providing the
target component with a set of attributes (name, value pairs) that characterize the difference between the
existing state and the desired state. This set of attributes, along with a unique identifier, is a
configuration. A configuration may be simple, consisting of only a few attributes, or it may be quite large
with hundreds of attributes.
Connections: System components operating in a distributed environment must be able to locate those
other components that they need to communicate with. In ATST, a connection service tracks the
locations and status of all system components and provides a name service that is consulted when a
connection needs to be established. If necessary, the connection service is capable of directing that a non-
running component be started in order to satisfy a connection request.
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Figure 5.1. CS-1: The classic container/Component Model
Event Notifications: Status information is communicated throughout ATST using the event service.
Events are messages that are broadcast from some source. System components that are interested in
particular events must subscribe to the appropriate event channel. Events are published by name and
contains sets of attributes as values. Subscription to events is also by name but wildcards may be used by
a subscriber to receive classes of events through a single channel. The ATST event service is reliable and
high-performance. Events from a given publisher are also delivered to subscribers in the same order in
which they are published. All events are time stamped and identify their source – both the generating
component and the configuration that was active in that component when the event was generated.
Alarms: System alarms have the same structure and distribution properties as events but are functionally
distinct. Alarms denote abnormal conditions that require operator intervention. Alarms are not
considered an integral part of the ATST safety system, however. Ensuring safety is solely the
responsibility of the Global Interlock System. Alarms are useful for monitoring safety status as well as
other abnormal conditions and software systems may be implemented to refuse many commands when
unchecked alarms exist. Alarms are tagged in the same manner as events.
Log Messages: Log messages are simple string messages that record system activity. As with events and
alarms, log messages are transmitted using a publish/subscribe mechanism and are time stamped and
source tagged. All log messages are categorized as one of debug, note, warning, and alarm (the alarm log
message category is not isomorphic to system alarms – all system alarms are logged using an alarm log
message but the handling of system alarms does not depend upon this logging). In addition, log messages
in the debug category have an associated level, and debug messages are only published if their level is
less than or equal to the current debug level of the originating component.
Persistent Stores Access: A great deal of information in ATST needs to be recorded for arbitrary periods
that are independent of the lifetimes of specific system components. In addition, system components
need access to initialization parameters on startup and reinitializations. Finally, information specific to an
experiment (virtual instrument details, science programs, configurations, and science header data) is
preserved. ATST uses various persistent stores for these types of information. System components have
access to these stores either directly or through database proxy services.
Alarms and log messages are always recorded in persistent stores. Events are not normally recorded but a
high-performance engineering archive is available for recording events upon demand. These persistent
stores are searchable using a general query mechanism under program control.
Application Framework: The Common
Services also provide the application
framework supporting consistent operation
in a distributed environment. The ATST
application framework is based on the
Container/Component Model made popular
by EJB (Enterprise Java Beans), Microsoft's
.NET initiative, and CORBA's CCM and
patterned after the model developed as part
of the ALMA project (Figure 5.1). The
OCS provides containers that wrap system
components in a common environment
providing uniform access to services.
Component developers focus on
implementing the functionality required of
each component and rely on access to a
container for basic services.
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Figure 5.2. OCS-1: Functional Categories of the OCS
5.2.2 Observatory Control System
The OCS has the following responsibilities:
Management of system resources
Management of experiments, science programs, observations, and configurations
Coordination of the TCS, ICS, and the DHS
Management of ATST systems during coordinated observing with other observatories
Essential services for software operations
User interfaces for observatory operations
In general the OCS assumes managerial responsibilities for the ATST system and directs the activities of
the remaining principal systems. Services that are central to the operation of ATST software are provided
by the OCS. The OCS acts as the interface between users and the ATST systems during normal
operation, allowing users to construct science programs and virtual instruments for use in an experiment,
monitor and control the experiment, and obtain science data from the experiment.
The OCS also provides basic services to support system maintenance and general system engineering
operations. This includes tools to examine system diagnostic information, handle alarm conditions,
monitor safety systems, and perform routine engineering tasks.
Functional Organization of the OCS:
The OCS can also be viewed as
organized hierarchically into broad
functional categories: application
support, experiment support, and
resource management. The top levels
of these categories are shown in Figure
5.2.
The application services provided by
the OCS include the event, alarm, log,
and persistent store services. The
application framework includes APIs
and libraries as well as a general
framework for building and deploying
ATST applications. The TCS, ICS and
DHS (as well as the OCS itself) are
resources that are managed by the OCS. The OCS provides for direct operator control of these resources
as needed. However, the normal operational model is to allow experiments as much resource control as
practical over the resources that are allocated to that experiment.
Performing Experiments with the OCS: Experiments are the heart of ATST operations, and the control
system is designed with this in mind. A laboratory-style environment provides flexible support to carry
out experiments that are likely not understood or defined at the time the laboratory itself is designed. An
experiment undertaken at the ATST requires a Virtual Instrument and a Science Program of
Observations. The OCS interacts with the ICS to create and manage virtual instruments. Science
program management is the sole responsibility of the OCS.
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Figure 5.3: DHS functional architecture
5.2.3 Data Handling System
The DHS is responsible for:
Bulk data transport
Quick look channels
Data storage, retrieval, and distribution
Data reduction pipelines support
The top-level functional architecture appears in figure 5.3.
The DHS manages the flow of scientific data collected by ATST instruments. The data reduction
pipelines support is a potential upgrade that is supported by the initial DHS design.
Because of the performance requirements placed on the DHS, parts of its functionality are distributed
across other system components. For example, instrument camera systems perform any data processing
required to reduce data output to meet bandwidth restrictions imposed by the implementation of the bulk
data transport. Similarly, instrument component developers are responsible for providing the processing
steps required to convert raw quick-look data into meaningful quality-control information.
Bulk Data Transport: The role of the bulk data transport is to reliably transfer science data from
scientific cameras sources to data store targets (Figure 5.4). Physically, the bulk data transport uses
multiple data channels on a high-performance switched network. The use of a switched network allows
for increased flexibility – data channels can be established between any data source and target.
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Figure 5.4. DHS-2: DHS 'pipeline' showing distributed functionality
Figure 5.5. DHS-2: Routing of Quick look data using publish/subscribe
The ATST scientific cameras are individually capable of generating large amounts of data quite rapidly.
This is compounded by the fact that multiple experiments may run simultaneously, each using multiple
cameras. The bulk data transport is to be implemented using the latest stable technology for high speed
data transfers and operates using data channels that are physically distinct from other system
communications. This ensures that system control and monitoring activities may continue unaffected by
bulk data transport loads.
Quick Look Channels: ATST cameras can generate quick-look images and post them onto quick look
data channel streams. The quick look facility is a publish/subscribe mechanism allowing applications to
accept, process, and display quick look data from any source (Figure 5.5). This allows, for example, an
operator’s GUI to display quick look data while a separate process performs automatic analysis of quick
look data with feedback into the ATST image quality control system. At the same time, a third process
may be sub sampling the same quick look data channel and recording selected images. The publish/
subscribe mechanism also simplifies display of quick look data at multiple stations in the observatory.
Data Storage, Retrieval and Distribution: ATST provides temporary storage for all scientific data
products and permanent storage for calibration data products. The temporary storage acts as cache
between the high-speed bulk data transport and slower distribution media (DVD, tapes, removable hard
drives, etc.). All science data, whether located in temporary or permanent storage, maintains associations
via a relational database with the configurations, observations, and experiments that were involved in the
creation of the data. Header information is also archived and associated with data products. Any header
attribute may be used as a key to retrieve one or more data products from the store.
When an experiment completes, the data products and all related ancillary products associated with that
experiment are retrieved from the store and made available on distribution media for that experiment’s
investigators.
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Figure 5.6. Telescope Control System.
Support for Data Reduction Pipelines: Support for on-line data reduction processing is a future upgrade
to the ATST system. The use of a switched network for bulk data transport and database access to all
data products simplifies the process of integrating this upgrade into ATST.
5.2.4 Telescope Control System
The TCS is the central coordination facility for the delivery of the solar image to the instrument. It is
responsible for the precise pointing and tracking calculations necessary to observe the sun. The TCS does
not itself operate any mechanical components;
rather it delegates this responsibility to the
various ATST telescope subsystems and
manages them according to the observation
requests. The TCS does interact with the other
principal systems, most notably the OCS and
ICS. Observation configurations generated by the
OCS are sent to the TCS for proper telescope
positioning and configuration. Coordinated
events are returned by the TCS so the OCS (and
associated observer) is informed about the
telescope's status. If an instrument uses the
telescope in any coordinated fashion, such as
scanning or calibration, the ICS and TCS
synchronize these activities through the TCS
interface (Figure 5.6).
The TCS also manages the wavefront image reconstruction process. The high-speed adaptive optics
corrections take place in the Adaptive Optics Control System (AOCS), a TCS subsystem. The TCS
manages the state of the AO system, including the offload of accumulated errors to other subsystems.
The TCS delegates the high-speed control loops of the telescope components to its subsystems. The green
elements in Figure 5.6 show the major TCS subsystems and their relationships. The scope of a
subsystem's functionality is limited both by construction and control. Generally, if both the error
detection and error correction of a simple control loop is handled locally, that loop is part of a subsystem.
For instance, the M1 mirror assembly has a number of axial force actuators that detect applied forces and
apply corrective forces to position the actuator at the correct position. The force map required to figure
the primary mirror is downloaded from the TCS to the M1 Control System (M1CS), but the M1CS is
responsible for positioning and maintaining the actuators in their proper positions. The TCS coordinates
the control loop by downloading “set-points”; the M1CS provides the actual control loop function.
Similar control methods are used for other subsystems like the AOCS and the Mount Control System
(MCS).
The description of the TCS subsystems can be found in their appropriate mechanical or optical system
description. These systems are developed and delivered by the subsystem vendor and follow an interface
to the TCS. A brief description of how they interact with the TCS is given here.
Enclosure Control System: The ECS operates the enclosure carousel and shutter drives to properly
position the entrance aperture. The enclosure needs to be moved to an accuracy of better than one-half
degree, and must avoid collisions with the telescope mount assembly due to the shape and dimensions of
the enclosure volume. The TCS provides a stream of altitude and azimuth trajectory data to the ECS.
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Mount Control System: The MCS operates the telescope altitude and azimuth drives to properly
position the telescope mount assembly. It also controls position of the Nasmyth and coudé rotators. The
TCS provides trajectory information at 20 Hz to the MCS.
M1 Control System: The M1CS controls the axial support actuators used to shape the figure of the
primary mirror. The TCS provides the default shape based upon the current position on the sky.
Additional shape information from the active optics may be delivered by the TCS.
M2 Control System: The M2CS operates the tip-tilt-focus actuators on the secondary mirror. The TCS
provides the default positions based upon the current position on the sky. Additional tip-tilt-focus
information from the adaptive optics may be delivered by the TCS.
Feed Optics Control System: The FOCS controls the smaller mirrors delivering the image to the coudé
or Nasmyth instruments, and it controls the calibration equipment located at the Gregorian Optical
Station. The TCS provides the required commands needed to position the optical elements.
Wavefront Correction Control System: The WCCS controls the wavefront correction hardware,
including the adaptive optics real-time controller and the active optics wavefront sensors. The TCS
manages the distribution of image correction data to the appropriate subsystems.
Acquisition Control System: The ACS operates the external acquisition telescope. The full-disk image
is used by the operator for target selection and positioning. The ACS provides 1 arc-second resolution in
four different imaging wavelength bands at rates up to 10 Hz.
Polarization, Analysis, and Calibration Package: The PAC operates the optical elements located at the
Gregorian Optical Station (GOS). The TCS configures the PAC for the appropriate observation type by
selecting suitable calibration optics and/or polarizers.
Heat Stop Assembly: The HSA absorbs the rejected solar light near the prime focus. The TCS controls
the associated Lyot stop and occulter mechanisms. It also monitors the thermal performance of the HSA.
Pointing and Tracking: The TCS has the responsibility for target acquisition and tracking. This requires
a number of simple yet important steps. First, the principal target for the ATST is the sun. This requires a
calculation of the solar position and rate. The second step is the conversion to an appropriate coordinate
system. Solar observations are usually carried out in either heliocentric or heliographic coordinate
systems.
Closing the loop on the TCS pointing is achieved by guide error signals from the adaptive optics system.
Figure 5.7 shows the TCS servo loops required to track features on the sun. Although the AO system has
a very good resolution of the positional error, the features tracked by the AO system may be moving with
relation to the solar center. The AO signal is useful to the TCS when the tracking of the telescope is to be
unlocked from the solar disk, thus allowing the telescope to guide on this moving feature.
Active and Adaptive Optics: The TCS is responsible for controlling the image quality parameters of the
telescope optics. The adaptive and active optics control systems provide residual wavefront error data in
the form of Zernike coefficients that is propagated to other telescope subsystems. Figure 5.7 shows the
schematic flow of Zernike coefficients from the adaptive optics system to the other TCS subsystems. Tip-
tilt-focus offloads at about 100 Hz are sent to the M2 mirror controller for removal of moderate
bandwidth errors (on the order of 10 Hz) such as wind shake. Any accumulated M2 bias needs to be
removed to prevent pupil wander; this error is further offloaded to the mount controller at about 1 Hz.
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5.2.5 Instrument Control System
The ICS is responsible for managing virtual instruments. The ICS provides the mechanisms for
associating components into virtual instruments, determining the availability of components, and holding
the representations of the virtual instruments. The ICS also maintains a set of active instruments: those
virtual instruments that are physically realized and actively sharing the light beam. Thus the ICS enables
multiple experiments to take place simultaneously.
The ICS assumes no active role during the carrying out of an experiment. The OCS directs control of a
virtual instrument during an experiment.
Requirements: The ATST is required to provide the flexibility inherent in a laboratory environment.
This is a key science requirement, and has a significant impact on the system design both in mechanical
systems and in software. The DST at Sunspot, NM, is specifically mentioned as a model that well
illustrates the desired flexibility. On the DST a series of optical benches on a protected rotating platform
provide the principal support for observing. Scientists can construct instruments specific to their
experimental needs from existing components. While a few instruments are “facility” and consist of a
fixed set of components, even these instruments may be combined with other components using
dichroics, beam splitters and slit-jaw reflections. The ATST mechanical systems provide a similar
flexibility through optical benches on a two level rotating coudé platform in the telescope pier.
The Experiment: Observers at ATST are
interested in performing Experiments (Figure
5.8). A central tenant of the ATST control
system model is that the system should be
adapted to the requirements of the
experiment. A laboratory environment
provides flexible support to carry out
experiments that are likely not understood or
defined at the time the laboratory is
designed. Consequently, experiments are a
formal concept within the model.
Figure 5.8. Information flow in an Experiment
Figure 5.7. Tracking and adaptive optics control loops controlled by the TCS.
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Figure 5.9. A possible instrument configuration
In ATST, an experiment includes a science
program of Observations and a Virtual
Instrument capable of performing those
observations. (The experiment also
ultimately includes Results but that aspect is
not relevant to this discussion.)
Observations contain sequences of
operational steps describing the behavior of
the instrument. Each operational step
consists of a set of configuration parameters
and a simple command describing a state
change within the instrument, collectively
referred to as a Configuration (Figure 5.9).
This use of science programs is typical of
modern observatory operations and matches
similar functionality provided at SOLIS,
Gemini, VLT, ALMA, and other
observatories.
What differentiates the ATST approach from these other observatories is that, instead of adapting
experiments to fit within the bounds imposed by instruments consisting of fixed components, the ATST
observer can construct a virtual instrument from available components to meet the needs of the particular
experiment. This provides a great deal of flexibility in the nature of experiments that can be performed at
ATST.
The Virtual Instrument: Instruments consist of one or more Components. Some components may be
purely mechanical with no associated software (e.g., a dichroic filter). Others may be purely software (a
sequencer). Most, however, include both mechanical and software aspects (cameras, scanners, etc.).
These last components are called Devices.
In a conventional instrument the set of components that comprise the instrument are fixed and
permanently associated with each other. Nevertheless, there is some software that understands these
associations. Thus the primary difference between a virtual instrument and a conventional instrument is
merely that the associations within a virtual instrument are not fixed but rather managed by software. A
subtle difference that is implemented in the ATST virtual instrument model is that telescope components
can also be associated as part of a virtual instrument. For example, an experiment that needs to perform
drift scanning across the solar disk can include the telescope mount as a component, while an experiment
performing coronal observations is likely to include the occulter as a component.
Scientists assemble the requisite components and combine them into the virtual instrument. Virtual
instruments are then named and saved for potential future use. Some virtual instruments are used so often
that the physical component associations are also maintained. This would be the case for ATST facility
instruments.
From a control perspective, once the component associations have been made the control of a virtual
instrument is identical to that of a conventional instrument. This simplifies the integration of instrument
operation within the otherwise conventional ATST control system.
Components are hierarchical and may be composed from other components. In particular, instruments
themselves are components. Composition of instruments is a common feature of operation of the DST
and is expected to be a key operational characteristic of ATST as well. With a few additional components
several facility instruments may be associated into a new virtual instrument.
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Figure 5.10. The lifecycle of a virtual instrument
Some experiments require cooperation between ATST and off-site observatories (including off-planet).
A virtual instrument can include special proxy components for coordination with off site facilities.
The Life Cycle of a Virtual Instrument: The first step when performing an
experiment with ATST is to construct a
virtual instrument (Figure 5.10). This can be
done either by browsing and selecting an
existing virtual instrument or constructing a
new virtual instrument from a catalog of
system components. The components are
then configured as needed for this
experiment. While many components can be
configured by setting a few parameters,
others – such as sequencing components –
may take more effort to configure, depending
on the requirements of the experiment. Once
the virtual instrument is defined, it is
registered with the ICS, which records the
instrument.
A science program that uses that instrument can now be constructed and added to the experiment.
Observations within the program may be controlled through sequences of configurations or interactively.
In either case the observations are scheduled with the OCS for execution.
As the time for observing approaches, the scientist lays out the physical systems associated with the
virtual instruments components and prepares for observing. The OCS also notifies the ICS that a
particular virtual instrument is needed for an upcoming observation and the ICS confirms that it is
available and enabled. When enabled, the virtual instrument assumes control over its components, and
observations proceed according to a prescribed sequence.
Once the observations in the science program have been completed, the OCS notifies the ICS that the
virtual instrument is no longer needed. The ICS then deactivates the instrument. Once deactivated, the
virtual instrument’s physical layout may be preserved for future use in other experiments or the
instrument's physical systems may be made available for use in other virtual instruments.
The Role of the OCS: The OCS acts as the interface between the scientist and the ATST control system
as a whole. The OCS maintains the science programs and sequences the observations of experiments by
providing configurations to the associated virtual instruments. The actual sequencing may be
accomplished using scripts or through graphical user interfaces for interactive observing.
Implementation of the Virtual Instrument Model: Virtual instrument component software is
constructed using a Container/Component Model as the basic framework. Containers provide access to
services required by the components and are responsible for managing component life cycles. This
allows component developers to concentrate on the functionality required in the component and provides
a common implementation for standard features. This approach also enhances the distributability of
components. Software only components can be instanced on arbitrary computer systems easily as long as
a container is available to hold the component.
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Figure 6.1. The Enclosure.
6. ENCLOSURE
The enclosure is comprised of five major components: (1) the
carousel; (2) the carousel drive system; (3) the lower
enclosure; (4) the enclosure thermal system, (5) the ancillary
mechanical systems; and (6) the enclosure control system.
These five items are described in detail, below. The external
enclosure elements appear in Figure 6.1.
6.1 ENCLOSURE DESIGN REQUIREMENTS
The ATST enclosure is comprised of the structural
components, drives, and thermal equipment that are used to
protect and track with the Telescope. The enclosure will meet
the following requirements:
Provide complete protection for the telescope and
optics under all weather conditions expected at the
Haleakalā Observatory site (survival & operations);
Point, track and slew along with the telescope over its
full required range of travel, while providing full
shading of the telescope structure;
Provide an unobstructed optical path from the sun to
M1, with acceptable seeing characteristics in and
around the Enclosure;
Provide a light-tight display to the surroundings when
closed at night; and
Provide the telescope with protection from wind-induced vibration and mirror buffeting, while
still allowing good flushing characteristics in and around the Telescope.
In addition to these top-level requirements, there are a number of second-level functions that the
enclosure provides. For example, the enclosure has a variety of safety systems and features included to
protect personnel and the telescope from damage (e.g., failsafe brakes; GIS interface, etc.). The complete
specifications and design for the enclosure, including all the top-level and second-level requirements, are
outlined in the Enclosure Design Requirements Document (ATST Document #SPEC-0010).
6.2 ENCLOSURE DESIGN DESCRIPTION
The enclosure design is critical to the performance of ATST. It must protect the telescope from wind,
weather, and direct sunlight (except on the primary mirror), and must do so without contributing
significantly to local seeing. Analyses have shown that even a white-painted enclosure requires active
skin cooling systems to keep from generating self-induced seeing. The design described below
incorporates active cooling of all critical insolated areas. Careful attention has also been paid to the
geometry of the structure for optimum performance and minimum operational cost.
6.2.1 Carousel
The carousel is the large structure that forms the basic envelope for telescope protection. It rotates about
an azimuth axis that is coincident with the telescope azimuth axis. It is a highly ventilated, but includes
cooled sun shades to prevent direct sunlight from entering the enclosure through the vent opening. The
enclosure and telescope can rotate independently. The design also features steeply sloped sides on either
side of the carousel entrance aperture that minimizes surface area normal or near-normal to the sun
(Figure 6.2). A 5° taper from front to back along the sides also contributes to maximizing surface area
that remains shaded through tracking operations.
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The carousel structure design is based on modern enclosure construction methods incorporating a large
steel truss ring for the base, supporting dual arch girders, along with intermediate framing and supports.
The plate coil panels (see section 6.2.4, Enclosure Thermal Control) will be used directly as the cladding
system in a standing seam or other weather-tight configuration. In areas that don’t require plate coil
coverage, panels designed for the same size and expansion properties will be used. This greatly simplifies
the detail work necessary to create a leak-free exterior envelope. All exterior surfaces are finished using
the thermal coating system detailed below. Foam insulation is applied to the interior surface to minimize
surface temperature variation. The structure is designed as an ‘exoskeleton’ with a weather-tight skin
installed on the inside of the structure. Fire-rated insulated steel panels, such as the DSL-FR model of the
Metecno-API line, are used as shown in an industrial application to the right.
Shutter Assembly: The shutter assembly is comprised of a primary and a secondary shutter (Figure 6.3).
As in the carousel, plate coil panels comprise all portions of the shutter skin that receive solar insolation
during tracking operations, with ‘dummy’ panels making up the rest. They are mounted on the exterior
side of a powder coated tubular steel frame which also serves as a mounting structure for the shutter
support and guide roller assemblies. The primary shutter is actually made in two pieces and bolted
together in a gasketed, flange-like, weather-tight arrangement so that when separated, starting from the
zenith-pointing position, half is left in place. The remaining, driven portion is then moved to the horizon-
pointing position, leaving a large opening to facilitate the staging and assembly of the telescope mount.
Carousel Entrance Aperture
Carousel Aperture Stop
Primary ShutterSecondary Shutters
Primary Shutter Flange
Secondary Shutter
6” diameterTelescopeAlignmentHole w/Plug
Shutter Seal
Shutter Stop
Figure 6.3. Shutter Assembly – on the left is a view from the front with the Shutter in the zenith-pointing position; on
the right is a view from the back with the Shutter in the horizon-pointing position.
Figure 6.2. Carousel geometry.
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The primary shutter requires an inner skin to provide a sealing surface between the various shutter
segments. Since the thermal coating on the outward facing surface of the plate coil on the shutters won’t
hold up to the friction of a seal scraping over it, the seal is made from below. Since there will be
conditions that cause the formation of condensation, the interior slopes to a drain. All drains are routed to
a sump where the condensate is pumped through the utility transfer system for disposal.
There is a 5.4-meter diameter carousel entrance aperture in the primary shutter is oversized with respect
to the ultimate requirements. This is to accommodate a separate carousel entrance aperture stop. A roll
up door, is provided in the shutter.
The secondary shutter segments are constructed similarly to the primary shutter. Plate coil panels
comprise all portions of the shutter skin that receive solar insolation during tracking operations, with
‘dummy’ panels making up the rest. They are mounted on the exterior side of a powder coated tubular
steel frame which also serves as a mounting structure for the shutter support and guide roller assemblies.
Unlike the primary shutter however, the secondary shutter segments are not driven; they are passively
lifted by the primary shutter using a grab bar arrangement.
Sun Shades: The sunshades over the vent gates are plate coil panels mounted on tubular steel frames
attached to the underlying carousel structure. When viewed from above, they provide shade to the entire
carousel surface below it down to the next lower sunshade. They also serve to channel the wind into the
interior of the enclosure.
Azimuth Track and Bogies: The azimuth track is a large hardened steel ring that is mounted to the top
of the stationary enclosure base. The rail is shaped and finished so the lateral guide rollers have stable,
smooth mating surfaces; there is provision for the seismic restraint system; and, the convex bogie wheels
are provided a stiff and smooth surface to traverse on the top. Each of the rail sections is individually
machined flat and subsequently bolted to the other sections with an integral shimming system. Once
assembled, and trued with the shims, a final in-place machining operation is performed to achieve the
required flatness if necessary.
To keep the carousel on the azimuth track in the event of an earthquake, seismic restraint brackets are
utilized. These brackets hang down from the underside of the carousel structure and extend underneath
the edge of the azimuth track. There is a small clearance between the extension portion of the bracket and
the underside of the azimuth track. If the enclosure experiences upward movement relative to the azimuth
track, the bracket minimizes the motion, thereby keeping the carousel on top of the track.
There are 16 support bogies arrayed on the underside of the carousel, four of which are drive units; the
other twelve bogies provide vertical support only. A series of guide rollers provide lateral definition. The
bogies are designed with a suspension system to minimize vibration transfer from the enclosure into the
foundation. Alignment of the bogie wheel is adjustable with respect to the center of rotation as well as
inclination with respect to vertical. Load cells are incorporated for bogie loading adjustments.
6.2.2 Carousel Drive System
The carousel drive system is comprised of the drive assemblies, encoders, and controllers that allow the
carousel and shutter to move in synch with the telescope during operations.
The altitude and azimuth axes of the enclosure feature closed loop three-phase permanent magnet AC
servomotors. All motors are NEMA premium efficiency motors. Using a modified flux vector control
algorithm and feedback from a motor mounted incremental encoder, the motor will behave very much
like a DC Motor but without the maintenance. Full torque at zero speed, ability to directly command
motor torque, tight speed regulation, quick acceleration, and fast positioning are all available. On the
azimuth axis, the 20-hp vector drives are the motive power for the cable-driven bogie assembly.
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The shutter drives are based on the Keck design, though there are some significant differences. Instead of
being mounted within the shutter proper, the ATST system mounts the drives at the azimuth mechanical
floor level at each end of the arch girders. This puts a significant potential source of heat further away
from the optic path and in a place where access for maintenance is less challenging.
The cables are attached to the primary shutter and are guided along each arch girder. There is a drive
motor on each end of each cable, keeping the cables in tension. Load cells on the cables along with torque
levels allow the servo control systems to maintain this balance. The Power Pac Wire Rope by Wire Rope
Industries consists of oval wire cables with an expected life of 30 years. Annual tensioning and
inspection will be required.
Fail-safe, redundant brakes are used on both the altitude and azimuth axes. In both cases, they are used as
parking brakes or in case of power outage or emergency stop only. In normal operation, the motors are
used for deceleration. As illustrated above for the shutter case, these units are spring set with the
activation of the electromagnet in the solenoid commercial units. The drive bogies have a similar
arrangement and in both cases, the discs are mounted directly on the drive shaft of the drive motors.
The encoders for the altitude and azimuth axes are high-resolution absolute linear non-contact encoders
(e.g., Stegmann KH 53 Pomux® Type B) that are matched to Omega Profile sections containing
permanent magnets as a unique position marker. In addition to these absolute encoders and the
incremental encoders installed on each drive motor, limit switches are used in discrete travel positions at
the end of motion range to act as a back up to the system and to supply position information to the ECS.
The altitude encoder Omega Profile sections are mounted with a non-ferrous OEM system 10-cm away
from the steel surface along each arch girder. The non-contacting read heads are attached to adjustable
brackets affixed to the primary shutter. The azimuth encoder Omega Profile sections are similarly
mounted to the inner diameter of the Lower Enclosure ring beam. The read heads are affixed to adjustable
brackets extending downward from the bottom of the azimuth mechanical level floor.
6.2.3 Lower Enclosure
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The lower enclosure is comprised of the stationary structure that supports the carousel and transfers its
loads to the ground (Figure 6.4). The lower enclosure includes foundations and interface to the soil. The
lower enclosure also includes the stationary floors, stairs, ladders, access doors, catwalk, and mounting
surface for the carousel azimuth track.
The foundations for the lower enclosure will be designed as a system along with the foundations for the
telescope pier and the support and operations building. They will be designed to minimize any vibration
transmission from the lower enclosure into the telescope structure. The lower enclosure structure is
conventional steel construction.
Half of the skin panels of the lower enclosure are actually integral to the lower enclosure cooling system.
More fully described below, they are perforated plate coated on the outside with the same thermal coating
system that covers all exterior components. The remaining panels are light gauge steel plate welded in
place to form full-size panels. Again, they are coated on the outside with the same thermal coating
system that covers all exterior components.
6.2.4 Enclosure Thermal System
The enclosure thermal system includes a passive ventilation system, an active ventilation system, the
carousel cooling system, the lower enclosure cooling system, and a thermal coating system.
Azimuth Track
Stationary side ofCarousel Seal
Ring Beam
Catwalk
Stationary side ofCable Chain carrier
Catwalk EmergencyEgress Ladder
Structural members(typical, non-optimized)
Area without flooring, left open for airplenums (see Sec. 7.4)
Personnel access
Support columnsand footers
Maintenance andequipment access
Figure 6.4. The Lower Enclosure.
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Passive Ventilation System: Passive flushing is provided by a combination of twenty independently
controllable vent gates along with the carousel rear access door and the carousel entrance aperture.
Outside wind blows through the vent gates and over the telescope structure, removing thermal turbulence
and assisting the thermal control of the mount and optics (Figure 6.5). When ambient conditions are
excessively windy, the vent gates can be partially closed to throttle the interior wind speed using
commercial roll-up shutters. They are opened prior to the observing day in order to equalize the inside
and ambient conditions. Assistance from the active ventilation system will be required during mornings
with no wind.
Active Ventilation System: Eighteen propeller fans, nine on each side of the enclosure, provide air
movement across the mirrors under no wind or low wind conditions. The fans are sized to provide
roughly 0.5m/s air flow across the mirrors’ travel paths. The system is designed to be used with the
ventilation doors open so that the fans draw in ambient air.
Carousel Cooling System: All surfaces of the carousel which receive sunlight are comprised of or
covered with plate coil heat exchangers containing a propylene glycol solution circulating through to
remove the resulting thermal load. The conventional chiller and circulation pump are remotely located in
the utility building to the west of the support and operations building. The chilled water is supplied at
4°C below the highest ambient temperature of any of the approximately 60 zones to all of the plate coil
heat exchangers. The volume/velocity is controlled for each zone so that the return does not exceed the
local ambient temperature. The control valves are all located at the azimuth mechanical level for
accessibility. The circulation can be shut down during periods of high ambient wind conditions when the
resulting fully developed forced convection is effective in removing the thermal load.
Lower Enclosure Cooling System: The lower enclosure cooling system has been optimized for times of
excellent seeing, which typically occur in the early morning. Data collected during the site survey
indicates that while there is a significant amount of excellent seeing during times of no or low wind, so
the lower enclosure cooling system is limited to those surfaces subject to insolation during the morning
hours. During times of moderate to high wind, the heat fluxes developed are removed by the wind.
The lower enclosure is cooled to within 1.5°C of ambient temperature by drawing ambient air through the
perforated metal skin and exhausting it west of the utility building. The 0.5-mm thick perforated plate has
a 2-3% open area using 1-mm diameter holes. The vane axial fans for this system are located in the
utility building.
Figure 6.5. Passive flows through the Enclosure.
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Thermal Coating System: The Thermal Coating System is based on AZW/11-LA Inorganic Low Alpha
White, non-specular thermal control coating manufactured by AZ Technology or IIT Research Institute.
The coating provides superior thermal protection by allowing only 8-12% of the solar radiation impinging
on the surface to be absorbed through to the interior systems while emitting 90-92% of the internal heat
generated. AZW-11LA incorporates a stabilized pigment system with a silicate binder.
6.2.5 Ancillary Mechanical Equipment
The ancillary mechanical systems include minor mechanical elements of the enclosure such as cranes,
sensor arrays, access doors, utility transfer systems, carousel entrance aperture cover, and other
miscellaneous elements of the enclosure.
The rear access door has two functions: access from the equipment lift for the primary mirror when
mounted on the mirror cart and additional passive ventilation capacity. The door is a standard dock-type,
wind-rated roll-up door. It is located at the back of the carousel. There is a matching door in the primary
shutter that coincides with it when the shutter is in the zenith pointing position. The door in the Carousel
is slightly larger than that in the shutter to provide access for maintenance to the shutter door.
The carousel entrance aperture provides a larger-than-necessary hole (5.4-meter diameter) in the shutter.
A smaller (movable) aperture stop is affixed to the shutter. Its cover is a smaller version of the
commercial roll-up shutters used for the vent gates.
A utility transfer system provides coolant, power, signal, and other utilities to each shutter segment. The
altitude wrap is provided by a series of hanging cable drapes. A guide trough is provided. A COTS-type
cable chain is used to support and manage the cables and utility lines. The azimuth cable wrap is
mounted just below the azimuth utility floor level. The wrap is non-powered; its movement is caused
directly by the movement of the carousel. Another COTS-type cable chain is utilized here as well.
7. SITE INFRASTRUCTURE AND SUPPORT FACILITIES
Site Infrastructure is the compilation of technical requirements, specific site characteristics and a
conceptual layout of the ATST facilities at the Haleakalā site. This includes the support and operations
building and certain equipment within that building, including a mirror cleaning and coating facility; and
mirror handling equipment. Each of these items is described in more detail below.
7.1 SITE SELECTION TECHNICAL DESCRIPTION AND IMPACT
During the D&D phase the ATST tested six candidate sites. We determined that it would be feasible to
build ATST at all six sites, but significant logistical and cost differences exist between them. These were
brought to the attention of project management and the Site Selection Working Group, and became part of
the information used to down-select to three sites. These are Big Bear Lake, CA; Haleakalā, HI; and La
Palma in the Spanish Canary Islands. The eventual recommendation of the Site Selection Working Group
was the Haleakalā site, based on its excellent seeing and low sky brightness.
7.1.1 Technical Site Requirements
Accessibility - A range of vehicles, from standard passenger cars to large construction cranes and flatbed
trucks must be able to reach the facility, both during construction and in long-term operation. This
applies to the roads leading to the site as well as to the local access in the immediate vicinity of the
telescope.
Dimensions – To accommodate the observatory structures a minimum horizontal area of approximately
200 ft. by 200 ft. is necessary, and topography that will allow the creation of a suitable platform. A site
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larger than the minimum would allow a more flexible site layout, and would also facilitate construction
staging.
Structural Characteristics – The soil/rock of the site must have sufficient bearing capacity to support
the loads imposed by the telescope pier and the building foundations while also allowing adequate
isolation between the two. Stiffer natural substrates that increase the lowest resonant frequency of the
telescope support system are considered very advantageous. The lateral force factors (seismic and wind)
inherent to the site must be of a magnitude that can be safely designed for without prohibitively expensive
structural measures.
Manageable permitting process – The environmental issues inherent to the site must be such that the
construction of ATST would not likely be precluded based on the applicable environmental protection
statutes. Any necessary construction permits issued by the regional authorities must also be obtainable.
Utility infrastructure – Sufficient electrical power, data/telephone connection, and domestic water/sewer
service must be achievable at the site. Existing infrastructure that can be extended to ATST and a low
cost connection to local utility company lines are considered very advantageous.
The Haleakalā site meets all of these requirements.
7.1.2 The Haleakalā site
This site is at Haleakala Observatory on the island of Maui, within two hours of coastal cites and less than
a one-hour drive from the observatory’s base lab facility (Figure 7.1). The entire compound, including a
large Air Force telescope complex, is
owned by the University of Hawaii and
managed by the Institute for Astronomy
(IfA). Two potential sites within this
compound have been identified for
ATST. The primary site is close to the
existing Mees Telescope. A rendering of
the ATST facility at this site appears in
Figure 7.2.
No local building permits are required for
construction at Haleakalā, however,
environmental permitting is a significant
cost, schedule, and public relations
factor. An Environmental Impact
Statement and a Conservation District
Use Permit are required, and the effort to
obtain these is underway.
Relatively little excavation will be required to create a suitable level platform for the ATST structure.
The volcanic gravel and cinder on the site has inherently low bearing capacity, so the pier and building
foundations will be wider than normal or extend down to more solid rock layers well below the surface.
The utility infrastructure at Haleakalā, especially electrical power and data connection, is well-developed
and has sufficient reserve capacity to serve ATST.
Figure 7.1. The location of ATST on Maui.
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7.2 SUPPORT AND OPERATIONS BUILDING
These two building elements (identified in Figure 7.4) are spatially and structurally contiguous and have
similar functional requirements. They are treated as a single entity for this analysis.
Structural Requirements: The structure of the Support and Operations Building carries the weight of
the roof and exterior walls; the interior floor loads of all levels; lateral seismic and wind loads; and the
dynamic loading of the rotating enclosure above. The building structure must be isolated from the
telescope pier to prevent unacceptable levels of vibration from reaching any critical optical elements. The
foundations must be sufficient to safely transmit all these loads to the bearing capacity of the soil.
Thermal Requirements: The design of the Support and Operations Building must minimize any
contribution to thermal turbulence in critical optical paths. This impacts the height of the support
building and its proximity to the enclosure, the appropriate location for heat generating mechanical
equipment, the selection of exterior materials and finishes, the potential need for active cooling of exterior
building surfaces, and the appropriate thermal separation of interior spaces. The CFD analysis used to
model the air flow performance of the enclosure allowed the project to thermally optimize the design and
orientation of the building once the site dependent variables of wind speed and direction, topography, and
available site space were established. The prevailing winds tend to come from the northeast at times of
excellent seeing. Seeing is also best in the early morning hours before heating disturbs the ground layer.
This led to the decision to build the support and operations building to the west of the telescope enclosure.
Functional Space Requirements: Table 7.1 lists the spaces defined within the support and operations
building and associated utility building. If remodeled space within the Mees building is included, the
ATST facility provides approximately 13,170 square feet of space.
Figure 7.2. ATST at the Mees site.
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Building Design: The building and layout is depicted in Figures 7.3 and 7.4. The basic construction of
the Support and Operations Building will be a steel-framed structure on concrete foundations with
standard metal panel roofing and siding. This is a conventional building system that is economical,
flexible, and adapts well to a variety of site conditions and lateral load cases. The exterior finish will be
determined by the best thermal performance (probably high-titanium white) with consideration given to
environmental impact issues as required. The interior build-out will be similar to standard office and light
commercial construction.
7.3 FACILITY EQUIPMENT
Facility equipment includes the outfitting and furnishing of the control room, shops, labs, offices and
other ancillary spaces. It also includes the special observatory-related mechanical and electrical
equipment that will be permanently installed in the facility to serve the utility needs of the telescope and
instruments, and to address the extensive cooling requirements of ATST. This does not include normal
building utility items such as lighting, domestic plumbing and general air conditioning, which are
incorporated into the budget and design of the building itself.
The following is a list of the necessary special utility equipment identified to date:
Electrical Generator – Capacity of ~200 KVA.
Uninterruptible Power Supplies – Two units serving a total load of ~50 KVA.
Chillers – Two units of ~30 ton capacity with appropriate temperature range to serve the cooling
requirements of optical components, the heat stop, the telescope mount, the enclosure, and other
special heat sources.
Space Description
ft2
m2 ft m
Control Room 650 60 9 2.7
Computer Room 280 26 9 2.7
Instrument Prep Lab 650 60 10 3.1
Site Manager's Office 130 12 8 2.4
Visiting Observer's Office 130 12 8 2.4
Shared Office Space (~4 people) 400 37 8 2.4
Kitchen/Break Area 250 23 8 2.4
Restrooms (3 - one on each level) 150 14 8 2.4
High-bay Receiving/ Mirror Prep 1,400 130 20 6.1
Mirror Coating Area 800 74 20 6.1
Entry vestibule to enclosure (4 levels) 1,380 128 varies
Platform Lift (4 major levels @460 sq.ft.) 1,840 171 76 23.2
Elevator (4 levels @ 80 sq.ft.) 320 30 65 19.8
Stairs (3 levels @ 160 sq.ft.) 480 45 varies
Machine and Service Rooms 250 23 9 2.7
Mechanical Equip. Space 700 65 N/A N/A
Total Net S&O Building Space 9,810 912
Utility Building 2,560 238 16 4.9
Remodeled Space in Mees Building 800 74 16 4.9
ATST Support Facility Space Requirements
Area Height
Summit Support Facilities
Table 7.1. Space Requirements
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Glycol (or other liquid coolant) – Supply and distribution piping as required.
Special Fans – For active ventilation of the telescope enclosure and adjacent spaces.
HEPA Filtration – Air handlers and filters as required for particulate control in the coudé labs,
mirror coating area and possibly in specified stationary lab areas.
Cryogens – Supply, distribution and processing for liquid nitrogen, compressed helium, or other
coolants as required for special instrument related systems.
Air compressor(s), vacuum pumps, and other equipment required for general utility use and
special telescope-related applications.
7.4 MIRROR COATING AND CLEANING FACILITIES
An appropriate area and the necessary equipment are included in the ATST facility for the handling,
cleaning and recoating of M1, M2, and the Nasmyth mirrors. This equipment includes the M1 assembly
handling cart, M1 lifter and coating plant. The availability of an appropriate existing coating facility or
the potential shared use and co-development of this facility with neighboring observatories is being
studied.
The other mirrors will be coated with protected silver to improve throughput at the coudé observing
station. Those relatively small mirrors will be sent out for coating.
Description: The coating plant utilizes an evaporative system to deposit a coating of pure reflective
aluminum. The coating plant itself is a large clam-shell stainless steel vacuum chamber used to apply the
coating to the mirror surface. The coating plant assembly includes vacuum pumping systems, chilled
water delivery, the magnetron assembly, and the vacuum tank itself. Additional associated equipment
fan
fan
tunnel
utility shaft
vehic
ula
r access?
ice
tanksgenerator
Existing
Cistern
support & operations building
Receiving &
Mirror Prep
offices
existing Mees solar observatory
kitchen
Expanded Shop
lab
utility building
existing main observatory road
service &
parking area
ple
num
Equipment
Area
Platform
Lift
Base of
Piers
concrete
pier
Mirror
Coating
Facility
ventilation
lower
enclosure
ups
ups
ups
cond.
n
ew
X-f
orm
er
hatch
chillers
1050
0 5
30 ft.
10 m
north
Notes:
- Building orientation and layout based on CFD analysis and site space restrictions.
- Elevation of ground floor level is 9983' (~4 ft. higher than Mees building floor level)
waste
treatment
plant
50' turning
radius
for trucks
34 m
11.8
m
control
dimensions
elev.
exterior
utility area
thermal
ground shield
(3 m high)
mirror box
20
Figure 7.3. ATST site plan.
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includes a drainage system and holding tank for stripping fluids, compressed air delivery, and gas
cylinder racks.
Operation: The upper cover of the chamber is lifted clear of the lower half on jacking screws so that the
lower half can be moved on floor rails to the mirror stripping/cleaning area. After stripping, the mirror is
lifted by a crane and held suspended while the mirror cart is moved out of the way and the lower half of
the coating chamber moved under the mirror. The mirror is lowered onto a turntable support frame in the
coating chamber lower half and the chamber is moved back under its cover. During the coating process
the mirror must be positioned central to the axis of rotation of the turntable and be retained in that
position. The front surface of the mirror must remain in a normal plane to the axis of rotation.
Additional Parameters:
The chamber interface to the coolant lines is by manually operated valves; there will be a flow
requirement and outflow temperature requirement.
For the installation of the plant the mobile section of the vessel will use air bearings and drives to
position the vessel within the coating plant chamber. Compressed air couplings normally used to
support the mirror handling cart will provide compressed air to the chamber air skates when the
chamber is installed in each observatory. Handling trolleys will be used for the maintenance and
installation of the magnetron systems.
A rack for 8 gas cylinders will be required for the Argon supply used in coating the mirror.
7.5 HANDLING EQUIPMENT
The most challenging requirement for material handling is transporting the primary mirror from the
telescope to the coating facility and back, which may occur as often as every six months. A platform lift
is provided for that purpose. The mirror in its cell and cart is approximately 5 m in diameter by 2 m high
Control
Room
Computer
Room
DN
Access Balco
ny
Coudé
Platform
WC
utility
closet
DN
UP
utility shaft
C1
C2
C3
C4
C5 C6
C7
C8
C9
C11
C10
C12
C13C14
8'-
2"
[248
9]
cle
artma
tma
enc
future
light feed
north
Advanced Technology
Solar Telescope
at
Haleakala Observatory
S&O BUILDING
COUDÉ LEVEL PLAN
sheet 12 of 20
scale: 1"= 10'-0"
drawn by Jeff Barr Jan 6, 2006
Notes:
For door & opening dimensions
refer to door schedule (sht 20)
For finish materials & ceiling heights
refer to room finish schedule (sht.20)
1050
0
20 ft.
5 m
roof over coating area
Platform Lift
maximum instrument
dimension
2.4
m
5 m
5' x 12' optical bench
when parked
lift serves as additional
lab floor space
8'-0
" window
chase
Mees
Bldg.
vestibule
non
-rotating zone
Instrument
Prep Lab
elevator
Figure 7.4. Coudé Level plan view.
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and weighs close to 15 tons. The capacity and dimensions of the lift are based on the size and weight of
the mirror. The design of the lift also accommodates the maximum defined volume and weight for
instruments that will be moved to and from the coudé stations.
For personnel and smaller equipment a standard building elevator is also provided that serves all five
levels of the Lower Enclosure and the two levels of the Support Facility. For disassembling the primary
mirror from its cell and for loading and unloading large instruments and other equipment, a bridge type
crane with ~20 ton capacity is provided in the high-bay receiving and mirror prep area. A smaller
capacity monorail crane is provided in the instrument lab. Appropriate hatches and removable flooring
are designed into the upper levels of the Lower Enclosure to allow the jib cranes on the Enclosure
(described elsewhere) to be used for material handling in that area. Appropriate additional equipment
(portable scissors lifts, fork lifts, special purpose hoists, etc.) will be provided as well.
All handling equipment, especially the lifts and cranes that are integral to the structural design of the
building, will have the highest affordable capacity and be configured for maximum flexibility as future
requirements are difficult to predict.
7.6 REMOTE OPERATIONS BUILDING
To augment the support and operations building, there is an identified need for a facility that would serve
ATST functions that do not require direct proximity to the observatory. This would allow for a smaller
structure and less heat generation adjacent to the telescope. The functional requirements for this facility
would be mostly for administrative offices with some auxiliary lab/shop space geared toward long-term
maintenance/storage of instruments and equipment. A high-speed data connection, for effective
teleconferencing, for real time communication with the observatory, and for transmission of data to home
institutions will be provided.