aura new initiatives office. larry stepp and brooke gregory the gsmt point design

AURA New Initiatives Office


Larry Stepp and Brooke Gregory

The GSMT Point Design


Advances of the Past Decade


What Lies Ahead?

Astronomers already see the need for more powerful O/IR telescopes, to:– Extend the reach of current ground-based O/IR facilities– Complement space-based telescopes (e.g. NGST)– Complement next generation radio facilities (ALMA; SKA)

What type of facility will provide the needed capabilities a decade hence?


Decadal Review

• In May 2000, the astronomy decadal review committee recommended, as its highest priority ground-based initiative, the construction of a 30-meter Giant Segmented Mirror Telescope (GSMT)

• In response, AURA formed a New Initiatives Office (NIO) to support scientific and technical studies leading to the creation of a GSMT

– Goal of ensuring broad astronomy community access to a 30m telescope contemporary with NGST.


AURA New Initiatives OfficeApproach to GSMT Design

Parallel efforts:• Understand the scientific context for GSMT in NGST era

– Develop the key science requirements

• Address challenges common to all ELTs– Site testing and selection

– Cost-effective mirror fabrication

– Characterization of wind loading

– Hierarchical control systems

– Adaptive optics

• Develop a Point Design – Approach integrates initial science goals & instrument concepts


What is a “Point Design”?

A point design is a learning exercise that:– Explores a single, plausible design

– Helps identify key technical issues

– Helps define factors important to the science requirements

– Provides an opportunity to develop necessary analytical methods

A point design is not:– A trade study that evaluates all possible options

– A design that anyone is proposing to build


GSMT Point Design: Scientific Motivations

• Provide a practical basis for wide-field, native seeing-limited instruments– Origin of large-scale structure in the universe

• Enable high-Strehl performance over ~ arc-minute fields– Stellar populations; galactic kinematics; chemical abundances

• Enable high sensitivity mid-IR spectroscopy– Detection of stars & planetary systems in formation


Key Point Design Features

• Fast aspheric primary– Stigmatic image after two reflections

• Radio telescope-type design– Structural advantages– Accommodates large instruments

• Adaptive secondary– Wind-buffeting compensation– Atmospheric correction in IR, with low emissivity– First stage in higher-order adaptive systems

• Prime focus instrument– Convenient plate scale for seeing-limited observations– Enables wide-field science


Optical Design

M1 diameter: 30 meters

M1 focal ratio: f/1

M2 diameter: 2 meters

M2 focal ratio: f/18.75

Optical design: Classical Cassegrain


Radio Telescope Structural Design Lightweight steel truss structure Fast primary focal ratio Small secondary mirror M2 supported on tripod structure Elevation axis behind M1

– Span between elevation bearings is less than M1 diameter

– Allows direct load path


Initial Point Design Structure

Concept developed by Joe Antebi of Simpson Gumpertz & Heger

• Based on radio telescope• Space frame truss• Single counterweight• Cross bracing of M2 support


Initial Point Design Structure

Typical 'raft', 7 mirrors per raft

Special raft - 6 places, 4 mirrors per raft

1.152 m mirror across flats

Circle, 30m dia.

Plan View of Structure Pattern of segments

Gemini


Primary Mirror Segments

• Segment dimensions

– 1.15-m across flats -- 1.33-m corner to

corner

– 50 mm thickness

• Number of segments: 618

• Maximum departure from sphere 110

microns

– Comparable to Keck

• Axial support is 18-point whiffletree– FEA Gravity deflection 15 nm RMS


Initial Structural Analysis

X Y

Z

Output Set: Mode 1, 2.156537 Hz, Deformed(0.0673): Total Translation

Horizon Pointing - Mode 1 = 2.16 Hz


Structural Analysis

• Total weight of elevation structure – 700 tonnes• Total moving weight – 1400 tonnes• Gravity deflections ~ 5-25 mm

– Primarily rigid-body tilt of elevation structure

• Lowest resonant frequencies ~ 2 Hz

Large size and low resonant frequency make wind buffeting a key issue.


Gemini 8-meter Telescope


Sensor Locations

Pressure sensors

Ultrasonic anemometer

Ultrasonic anemometer


Simultaneous Animations (c00030oo)

Wind Pressure (N/m2) Mirror Deformation (microns)

Wind Speed at 5 Locations (m/sec)


Response of structure to wind


Controllable Elements

Active Systems:

• Active structural elements– Active alignment– Active damping

• M2 rigid body motion– ~ 5-10 Hz

– Five axes

• M1 segment figure control– Based on look-up table ~ 0.1 Hz

– Astigmatism, focus, trefoil, coma

• M1 segment rigid body position – ~ 1 Hz

– Piston, tip & tilt


Controllable Elements

Adaptive Systems:

• High-order narrow-field conventional AO– ~ 10,000 – 50,000 actuators

• Multi-conjugate wide-field AO– ~ 3 DMs

– Laser Guide Stars

• Adaptive secondary mirror– ~ 20-50 Hz

– ~ 1000-10,000 actuators

• Adaptive mirror in prime focus corrector


0.001 0.01 0.1 1 10100

Ze

rnik

e m

od

e s

Bandwidth [Hz]

~100

~50

~20

~10

2

Controls Approach:Hierarchical Subsystems

aO (M1)

AO (M2)

Main Axes

LGS MCAO

Secondary rigid body

temporal avg

spatial & temporal avg



spatial avg

spatial avg


Active and Adaptive Optics will be integrated into Telescope and Instrument concepts from the start.


Instruments

• NIO team currently developing design concepts for 4 instruments:– Multi-Object, Multi-Fiber, Optical Spectrograph – MOMFOS– Near IR Deployable Integral Field Spectrograph – NIRDIF– MCAO-fed near-IR imager– Mid-IR, High Dispersion, AO Spectrograph – MIHDAS

Paper by Sam Barden et al immediately after this one.


Instrument Locations on Telescope

Prime Focus

Fiber-fed Nasmyth

Direct-fed Nasmyth

Co-moving Cass


Instrument Locations on Telescope

Prime Focus

Fiber-fed Nasmyth

Direct-fed Nasmyth

Fixed Gravity Cass

Co-moving Cass


MCAO System


Paper on:

Adaptive optics requirements, concepts and performance estimates for Extremely Large telescopes

by Brent Ellerbroek and Francois Rigaut at 1:10.


Mayall, Gemini and GSMT Enclosuresat same scale

Mayall Gemini GSMT


McKale Center – Univ of Arizona


GSMT – at same scale


Some Possible GSMT Enclosure Designs


Summary: Key Point-Design Features

• F/1 primary mirror– Advantages:

• Reduces size of enclosure

• Reduces flexure of optical support structure

• Reduces counterweights required

– Disadvantages:• Increased sensitivity to

misalignment

• Increased asphericity of segments



• Paraboloidal primary– Advantages:

• Good image quality over 10-15 arcmin field with only two reflections

• Lower emissivity for mid-IR

• Compatible with laser guide stars

– Disadvantages:

• Higher segment fabrication cost

• Increased sensitivity to segment alignment



• Radio telescope structure– Advantages:

• Direct load path to elevation bearings • Can have short back focal distance• Allows small secondary mirror – can

be adaptive• Allows MCAO system ahead of

Nasmyth focus• Allows many gravity-invariant

instrument locations – Disadvantage:

• Requires counterweight• Sweeps out larger volume in enclosure



• 2m diameter adaptive secondary mirror– Advantages:

• Correction of low-order M1 modes

• Enhanced native seeing

• Good performance in mid-IR

• First stage in high-order AO system

– Disadvantages:• Increased difficulty (i.e., cost)



• Prime focus location for MOMFOS– Advantages:

• Fast focal ratio leads to reasonably-sized instrument

• Adaptive prime focus corrector allows enhanced seeing performance

– Disadvantages:• Issues of interchange with M2

• Requires fibers instead of slits


Plans for Next 15 Months

Involve community in defining GSMT scientific context Continue structural analysis Construct hierarchical system control model Simulate system performance in presence of

disturbances Extend AO development efforts Continue site testing Develop cost-reduction strategies

– Segment fabrication– Telescope structure– Adaptive optics– Instrument technologies– Enclosures


Acknowledgements

George Angeli Joe Antebi and Frank Kan

of SG&H Sam Barden Dick Buchroeder Myung Cho Brent Ellerbroek Paul Gillett Brooke Gregory Charles Harmer

Many people have contributed to this work, including:

Ming Liang Matt Mountain Joan Najita Jim Oschmann Jennifer Purcell Francois Rigaut Rick Robles Mike Sheehan David Smith of MERLAB Steve Strom

Plus many NOAO & Gemini scientists working on the GSMT science case


Information on AURA NIO activities is available at:

www.aura-nio.noao.edu

aura new initiatives office. larry stepp and brooke gregory the gsmt point design

Documents

gsmt point design slide

new initiatives office

point design approach

plausible design

gsmt design parallel

widefield science slide

past decade slide

classical cassegrain