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Outline
• Overview of instrument
• CCDs
• mechanics
• instrument control
• performance
• construction anecdotes
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Apache Point Observatory
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3.5-meter telescope
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Light Path
primary mirror
tertiary mirror
rotator
SPIcam
secondary mirror
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camera control electronics
axis controllers
data transmission
remote workstation
telescope controlcomputer
control computer
data interface
closed cycle cooler(CryoTiger)
temperature control(Lakeshore)
dewar
CCD
cooling head
waste heat
rotator
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Charge-Coupled-Devices (CCDs)
near-perfect detectors for optical radiation
• high quantum efficiency
• 100% fill-factor
• large linear dynamic range
• ∼ few electrons (photons) read-noise
• negligible dark-current
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CCD structure
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CCD layout
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Quantum Efficiency
• blue cut-off results from short penetration depth of
photons through gate structure
• red cut-off results from band-gap of silicon (1.14 eV =
1085 nm, at 173 K)
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CCD quantum efficiency
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Front-side vs. back-side illumination
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Dynamic Range
• determined by the “full well depth” of the device
• scales approximately with pixel volume
• ∼ 200, 000 e− for SPIcam CCD (∼ 60, 000ADU)
• newer CCDs with read noise ∼ 1e− (> 16-bit dynamic
range)
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Read Noise
• usually dominated by the properties of the on-chip am-
plifier
• scales as√readout− rate
• typically 3-8 e− for scientific CCDs
• newer devices are approaching sub-electron read noise
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CCD readout
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Dark Current
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Mechanical Design: Internal
• position detector rigidly with respect to optical path
• transfer mechanical registration from inside to outside
of dewar
• thermally isolate detector from environment
• keep vacuum environment as clean as possible
• bring first-stage output amplifier as close to the detector
as possible
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cold head
cold strap
cooling block
window
CCD
reference surface
fiberglassspring
dewar lid
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positioning detector
• f/10 beam of 3.5-meter has ∼ 700µm depth-of-focus
• calibrated contact to cooling head
• allow for thermal contraction on cooling =⇒ fiberglass
spring
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Cryogenics
• cool to ∼ −100◦C
• eliminate need for liquid nitrogen
• ion-pump is useful
• reduce workload on observatory staff
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CryoTiger
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CryoTiger
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Mechanical Design: External
• shutter
• filter wheel
• electronics
• waste-heat control
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Rotating-Wheel Shutter
CCDsmall slit
large slit
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Shutter Timing
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Filter Wheel
• must work
• read-back of wheel position
• holds 6 filters
• position filters to ∼ few microns
• minimize handling of filters
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Electronics Packaging
• robust against electrical interference
• minimize cable lengths to dewar
• lightning protection
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Simplified Grounding Scheme
controlcomputer
Dewar
CCD
"earth ground"
"earth ground"
analogelectronics
fiber−optic link
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Waste-Heat Removal
• forced-air heat removal from electronics packages
• dump heat to mid-level
• makes a good vacuum-cleaner
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Electronics
• based on architecture developed by Peter Doherty at
Photometrics
• 6811 =⇒ DSP =⇒ level-shifter =⇒ clock wave-forms
• 6811 also controls shutter and filter-wheel
• 40 kHz pixel rate
• parallel data is serialized for transmission to control
computer
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Software control
• unix workstation for ease of networking
• command-line interface
• scripting in “mana” (Gene Magnier)
• instrument sends commands directly to TCC
• scripts for taking focus images, sky-flats
• automatic focus adjustment when changing filters
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Software architecture
telescope controlcomputer (TCC)
motor controllers
dryrot.apo.nmsu.edu(Sparc 5)
electronics package
remote workstation
Master Control Computer(MC)
Remark Interface(Mac)
SPIcam
data onfiber
internet
serial on fiber
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Performance
• 25 sec. full-frame read-time (binned 2× 2)
• 4.78 arcminute F.O.V. at 0.14 arcsec per pixel
• 3.37 e− / ADU sensitivity
• 5.7 e− read-noise, 2.7 e−/hr dark current
• 0.999999 CTE in both serial and parallel directions
http://www.apo.nmsu.edu/Instruments/SPIcam/
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Sensitivity - Sloan
Filter Star Sky (per pixel)
u* 10.1 0.7
g* 303 12.8
r* 310 18.2
i* 259 25.6
z* 77.5 32.0
m = 20, 1 sec. integration, binned 2× 2, area is 4πσ2 for Gaussian
PSF
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Sensitivity - Johnson-Cousins
Filter Star (e−/s) Sky (e−/s/pixel)
U 20.2 2.4
B 189 3.7
V 303 7.4
R 256 12.1
I 216 22.9
m = 20, 1 sec. integration, binned 2× 2, area is 4πσ2 for Gaussian
PSF
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Anecdotes from SPIcam construction
• know when to “wing it”
• monitor everything
• efficiency of operation is critical
• always carry tools
• roads in New Mexico are rough