Transcript Adaptive Optics Update - Home: GSMT Program Office

Gemini AO Program
Adaptive Optics Requirements,
Concepts, and Performance Predictions
for Extremely Large Telescopes
Brent Ellerbroek and Francois Rigaut
Gemini Observatory
SPIE Opto-Southwest / Astronomical Optics
September 17, 2001
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
1
Gemini AO Program
Presentation Outline
•
•
•
•
Requirements and AO operating modes
Implementation concepts and issues
Towards performance estimates
Summary
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
AO Performance Objectives for GSMT
Narrow
MCAO Low
field AO
order AO
10”
2’
2’
Field-of-view
Wavelength range, mm
Resolution, arc sec
Strehl, %
Emissivity, %
Stability over 3600 s, %
Astrometric accuracy, arc sec
1.0-2.5
DL
80 (90)
20
1.0-2.5
DL
50
20
1.0-20
0.1-0.2
10
1
1e-4
5
1e-3
2
1e-2
1
5
2
Photometric accuracy, %
(DL: Diffraction limited)
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
Representative Parameters for
Narrow Field AO (NFAO)
Target Strehl at l = 1 mm
# Actuators
Servo BW, Hz
FoV diameter, arc sec
NGS magnitude (Hartmann WFS)
Implementation errors, nm
0.8
0.9
59600 147000
80
125
1.75
1.1
9.3
8.0
34
23
• Standard scaling law analysis with nominal Cerro
Pachon atmosphere
• Strehl budget divided equally into: fitting, servo lag,
anisoplanatism, WFS noise, implementation errors
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
Representative Parameters for MCAO
l, mm
Target Strehl
1.65
0.4
Target FoV, arc sec
1
# of DM’s
3
Actuator pitch, m
# of sodium LGS or NGS
Power/LGS, W
0.5
5
30?
Servo Bandwidth, Hz
40
Implementation errors, nm
90
SPIE Opto-Southwest
September 17, 2001
1.0 • Gemini-South design
scalable to D = 30 m
0.5
…
2 • … with increased
laser power
5-9?
0.25
– Increase between
O(D) and O(D2)
9? • Better correction over
wider FoV implies
120?
more DM’s, WFS’s
40
50
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
– Scaling not yet fully
quantified
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Gemini AO Program
Representative Parameters for Low
Order AO (LOAO)
• Standard Strehl scaling laws break down for
low-order systems
• Initial parameter estimates can be derived from
Hokupa’a performance on Gemini-North
– Number of actuators: 500-1000
– Limiting magnitude: 15-17
– Will achieve near-diffraction limited performance for
l > 5 mm
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
LLT
MOMFOS
LOAO
AO Implementation
concept
for GSMT
Prime
Focus
Adaptive M2
(LOAO)
LLT?
LLT?
M1
MCAO
Elevation
Axis
Inst. NFAO
Inst.
SPIE Opto-Southwest
September 17, 2001
M1
Support
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
Inst.
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Gemini AO Program
Ideas on NFAO Implementation
• NGS AO on bright guidestars
• Woofer/tweeter wavefront correction
– Adaptive M2 (low order, large stroke)
– Fast tip/tilt mirror
– MEMS or other high-order DM (limited stoke)
• Shack-Hartmann wavefront sensor or point diffraction
interferometer (PDI)
– PDI would eliminate reconstructor and reduce detector count, but has
limited dynamic range
– Two-stage wavefront sensing possible
• Atmospheric dispersion compensation requirements and
implementation TBD
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
8
Gemini AO Program
Guide Star Options for MCAO
1) Natural
Pros
Cons
No lasers!
Questionable sky coverage (at desired
Strehls w/o a revolutionary WFS)
2) Rayleigh 1.
2.
3.
Mature laser tech.
Safe for satellites
Fixed LGS range
1.
2.
3.
Performance questions
Many guide stars required
Optical design questions
3) Sodium
Performance
predictions
Few (5) guidestars
required
1.
2.
3.
4.
LGS elongation
Immature laser tech.
Variable LGS range
Satellite predictive avoidance
1.
2.
4) Sodium
+ loworder
Rayleigh
Enhanced sky coverage (tilt
aniso. compensation with a
single tip/tilt NGS)
SPIE Opto-Southwest
September 17, 2001
2) 3 and 3) 1-4
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
Coping with LGS Elongation
• LGS Elongation proportional to aperture radius
– Increases WFS spot size by factor of ~3 for D = 30 m
• Increases laser power requirements
– Factor of ~32 for a Hartmann WFS with 22 pixels per
subaperture
– Factor of ~3 for a Shack-Hartmann WFS with many pixels
per subaperture
• Increased sensitivity to sodium column fluctuations
• Alternate approaches:
– Track short laser pulses through sodium layer?
• Need ~ 1 msec pulses, duty cycle of 0.05-0.10
– Multiple lasers and launch telescopes per LGS?
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
MCAO Optical Design
• Derived from Gemini-South
– Collimated space for DM’s, ADC’s, beamsplitters between
off-axis parabolas (OAP’s)
• Design constrained by magnification ratio tradeoffs
– Low magnification permits near-normal angles of
incidence on DM’s, but …
• Requires large diameters for TTM, ADC
• Increases overall size, complicates packaging
– Current design uses M=60 (0.5 m beam diameters)
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
MCAO Opto-Mechanical Layout
Collimated space for
ADC’s, beamsplitters
DM’s &
TTM
OAP’s
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
Issues for Further Work
• LGS WFS optical path
– Noncommon path aberrations and pupil distortion with
variable LGS range
• Tip/tilt mirror requirements
• Explore alternative approaches with reduced surface
count
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
13
Gemini AO Program
AO Modeling Issues and Methods
• First-order system sizing and performance analysis
– Number/geometry of guidestars, DM’s, WFS
– Performance vs WFS noise, control loop bandwidth
 Classical linear systems models
• Higher order effects and implementation errors
– Wave optics propagation (atmosphere, optical train, WFS)
– Extended (three-dimensional) laser guide stars
– DM-to-WFS misregistration, noncommon path errors,…
 Wave optics propagation simulations
• Computation requirements for ELT’s??
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
NGS’s
LGS’s
Science Fields
Turbulence
Wave
Optics
Minimal Variance
Simulation
Shackfor
Hartmann
Evaluation
• Geometric or
Wave
Optics
of
Higher• Gain/bias
Order
calibration
• 3-D LGS
Effects
• Photon + Read
•
Noise
• Misregistration
- Filtered white noise
- Taylor hypothesis
LGS
Pointing
Tip/Tilt
Offload
Reconstructor
LGS + NGS
WFS’s
DM’s
TTM
• Zonal
• 2nd order Dynamics
• Misregistration
Common- and
Noncommon
Path Errors
Science
Instrument
Strehl Histories
Mean PSF’s
Gemini AO Program
Sample Modeling Results for MCAO
on Gemini-South
Linear Systems Analysis
Wave Optics Simulation
Strehl Ratio
Center Edge Corner
FoV
FoV FoV
J band 0.590
H band 0.739
K band 0.843
0.478 0.475
0.653 0.649
0.787 0.783
D = 8 m, median Cerro Pachon Seeing,
y = 00, 1’ square FoV, no WFS noise
Solid: Extended laser beacons and wave
optics propagation
Dashed: WFS noise, DM misregistration
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
Computation Requirements vs.
Aperture Diameter
• Scaling with diameter D for a fixed density of DM
actuators and WFS subapertures
Optical propagation
O(1)
(assumes fixed phase screen size of at least twice outer scale)
Compute WFS measurements
Compute DM actuator commands
Compute DM surface figures from commands
 Compute estimation matrix before simulation
O(D2)
O(D4)
O(D2)
O(D6)
Conventional matrix computation
Factor of 4096 in scaling from D = 8m to D = 32m !
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
Sparse Matrix Methods Reduce
Computation Requirements
• First studied in 1983-84 for NGS AO with classical least
squares estimators
– Scaling with aperture diameter ~O(D3), not O(D6)
• Features of more elaborate AO configurations and
estimators require special treatment
– Turbulence-related matrices appearing in optimal estimators
– LGS tip/tilt removal and NGS tip/tilt measurements
• Method to include these terms recently developed
– Computing LOAO / NFAO estimators for GSMT almost trivial
– Computing MCAO estimators at least feasible
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
MCAO Results (and Predictions)
Using Sparse Matrix Methods
NGS MCAO
Aperture, m
8
Actuators
789
Subapertures
Optimal
RMS WFE, mm
Time, hours
RAM, MB
Sparse
RMS WFE, mm
Time, hours
RAM, MB
SPIE Opto-Southwest
September 17, 2001
1020
0.138
16
LGS MCAO
32
8
16
32
(9700)
789
2417
(9700)
4280 (17120)
1024
2417
-
- 0.149
4284 (17124)
-
-
0.51 (32) (2103) 0.51 (32) (2103)
Equivalent Performance
13.7 (176)
(2827)times
13.8
(177) (2848)
Computation
reasonable
at least
feasible 0.139 Memory
0.137 requirements
0.148
0.149
0.19 1.92 (19.61) 0.21 1.68 (13.44)
Performance a weak function of D
111.9 679.7 (4128) 98.5 560.5 (3189)
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
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Gemini AO Program
Summary
• Three AO operating modes defined for GSMT
– NFAO with a bright NGS (High Strehls in NIR, 1-10” field)
– MCAO using multiple sodium LGS (Good Strehls in NIR, 1-2’
field)
– LOAO using a dim NGS (Improved resolution in near- and
thermal IR, 2’ field, low emissivity)
• Top-level implementation approach defined
• Work on MCAO and NFAO designs in progress
• Estimators can be efficiently computed/evaluated using
sparse matrix methods
SPIE Opto-Southwest
September 17, 2001
Ellerbroek/Rigaut [SW01-114]
AO … for ELT’s
21