Post on 13-Dec-2015
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
1
Gemini AO Program
Adaptive Optics Requirements, Adaptive Optics Requirements, Concepts, and Performance Predictions Concepts, and Performance Predictions
for Extremely Large Telescopesfor Extremely Large Telescopes
Brent Ellerbroek and Francois RigautBrent Ellerbroek and Francois RigautGemini ObservatoryGemini Observatory
SPIE Opto-Southwest / Astronomical OpticsSPIE Opto-Southwest / Astronomical OpticsSeptember 17, 2001September 17, 2001
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
2
Gemini AO Program
Presentation OutlinePresentation Outline
• Requirements and AO operating modes• Implementation concepts and issues• Towards performance estimates• Summary
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
3
Gemini AO Program
AO Performance Objectives for GSMTAO Performance Objectives for GSMT
Narrow field AO
MCAO Low order AO
Field-of-view 10” 2’ 2’
Wavelength range, m 1.0-2.5 1.0-2.5 1.0-20
Resolution, arc sec DL DL 0.1-0.2
Strehl, % 80 (90) 50 -
Emissivity, % 20 20 10
Stability over 3600 s, % 1 5 2
Astrometric accuracy, arc sec 1e-4 1e-3 1e-2
Photometric accuracy, % 1 5 2
(DL: Diffraction limited)
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
4
Gemini AO Program
Representative Parameters for Representative Parameters for Narrow Field AO (NFAO)Narrow Field AO (NFAO)
• Standard scaling law analysis with nominal Cerro Pachon atmosphere
• Strehl budget divided equally into: fitting, servo lag, anisoplanatism, WFS noise, implementation errors
Target Strehl at m 0.8 0.9
# Actuators 59600 147000
Servo BW, Hz 80 125
FoV diameter, arc sec 1.75 1.1
NGS magnitude (Hartmann WFS) 9.3 8.0
Implementation errors, nm 34 23
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
5
Gemini AO Program
• Gemini-South design scalable to D = 30 m …
• … with increased laser power– Increase between
O(D) and O(D2)
• Better correction over wider FoV implies more DM’s, WFS’s– Scaling not yet fully
quantified
Representative Parameters for Representative Parameters for MCAOMCAO
, m 1.65 1.0
Target Strehl 0.4 0.5
Target FoV, arc sec 1 2
# of DM’s 3 5-9?
Actuator pitch, m 0.5 0.25
# of sodium LGS or NGS 5 9?
Power/LGS, W 30? 120?
Servo Bandwidth, Hz 40 40
Implementation errors, nm 90 50
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
6
Gemini AO Program
Representative Parameters for Low Representative Parameters for Low Order AO (LOAO)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 > 5 m
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
7
Gemini AO Program
AO Imple-AO Imple-mentation mentation concept concept for GSMTfor GSMT
MCAO
Inst.
LLT
LLT? LLT?
Inst.
Adaptive M2 (LOAO)
LOAO
MOMFOS
ElevationAxis
M1
M1Support
PrimeFocus
Inst. NFAO
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
8
Gemini AO Program
Ideas on NFAO ImplementationIdeas 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-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
9
Gemini AO Program
Guide Star Options for MCAOGuide Star Options for MCAO
Pros Cons
1) Natural No lasers! Questionable sky coverage (at desired Strehls w/o a revolutionary WFS)
2) Rayleigh 1. Mature laser tech.
2. Safe for satellites
3. Fixed LGS range
1. Performance questions
2. Many guide stars required
3. Optical design questions
3) Sodium 1. Performance predictions
2. Few (5) guidestars required
1. LGS elongation
2. Immature laser tech.
3. Variable LGS range
4. Satellite predictive avoidance
4) Sodium + low-order Rayleigh
Enhanced sky coverage (tilt aniso. compensation with a single tip/tilt NGS)
2) 3 and 3) 1-4
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
10
Gemini AO Program
Coping with LGS ElongationCoping 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 sec pulses, duty cycle of 0.05-0.10
– Multiple lasers and launch telescopes per LGS?
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
11
Gemini AO Program
MCAO Optical DesignMCAO 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-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
12
Gemini AO Program
MCAO Opto-Mechanical LayoutMCAO Opto-Mechanical Layout
OAP’s
Collimated space forADC’s, beamsplitters
DM’s &TTM
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
13
Gemini AO Program
Issues for Further WorkIssues 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-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
14
Gemini AO Program
AO Modeling Issues and MethodsAO Modeling Issues and Methods
• First-order system sizing and performance analysis– Number/geometry of guidestars, DM’s, WFS
– Performance vs WFS noise, control loop bandwidthClassical 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??
Gemini AO Program
ScienceInstrument
LGS + NGSWFS’s
Turbulence- Filtered white noise- Taylor hypothesis
Science FieldsLGS’s
NGS’s
LGSPointing
Tip/TiltOffload
DM’sTTM
Recon-structor
Common- andNoncommonPath Errors
Strehl HistoriesMean PSF’s
Wave Wave Optics Optics Simulation Simulation for for Evaluation Evaluation of Higher-of Higher-Order Order EffectsEffects
• Shack-Hartmann• Geometric or Wave Optics• Gain/bias calibration• 3-D LGS• Photon + Read Noise• Misregistration
• Zonal• 2nd order Dynamics• Misregistration
Minimal Variance
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
16
Gemini AO Program
Sample Modeling Results for MCAO Sample Modeling Results for MCAO on Gemini-Southon Gemini-South
Strehl Ratio
Center FoV
Edge FoV
Corner FoV
J band 0.590 0.478 0.475
H band 0.739 0.653 0.649
K band 0.843 0.787 0.783D = 8 m, median Cerro Pachon Seeing, = 00, 1’ square FoV, no WFS noise
Linear Systems Analysis Wave Optics Simulation
Solid: Extended laser beacons and wave optics propagationDashed: WFS noise, DM misregistration
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
17
Gemini AO Program
Computation Requirements vs. Computation Requirements vs. Aperture DiameterAperture Diameter
• Scaling with diameter D for a fixed density of DM actuators and WFS subaperturesOptical propagation O(1)
(assumes fixed phase screen size of at least twice outer scale)
Compute WFS measurements O(D2)
Compute DM actuator commands O(D4)
Compute DM surface figures from commands O(D2)Compute estimation matrix before simulation O(D6)
Conventional matrix computation
Factor of 4096 in scaling from D = 8m to D = 32m !
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
18
Gemini AO Program
Sparse Matrix Methods Reduce Sparse Matrix Methods Reduce Computation RequirementsComputation 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-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
20
Gemini AO Program
MCAO Results (MCAO Results (and Predictionsand Predictions) ) Using Sparse Matrix MethodsUsing Sparse Matrix Methods
NGS MCAO LGS MCAO
Aperture, m 8 16 32 8 16 32
Actuators 789 2417 (9700) 789 2417 (9700)
Subapertures 1020 4280 (17120) 1024 4284 (17124)
Op
timal
RMS WFE, m 0.138 - - 0.149 - -
Time, hours 0.51 (32) (2103) 0.51 (32) (2103)
RAM, MB 13.7 (176) (2827) 13.8 (177) (2848)
Sparse
RMS WFE, m 0.139 0.137 - 0.148 0.149 -
Time, hours 0.19 1.92 (19.61) 0.21 1.68 (13.44)
RAM, MB 111.9 679.7 (4128) 98.5 560.5 (3189)
Equivalent Performance
Performance a weak function of D
Computation times reasonableMemory requirements at least feasible
SPIE Opto-SouthwestSeptember 17, 2001
Ellerbroek/Rigaut [SW01-114]AO … for ELT’s
21
Gemini AO Program
SummarySummary
• 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