adaptive optics systems for the thirty meter telescope
DESCRIPTION
Adaptive Optics Systems for the Thirty Meter Telescope. Brent Ellerbroek Thirty Meter Telescope Observatory Corporation Adaptive Optics for Extremely Large Telescopes Paris, June 23, 2009. Presentation Outline. AO requirements flowdown Top-level science-based requirements for AO at TMT - PowerPoint PPT PresentationTRANSCRIPT
TMT.AOS.PRE.09.027.REL01Ellerbroek, AO4ELT, Paris, June 23 2009
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Brent Ellerbroek
Thirty Meter Telescope Observatory Corporation
Adaptive Optics for Extremely Large Telescopes
Paris, June 23, 2009
Adaptive Optics Systems for the Thirty Meter Telescope
TMT.AOS.PRE.09.027.REL01Ellerbroek, AO4ELT, Paris, June 23 2009
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Presentation Outline
AO requirements flowdown– Top-level science-based requirements for AO at TMT– Derived requirements and design choices– First light AO architecture summary
Subsystem designs– Narrow Field Infra-Red AO System (NFIRAOS)– Laser Guide Star Facility (LGSF)
System performance analysis
Component requirements and prototype results
Lab and field tests
Upgrade paths
Summary
TMT.AOS.PRE.09.027.REL01Ellerbroek, AO4ELT, Paris, June 23 2009
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Top-Level Requirements at First Light
Derived to enable diffraction-limited imaging and spectroscopy at near IR wavelengths:
Throughput > 85% from 0.8 to 2.5 m
Thermal Emission < 15% of background from sky + telescope
Wavefront Quality 187 nm RMS on-axis*
191/208 nm RMS on a 10”/30” FoV
Sky Coverage > 50 % at the Galactic Pole
Photometry 2% differential accuracy (10 min exposure, 30” FoV)
Astrometry 50 as differential accuracy (100 sec exposure, 30” FoV)
Acquisition time < 5 minutes to acquire a new field
Reliability < 1% unscheduled downtime
*Yields Strehl ratios of 0.41, 0.60, and 0.75 in J, H, and K bands
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Implied AO Architectural Decisions
High Throughput
Low Emission
Minimal Surface Count; AR coatings
Cooled Optical Path (-30° C)
Diffraction-Limited Image Quality
High Sky Coverage
10-30” Corrected FoV
Very High Order AO (60x60)
Tomography (6 GS) + MCAO (2 DMs)
(Sodium) Laser Guide Stars
MCAO to “Sharpen” NGSLarge Guide Field (2’)
Near IR (J+H) Tip/Tilt NGS
Multiple (3) NGS to Correct Tilt Aniso.
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Technology and Design Choices (I)
Utilize existing or near-term approaches whenever possible
Solid state, CW, sum-frequency (or frequency doubled) lasers for bright sodium laser guidestars– Located in telescope azimuth structure with a fixed gravity vector
Impact of guidestar elongation is managed by:– Laser launch from behind secondary mirror– “Polar coordinate CCD” with pixel layout matched to elongation– Noise-optimal pixel processing, updated in real time
Mirror-based beam transport from lasers to launch telescope is current baseline
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Technology and Design Choices (II)
Piezostack DMs for high-order wavefront correction– “Hard” piezo for large stroke, low hysteresis at low temperature– 5 mm inter-actuator pitch implies a large AO system
Surface count minimized to improve throughput and emissivity– Tip/tilt correction using a tip/tilt stage, not separate mirror– Field de-rotation at instrument-AO interface (no K-mirror)
Tomographic wavefront reconstruction implemented using efficient algorithms and FPGA/DSP processors
Tip/tilt/focus NGS WFSs located in science instruments– Baseline detector is the H2RG array
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AO Architecture Realization
Narrow Field IR AO System (NFIRAOS)– Mounted on Nasmyth
Platform
– Ports for 3 instruments
Laser Guide Star Facility (LGSF)– Lasers located within
TMT azimuth structure
– Laser launch telescope mounted behind M2
– All-sky and bore-sighted cameras for aircraft safety (not shown)
AO Executive Software (not shown)
Lasers
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NFIRAOS on Nasmyth Platform with Client Instruments
Electronics Enclosure
Nasmyth Platform Interface
LGS WFS Optics
Instrument Support Structure
NFIRAOS Optics Enclosure
Future (third) Instrument
Nasmyth Platform
Laser Path IRMS
(and on-instrument WFS)
IRIS (and on-instrument WFS)
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NFIRAOS Science Optical Path
DM0/TTS
• 1-1 OAP optical relay
• DMs located in collimated path
WFS Beam-splitter
Light From
TMT
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OAP2
IR Acquisition camera
2 Truth NGS WFSs1 60x60 NGS WFS
OAP1
6 60x60 LGS WFSs
63x63 DM at h=0kmOn tip/tilt platform
(0.3m clear apeture)
76x76 DM at h=11.2km
Output to science instruments and IR T/T/F WFSs
Input from
telescope
NFIRAOS Opto-mechanical Layout
AO and science calibration units not illustrated
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Laser Guide Star FacilityConservative Design Approach
Approach based upon existing LGS facilities (i.e. Gemini North and South)
Laser system– Initially 6 25W solid state, CW laser devices with one spare
– Space for future upgrades to additional or more advanced lasers
Beam transfer optics– Azimuth structure path
– “Deployable” path to transfer beams to elevation structure along telescope elevation axis
– Elevation structure path, including pupil relay optics and pointing/centering mirrors for misalignment compensation
– Top-end beam quality, power, and alignment sensors
– Optics for asterism generation, de-rotation, and fast tip/tilt correction
Laser launch telescope– 0.5m unobscured aperture and environmental window
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Approach to Performance Analysis
Key requirement is 187 nm RMS wavefront error on-axis– 50% sky coverage at Galactic pole– At zenith with median observing conditions– Delivered wavefront with all error sources included
Performance estimates are based upon detailed time-domain AO simulations– Physical optics WFS modeling with LGS elongation– Telescope aberrations and AO component effects included– Actual RTC algorithms for pixel processing and tomography– “Split” tomography enables simulation of 100’s of NGS asterisms
Simulated disturbances are based upon TMT site measurements, sodium LIDAR data, telescope modeling
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Examples of AO Simulation Data and Intermediate Results
Atmospheric phase screen
TMT aperture function
M1 phase map M1+M2+M3 on-axis phase map
Sodium layer profile
Input Disturbance:
AO System Responses:
LGS sub-aperture image
Polar coordinate CCD pixel intensities DM phase maps Residual error
phase map
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Example NGS Guide Field from Monte Carlo Sky Coverage Simulation
Sample Asterism near 50% Sky Coverage (Besançon Model, Galactic Pole)
Tip/Tilt NGS
Tip/Tilt/Focus NGS
Tip/Tilt NGS
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Performance Estimate Summary
178 nm RMS error in LGS modes– 127 nm first order, 97 nm AO
components, 79 nm opto-mechanical
47.4 nm tip/tilt at 50% sky coverage63.4 nm overall error in NGS modes187 nm RMS total at 45% sky coverageNGS Algorithm optimization and detector characterization still underway
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Sky Coverage Results for Enclosed Energy on a 4 mas Detector
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Key AO Component Technologies
Component Key Requirements
Sodium guidestar lasers
25W
Coupling efficiency of 130 photons-m2/s/W/atom
Deformable mirrors 63x63 and 76x76 actuators
10 m stroke and 5% hysteresis at -30C
Tip/tilt stage 500 rad stroke with 0.05 rad noise
20 Hz bandwidth
NGS WFS detector 240x240 pixels
~0.8 quantum efficiency,1 electron at 10-800 Hz
LGS WFS detectors 60x60 subapertures with 6x6 to 6x15 pixels each
~0.9 quantum efficiency, 3 electrons at 800 Hz
Low-order IR NGS WFS detectors
1024x1024 pixels
~0.6 quantum efficiency, 5 electrons at 10-800 Hz
Real time control electronics/algorithms
Solve 35k x 7k reconstruction problem at 800 Hz
TMT.AOS.PRE.09.027.REL01Ellerbroek, AO4ELT, Paris, June 23 2009
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Laser Systems
50W+ power successfully demonstrated by a prototype Nd:YAG, sum frequency, CW laser
Development of a facility class 25W design now underway at ESO, with AURA/Keck/GMT/TMT support for prototyping
Sodium layer coupling of ~260 photons–m2/s/W/atom demonstrated, but issues remain– Magnetic field orientation, photon recoil, inaccessible ground states– coupling of ~ 70 photons-m2/s/W/atom predicted at ELT sites
Possible solutions include combined D2a/D2b pumping and multiple (3-5) laser lines– Performance penalty is ~40 nm RMS without laser improvements
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Wavefront Correctors: Prototyping Results
Prototype
Tip/Tilt StageSimulated DM Wiring included in
bandwidth demonstration
Low hysteresis of only 5-6% from -40° to 20° C
Subscale DM with 9x9 actuators and 5 mm spacing
-3dB TTS bandwidth of 107 Hz at -35C
20 Hz
Req’t
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“Polar Coordinate” CCD Array Concept for Wavefront Sensing with Elongated Laser
Guidestars
D = 30m D = 30m
Elongation Elongation 3- 3-4”4”
TMT
sodium layer ΔH =10km
H=100km
Fewer illuminated pixels reduces pixel read rates and readout noise
AODP Design
LLT
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Laser Guide Star (LGS) WFS Detector Requirements
Parameter Requirement CommentsArray Geometry “Polar Coordinate” Matched to LGS elongation
Number of subapertures 60x60 For NFIRAOS
Pixels/subaperture 6x6 up to 15x6 205k total pixels
Frame rate
Readout time
800 Hz
500 sec
Read noise at 800 Hz 3 electrons Derived from measured planar JFET performance (CCID-56 CCD)
Quantum efficiency 0.9 at 0.589 m Narrow-line optimized
Now waiting to fabricate and test the 1-quadrant prototype design developed under AO Development Program (AODP) funding
TMT.AOS.PRE.09.027.REL01Ellerbroek, AO4ELT, Paris, June 23 2009
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Real Time Controller (RTC): Requirements and Design Approach
Perform pixel processing for LGS and NGS WFS at 800 Hz
Solve a 35k x 7k wavefront control problem at 800Hz– End-to-end latency of 1000s (strong goal of 400 s)
Update algorithms in real time as conditions change
Store data needed for PSF reconstruction in post-processing
Using conventional approaches, memory and processing requirements would be >100 times greater than for an 8m class MCAO system
Two conceptual design studies by tOSC and DRAO provide effective solutions through computationally efficient algorithms and innovative hardware implementations
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Lab Tests and Field Measurements
University of Victoria Wavefront Sensor Test Bench– Tests of matched filter
wavefront sensing with real time updates as sodium layer evolves
University of British Columbia sodium layer LIDAR system– 5W laser, 6m receiver– 5m spatial resolution at 50
Hz
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Options for First Decade AO Upgrades and Systems
MEMS-based MOAO in future NFIRAOS instruments– Increased sky coverage via improved NGS sharpening– Multiple MOAO-fed IFUs on a 2 arc minute FoV– Order 120x120 wavefront correctors for ~130 nm RMS WFE (with
upgraded lasers, wavefront sensors, and RTC)– MEMS correct NFIRAOS residuals; simplified stroke/linearity requirements
Additional AO systems for “first decade” instrumentation:– Mid-IR AO (Order 30x30 DM, 3 LGS)– MOAO (Order 64x64 MEMS, 5’ field, ~8 LGS)– ExAO (Order 128x128 MEMS, amplitude/phase correction for M1
segments, advanced IR WFS, post-coronagraph calibration WFS)– GLAO (Adaptive secondary to control ~500 wavefront modes, 4-5 LGS)
Adaptive secondary mirror could be useful for all systems– Only corrector needed for GLAO and Mid-IR AO– Large-stroke “woofer” for MOAO, ExAO, and NFIRAOS+
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Summary
TMT will be designed from the start to exploit AO– Facility AO is a major science requirement for the observatory
An overall AO architecture and subsystem requirements have been derived from the AO science requirements– Builds on demonstrated concepts and technologies, with low risk
and acceptable cost
AO subsystem designs have been developedDesigns and performance estimates are anchored by detailed analysis and simulationComponent prototyping and lab/field tests are underwayConstruction phase schedule leads to AO first light in 2018Upgrade paths are defined for improved performance and new AO capabilities during the first decade of TMT
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Additional Posters and Talks
Presenter Topic TimeNelson Science and top-level AO requirements 0930 Tuesday
Herriot NFIRAOS 1640 Tuesday
Travouillon Turbulence and windspeed models 1040 Wednesday
Wang Sky coverage analysis 1120 Wednesday
Pfrommer UBC LIDAR system 1410 Wednesday
Boyer Laser Guide Star Facility 1600 Wednesday
R. Conan UVic LGS WFS Test Bench 1410 Thursday
Loop IRIS on-instrument WFS 1430 Thursday
Sinquin Wavefront correctors 1720 Thursday
Gilles Tomographic wavefront reconstruction 0850 Friday
Browne Real-time control electronics 1040 Friday
Hovey Real-time control electronics 1100 Friday
Andersen NFIRAOS operating temperature 1720 Tuesday (poster)
Lardier UVic LGS WFS Test Bench 1800 Thursday (poster)
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Acknowledgements
The authors gratefully acknowledge the support of the TMT partner institutions
They are– the Association of Canadian Universities for Research in Astronomy (ACURA)– the California Institute of Technology – and the University of California
This work was supported as well by– the Gordon and Betty Moore Foundation– the Canada Foundation for Innovation– the Ontario Ministry of Research and Innovation– the National Research Council of Canada– the Natural Sciences and Engineering Research Council of Canada– the British Columbia Knowledge Development Fund– the Association of Universities for Research in Astronomy (AURA)– and the U.S. National Science Foundation.