TMT.AOS.PRE.09.027.REL01

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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

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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

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Top-Level Requirements at First Light

• Derived to enable diffraction-limited imaging and
  spectroscopy at near IR wavelengths

Throughput gt 85 from 0.8 to 2.5 mm
Thermal Emission lt 15 of background from sky telescope
Wavefront Quality 187 nm RMS on-axis 191/208 nm RMS on a 10/30 FoV
Sky Coverage gt 50 at the Galactic Pole
Photometry 2 differential accuracy (10 min exposure, 30 FoV)
Astrometry 50 mas differential accuracy (100 sec exposure, 30 FoV)
Acquisition time lt 5 minutes to acquire a new field
Reliability lt 1 unscheduled downtime
Yields Strehl ratios of 0.41, 0.60, and 0.75 in J, H, and K bands 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
Diffraction-Limited Image Quality
Very High Order AO (60x60)
10-30 Corrected FoV
Tomography (6 GS) MCAO (2 DMs)
High Sky Coverage
(Sodium) Laser Guide Stars
Near IR (JH) Tip/Tilt NGS
MCAO to Sharpen NGS
Large Guide Field (2)
Multiple (3) NGS to Correct Tilt Aniso.
High Throughput
Minimal Surface Count AR coatings
Low Emission
Cooled Optical Path (-30 C)
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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)

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NFIRAOS on Nasmyth Platform with Client
Instruments
Future (third) Instrument
NFIRAOS Optics Enclosure
Instrument Support Structure
LGS WFS Optics
Nasmyth Platform Interface
Nasmyth Platform
Electronics Enclosure
Laser Path
IRIS (and on-instrument WFS)
IRMS (and on-instrument WFS)
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NFIRAOS Science Optical Path

• 1-1 OAP optical relay
• DMs located in collimated path

Light From TMT
WFS Beam-splitter
DM0/TTS
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NFIRAOS Opto-mechanical Layout
2 Truth NGS WFSs 1 60x60 NGS WFS
IR Acquisition camera
Input from telescope
OAP1
OAP2
76x76 DM at h11.2km
63x63 DM at h0km On tip/tilt platform (0.3m
clear apeture)
Output to science instruments and IR T/T/F WFSs
6 60x60 LGS WFSs
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 100s 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
Input Disturbance
Atmospheric phase screen
TMT aperture function
M1 phase map
M1M2M3 on-axis phase map
Sodium layer profile
AO System Responses
LGS sub-aperture image
Polar coordinate CCD pixel intensities
Residual error phase map
DM phase maps
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Example NGS Guide Field from Monte Carlo Sky
Coverage Simulation
Tip/Tilt NGS
Tip/Tilt/Focus NGS
Tip/Tilt NGS
Sample Asterism near 50 Sky Coverage (Besançon
Model, Galactic Pole)
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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 coverage
• 63.4 nm overall error in NGS modes
• 187 nm RMS total at 45 sky coverage
• NGS 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 mm stroke and 5 hysteresis at -30C
Tip/tilt stage 500 mrad stroke with 0.05 mrad 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
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Laser Systems

• 50W power successfully demonstrated by a
  prototype NdYAG, 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 photonsm2/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 Stage
Simulated DM Wiring included in bandwidth
demonstration
Subscale DM with 9x9 actuators and 5 mm spacing
20 Hz Reqt
-3dB TTS bandwidth of 107 Hz at -35C
Low hysteresis of only 5-6 from -40 to 20 C
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Polar Coordinate CCD Array Concept for
Wavefront Sensing with Elongated Laser Guidestars
Fewer illuminated pixels reduces pixel read rates
and readout noise
sodium layer ?H 10km
D 30m ? Elongation ? 3-4
H100km
LLT
TMT
AODP Design
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Laser Guide Star (LGS) WFS Detector Requirements
Parameter Requirement Comments
Array 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 msec
Read noise at 800 Hz 3 electrons Derived from measured planar JFET performance (CCID-56 CCD)
Quantum efficiency 0.9 at 0.589 mm Narrow-line optimized
Now waiting to fabricate and test the 1-quadrant
prototype design developed under AO Development
Program (AODP) funding
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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 1000?s (strong goal of 400
  ms)
• 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 gt100 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 developed
• Designs and performance estimates are anchored by
  detailed analysis and simulation
• Component prototyping and lab/field tests are
  underway
• Construction phase schedule leads to AO first
  light in 2018
• Upgrade 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 Time
Nelson 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.