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
> 85% from 0.8 to 2.5 mm
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 mas 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
Diffraction-Limited Image Quality
10-30” Corrected FoV
Very High Order AO (60x60)
Tomography (6 GS) + MCAO (2 DMs)
(Sodium) Laser Guide Stars
High Sky Coverage
Near IR (J+H) Tip/Tilt NGS
Large Guide Field (2’)
MCAO to “Sharpen” NGS
Multiple (3) NGS to Correct Tilt Aniso.
High Throughput
Low Emission
Minimal Surface Count; AR coatings
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)
Lasers
AO Executive Software
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(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
IRMS
IRIS (and on-instrument WFS)
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(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
Beamsplitter
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 h=11.2km
63x63 DM at h=0km
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 Facility
Conservative 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 timedomain 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
Input Disturbance:
Atmospheric
phase screen
TMT aperture
function
M1 phase map
M1+M2+M3 onaxis phase map
Sodium layer
profile
AO System Responses:
LGS subaperture image
Polar coordinate
DM phase maps
CCD pixel intensities
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Residual error
phase map13
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 optomechanical
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
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 Stage
Simulated DM Wiring included in
bandwidth demonstration
Subscale DM
with 9x9
actuators and 5
mm spacing
20 Hz
Req’t
-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
ΔH =10km
H=100km
D = 30m
 Elongation  3-4”
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 1000ms (strong goal of 400 ms)
Update algorithms in real time as conditions change
Store data needed for PSF reconstruction in postprocessing
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 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
Ellerbroek,
AO4ELT, Paris, June 23 2009
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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.
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