adaptive optics systems for the thirty meter telescope

TMT.AOS.PRE.09.027.REL01Ellerbroek, AO4ELT, Paris, June 23 2009

1

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


2

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


3

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


4

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.


5

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


6

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


7

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


8

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)


9

NFIRAOS Science Optical Path

DM0/TTS

• 1-1 OAP optical relay

• DMs located in collimated path

WFS Beam-splitter

Light From

TMT


10

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


11

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


12

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


13

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


14

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


15

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


16

Sky Coverage Results for Enclosed Energy on a 4 mas Detector


17

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


18

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


19

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


20

“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


21

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


22

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


23

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


24

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+


25

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


26

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)


27

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.

adaptive optics systems for the thirty meter telescope

Documents

instrumentao interface

large ao systemsurface

ngslarge guide field

tmtderived requirements

ir j h tiptilt ngsmultiple

science calibration

telescope azimuth structure

near ir wavelengths