Ellerbroek AFOSR PRET

Thirty Meter Telescope (TMT) Adaptive Optics Overview: 

Thirty Meter Telescope (TMT) Adaptive Optics Overview AFOSR SSA PRET Review January 18, 2005 Brent Ellerbroek Adaptive Optics Group Lead Thirty Meter Telescope Project

Presentation Outline: 

Presentation Outline TMT Project Overview Organization Telescope Science Cases Science instruments and associated AO requirements Adaptive Optics Overview System architecture Facility AO Requirements and design concepts First order performance estimates and special issues Specialized instrument AO systems Extreme, Multi-Object, and Mid Infrared AO Program Plans

An (Almost) Equation-Free Zone: 

An (Almost) Equation-Free Zone =

The TMT Project: 

The TMT Project Seeks to design and build a thirty-meter-diameter telescope Is a collaboration of: The Association of Universities for Research in Astronomy (AURA) The Association of Canadian Universities for Research in Astronomy (ACURA) The University of California The California Institute of Technology Is now commencing a Design Development Phase (DDP) to Establish a management structure and staff the project Collect data to collect the site Complete the conceptual design of the telescope, adaptive optics systems, and an initial suite of instruments Establish a cost estimate with uncertainties on the order of 10% Complete a Conceptual Design Review

Telescope Reference Design: 

Telescope Reference Design Synthesis of design concepts from ACURA, AURA, CELT D = 30 m, f/1 primary 1.2 to 2 m segments 3.5 m concave secondary f/15 output focal ratio 20 arc min FOV Elevation axis above primary Nasmyth-mounted instrumentation

Science Objectives: 

Science Objectives Understanding the emergence of large scale structure in the universe Understanding how galaxies assemble and evolve Mapping stellar populations in nearby galaxies Understanding where, when and how often planets form Characterizing planets via imaging and spectroscopy

Proposed Scientific Capabilities: 

Proposed Scientific Capabilities Visible, seeing-limited spectroscopy Wide field (20’) High [spectral] resolution Near infra-red (0.8-2.5 mm), diffraction-limited imaging Narrow field (10”) “Wide” field (30”) Near infra-red (0.8-2.5 mm) spectroscopy Narrow field (2”), diffraction limited Near diffraction-limited multi-object spectroscopy of many small (2”) objects in a large (5’) field-of-regard Mid infra-red (7-18 mm) diffraction-limited spectroscopy and imaging Planet formation imaging and spectroscopy

Fundamental AO Design Issues: 

Fundamental AO Design Issues Pacing the design and development effort Scientific utility, cost, technical risk Mix of facility and dedicated AO systems Wavefront correction Wavefront sensing Laser guide star (LGS) generation and projection Large-stroke, high-order wavefront correction Large stroke, high order deformable mirrors (DMs) “Woofer-tweeter” DM configurations Piezostack, MEMs, and/or an adaptive secondary Defeating LGS elongation Maximizing sky coverage

First Light (2014) AO Architecture: 

First Light (2014) AO Architecture LGS Facility (Active) Secondary Narrow-Field IR AO System Narrow-Field Near IR Instruments

Comprehensive AO Architecture: 

Comprehensive AO Architecture LGS Facility Secondary Multi-Conj. AO System Narrow-Field Near IR Instruments “Wide-Field” Near IR Imager Multi-Object AO System Multi-Object Spectrograph Mid Infrared AO System Mid IR Instrument(s) Extreme AO System Planet Formation Imager/Spectrometer

BaselineAO Component Summary: 

BaselineAO Component Summary LGS AO NGS AO

Narrow Field IR AO System (NFIRAOS) Specifications: 

Narrow Field IR AO System (NFIRAOS) Specifications

NFIRAOS Strawman Design Concept: 

Low-order, large-stroke DM (if needed) NFIRAOS Strawman Design Concept z LGS WFS Narrow field IR instrument(s) NGS WFS Telescope focal plane Off-axis parabola relay High-order DM Dichroic beamsplitter Field derotation Field stop mirror Science NGS LGS

NFIRAOS/MCAO Strawman Layout: 

NFIRAOS/MCAO Strawman Layout

Notional MCAO Error Budget: 

Notional MCAO Error Budget

MCAO Performance vs. Number of DMs: 

MCAO Performance vs. Number of DMs 1 DM 2 DMs 3 DMs

Important Second-Order Issues: 

Important Second-Order Issues DM stroke requirements Important at D = 30 m Sodium LGS elongation Critical at D = 30 m Sky coverage Potential improvement with infra-red tip/tilt sensing

DM Stroke Requirements: 

Driven by RMS optical path difference due to turbulence Aperture-averaged values with a Kolmogorov spectrum are well known: Tilt included: sTI2 = 1.03 (D/r0)5/3 Tilt removed: sTR2 = 0.13 (D/r0)5/3 Formulas for the un-averaged values with a finite outer scale are more involved: DM Stroke Requirements

Mean-Square Phase Variance vs.Aperture Coordinate and Outer Scale: 

Mean-Square Phase Variance vs. Aperture Coordinate and Outer Scale Piston-Removed Variance Tip/Tilt/Piston-Removed Variance Outer scale (L0) could impact actuator stroke requirements by up to a factor of (0.25/0.07)1/2 = 1.85 -- 7.5 mm vs. 14 mm of stoke for 5s correction of turbulence with r0=10 cm

Sodium Laser Guide Star Elongation: 

Sodium Laser Guide Star Elongation Guidestars appear elongated due depth of sodium layer First-order elongation given by Will significantly degrade LGS WFS accuracy using standard designs and algorithms r H h q Transmitter-subaperture offset Sodium layer range Sodium layer depth

Options for Defeating LGS Elongation: 

Options for Defeating LGS Elongation Buy a (much) more powerful laser Develop short-pulse lasers and track short pulses though the sodium layer “Dynamic refocusing” Develop improved processing algorithms Matched filter wavefront sensing Noise weighted wavefront reconstruction … and buy a (somewhat) more powerful laser

Dynamic Refocusing via Charge Shifting on a Radial CCD Array: 

Dynamic Refocusing via Charge Shifting on a Radial CCD Array WFS Pupil Plane Lenslets Pupil WFS Focal Plane r = 85 km r = 100 km r = 100 km r = 85 km

Modeling LGS Spot Elongation: 

Modeling LGS Spot Elongation Convolution of 3 terms: 2-d Guidestar on sky Subaperture PSF Sodium layer profile Image vs. transmitter-to-subaperture separation: 0 m 4 m 16 m * *

Spot Displacement Estimation via Matched Filtering: 

Spot Displacement Estimation via Matched Filtering Model Shack-Hartmann spot I as a first order function of displacement q plus additive noise Noise-optimal displacement estimate is Estimation error covariance matrix is May define an “effective image spot size” qB by

Effective Spot Size vs. Spot Sampling: 

Effective Spot Size vs. Spot Sampling Laser power requirement will scale between qB and qB2

Noise Propagation Through Wavefront Reconstuction: 

Noise Propagation Through Wavefront Reconstuction WFS measurement model Least-squares wavefront reconstruction Noise-weighted least-squares reconstruction

Estimation Error Due to Noise: 

Estimation Error Due to Noise Instantaneous error in terms of DM actuators Where R is either of the above reconstructors Instantaneous wavefront error profile due to noise Mean-square (aperture averaged) phase error due to noise

Low-Order Mode Removal (Tip/Tilt/Piston for LGS AO): 

Low-Order Mode Removal (Tip/Tilt/Piston for LGS AO) Wavefront error with modes v1,…,vn removed Mean-square phase error with low-order modes removed

Mean-Square Phase Error Due to Noise: 

Mean-Square Phase Error Due to Noise With zero-mean measurement noise n

Impact of LGS Elongation on Wavefront Reconstruction Error Due to Noise: 

Impact of LGS Elongation on Wavefront Reconstruction Error Due to Noise

Required Increase in LGS WFS SNR to Compensate for LGS Elongation: 

Required Increase in LGS WFS SNR to Compensate for LGS Elongation Required laser power increases by about a factor of 1.32=1.69 with photon limited sensing (less with detector read noise)

Tip/Tilt Wavefront Sensing and Sky Coverage: 

Tip/Tilt Wavefront Sensing and Sky Coverage Laser guidestars cannot sense overall wavefront tip/tilt (line-of-sight) due to guidestar position uncertainty Would like to use the dimmest possible natural stars for this purpose to maximize sky coverage What wavelength should be used for tip/tilt sensing? Visible wavelength advantages: Developed detector technology Darker sky backgrounds Near infra-red advantages: Higher guidestar densities Image sharpening by the AO system

Guide Star Densities and Sky Backgrounds: 

Guide Star Densities and Sky Backgrounds Bahcall-Soniera guidestar density model at the galactic pole Zeropoints of 9.71e9 (V) and 5.52e9 (J) phot/m2/sec Sky background of [20.18,21.03,21.51] (V) and 16.5 (J) mag/arcsec2

MCAO-Compensated Point SpreadFunctions in V and J Band: 

MCAO-Compensated Point Spread Functions in V and J Band V band J band Strehls much higher in J band, particularly off-axis

Tip/Tilt Sensing Errors Due to Noisewith Ideal Detectors: 

Tip/Tilt Sensing Errors Due to Noise with Ideal Detectors Matched filter tip/tilt estimation Infinitesimal pixels, zero read noise No saturation or quantization J-band sensing approximates desired performance 3 milli arc second tip/tilt error for 2k stars/deg2 with a 30 arc second offset

Instrument-Specific AO Systems: 

Instrument-Specific AO Systems Mid-IR AO (Mid IR Spectrograph) Minimal warm surfaces to reduce emissivity in 7-18 mm band Requires either an adaptive secondary or a cryogenic DM Extreme AO (Planet Formation Imager) Stable, well calibrated AO performance to detect companions at contrast ratios of 10-6 to 10-8 High order AO + coronography + multi-wavelength detection Multi-Object AO (IR Multi-Object Spectrograph) Compensate multiple small fields of view with a large field-of-regard using independent deformable mirrors Avoids large number of serial DMs (and reflections) that would be required using MCAO Requires open-loop control of each DM

Sample MOAO Instrument Concept: 

Sample MOAO Instrument Concept

Project Responsibilities and Schedules: 

Project Responsibilities and Schedules Narrow-Field IR AO System Instrument design team responsibility, with project office coordination and direction Conceptual design 2005-2006; preliminary design 2006-2007 Mid IR, Extreme, and Multi-Object AO systems Instrument design team responsibilities Feasibility studies 2005-2006; conceptual design studies 2006-2007 Laser guide star facility and adaptive secondary Project office responsibilities (with subcontracts as needed) Conceptual design 2005-2006; preliminary design 2006-2007

Supporting Activities: 

Supporting Activities AO component development Deformable mirrors (all flavors) Fast, quite IR detectors for wavefront sensing Longer-term projects in guidestar lasers and visible detectors (following completion of AODP and other contracts) Analysis, modeling, and algorithm development Lab and field tests UCSC and University of Victoria AO labs Palomar AO system and multiple guide star unit Keck and Lick LGS AO systems Gemini-North AO system