Thirty Meter Telescope TMT Adaptive Optics Overview

1
Thirty Meter Telescope (TMT) Adaptive Optics
Overview

• AFOSR SSA PRET Review
• January 18, 2005
• Brent Ellerbroek
• Adaptive Optics Group Lead
• Thirty Meter Telescope Project

2
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

3
An (Almost) Equation-Free Zone

4
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

5
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

6
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

7
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

8
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

9
First Light (2014) AO Architecture
LGSFacility
(Active)Secondary
Narrow-FieldIR AO System
Narrow-FieldNear IR Instruments
10
Comprehensive AO Architecture
LGSFacility
Wide-Field Near IR Imager
Secondary
Multi-Conj.AO System
Narrow-FieldNear IR Instruments
Multi-ObjectAO System
Multi-ObjectSpectrograph
Mid InfraredAO System
Mid IR Instrument(s)
Extreme AO System
Planet FormationImager/Spectrometer
11
BaselineAO Component Summary
NGS AO
LGS AO
12
Narrow Field IR AO System (NFIRAOS) Specifications
13
NFIRAOS Strawman Design Concept
z
LGSWFS
NGSWFS
Dichroic beamsplitter
Fieldderotation
Narrow fieldIR instrument(s)
High-orderDM
Fieldstopmirror
Low-order,large-strokeDM (if needed)
Science NGS LGS
Telescopefocalplane
Off-axis parabolarelay
14
NFIRAOS/MCAO Strawman Layout
15
Notional MCAO Error Budget
16
MCAO Performance vs. Number of DMs
1 DM 2 DMs 3 DMs
17
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

18
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

19
Mean-Square Phase Variance vs.Aperture
Coordinate and Outer Scale
Tip/Tilt/Piston-Removed Variance
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 r010 cm

20
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

Sodium layer depth
h
q
Sodium layer range
H
r
Transmitter-subaperture offset
21
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

22
Dynamic Refocusing via Charge Shifting on a
Radial CCD Array
WFS Pupil Plane
WFS Focal Plane
r 85 km
r 100 km
r 85 km
r 100 km
Lenslets
Pupil
23
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
24
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

25
Effective Spot Size vs. Spot Sampling

• Laser power requirement will scale between qB
  and qB2

26
Noise Propagation Through Wavefront Reconstuction

• WFS measurement model
• Least-squares wavefront reconstruction
• Noise-weighted least-squares reconstruction

27
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

28
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

29
Mean-Square Phase Error Due to Noise

• With zero-mean measurement noise n

30
Impact of LGS Elongation on Wavefront
Reconstruction Error Due to Noise
31
Required Increase in LGS WFS SNR to Compensate
for LGS Elongation
Required laser power increases by about a factor
of 1.321.69 with photon limited sensing (less
with detector read noise)
32
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

33
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

34
MCAO-Compensated Point SpreadFunctions in V and
J Band
V band
J band

• Strehls much higher in J band, particularly
  off-axis

35
Tip/Tilt Sensing Errors Due to Noisewith 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

36
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

37
Sample MOAO Instrument Concept
38
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

39
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