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Adaptive Optics for ATST
ApproachIssuesPlans
Requirements:see SRD
• The ATST shall provide diffraction-limited observations (at the detector plane) with high Strehl(S > 0.6 (goal S>0.7) during good seeing conditions (r0(500nm) > 15cm); S> 0.3 during median seeing (r0(500nm) = 10cm) ) at visible and infrared wavelength.
• The wavefront sensor must be able to lock on granulation and other solar structure, such as pores and umbral and penumbral structure.
• Time sequences of consistent image quality are required for achieving many of the science goals.
• Robust operations.
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SRD: 99% of flux within 0.”3
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Nordlund, Stein Keller simulations
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Scatter Plots: Stokes I
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Scatter Plots: Stokes V
Scatter Plots: Stokes V
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Solar AO
•Small r0 (visible&day-time seeing)
•Near-ground turbulence
•High temporal frequencies
•Extended object
•Object evolves in time (sec –min)
• Photons are plentiful (broad-band)
Day-time vs. Night time seeing
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Wavefront Sensor Noise
Night time AO:•S/N limited by # of photons collected and detector noise (<1-3e-)
•Limiting magnitude
•For faint objects: laser guide stars required
Solar AO:•S/N limited by image contrast (Michau et al 1992) - granulation 1.5 –2 % contrast for d ~10cm
•Larger FOV to track on large scale structure: Yes but, average over many isoplanatic patches > only turbulence near telescope is corrected
•Flat Field Problems are deadly!! Partially filled apertures are problematic!
•Background: Photon noise dominates - Detector noise is not an issue. CCDs with large wells are prefered.
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WFS noise vs. image contrast
Pore: 8% contrast
Gran: 1.5% cont.
Flux Tubes:
DST w/ AO
6 sec exposure G-band
Not bad, isn’t it?
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Disk Center
Intensity &Magentogram:
6302 A
Exp: 18 sec
Intensity &Dopplergram:
Exp: 18 sec
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FeI 5576A
Intensity &
Dopplergram
Exp.: 30sec
FeI 5380 A:
Intensity &
Dopplergram
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Broad-band filtergrams at 500 nm
2.5 hours sequence of 1sec exposures
AO at the DST:
ATST AO PERFORMANCE
Fitting error & Bandwidth error only
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Adaptive Optics for the ATST
NIR (1.6 micron)
High Strehls should be fairly easy to achieve!
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Implementation
200 Pixels
200 Pixels
20 Pixels
20 Pixels
Pupil image & lenslet
Subaperture images 2-d x-correlations Camera arrangement
High-order AO designParallel Approach!
Host Computer
LinkPort
toRS422
Monitor
Keyboard
CameraLink
toLinkPort
Camera
NSOPhotoBit
200x200
33MHz per
channel
2500fps
Ch0
Control
Ch1
Ch2
Ch3
Ch4
4 DSP Cluster
4 DSP Cluster
4 DSP Cluster
4 DSP Cluster
4 DSP Cluster
4 DSP Cluster
4 DSP Cluster
4 DSP Cluster
4 DSP Cluster
4 DSP Cluster
40 DSP Block Diagram
Tip/TiltMirror
Deformable Mirror
Ch5
Ch6
Ch7
Ch8
Ch9
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DSP WFS&Reconstructor
Camera & DM interfaces
Sub-aperture sorting DSPs
WFS/Reconstructor DSPs
I/O capabilities!
Scalable!
Expect to do ~10 apertures on 1 DSP > ~100 DSPs
Or there may be something else by the time
WFS Camera
SH-WFS Camera
AO System with O(1000) DOF for ATST
Need: ~ 6002 – 8002 pixel camera
> 2000 fps
Custom Camera:
• CCD or CMOS
• Many parallel readouts
• Design Contract during D&D phase
Alternative: split optically (e.g., prisms). Alignment? Stability?
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AO Performance
• The site is the most important factor• The site will ultimately determine the performance• Cost, Complexity scale with (D/r0)2
• Subabperture size ~ r0:• Contrast in subaperture images > WFS noise• Isoplanatic angle > FOV for correlation tracking >
WFS noise and s 2 ~ (?/ ?0)5/3
• Bandwidth: fG ~ v/r0 ; s 2 ~ (fG/fs) 5/3
Sum of 11 one sec. exposures
Destretched before averaged
Long exposure w/AO at DST
Fair Seeing
High altitude seeing
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Sum of 11
No destretch
Long exposure w/AO at DST
Good seeing
Good high altitude conditions
Optics
• Where do(es) the wavefront sensor(s) go?• Tied in with instrumentation, scanning,
modulator, analyser• Complexity due to simultaneous observations
requirement
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AO & Diffraction Limited Spectro-
PolarimeterOther Instruments:•Spectrograph
•UBF
•Fabry-Perot
•VIS/IR Polarimeter
•VIS/IR CCDs
•“bring your own”
4853-57
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Reconstruction
• Modal Reconstruction• Simple Zonal Approach won’t work because of
rotation between WFS and DM• Methods very much the same as in night time AO• Issues:
– Alignment of WFS and DM actuator grid– Pupil wobble– Develop optimized reconstruction algorithms– Continuously update of reconstruction matrix
PSF Estimation
• Needed for quantitative analysis. E.g. Photometry
• Important in particular for extended objects• Interpretation of low Strehl observations• Should be/Will be standard product of AO
system• Current status: under development
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PSF MTF
Estimation of long exposure PSF from wavefront sensor statistics.
Implement as standard feature!
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Low-order AO1.5sec exposure
Reconstructed image
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Calibration of Optical Path
Recommended
