Download - Adaptive Optics
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Adaptive Optics
AO Team
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Outline
• Solar AO – What is different?• High order AO development – a prototype for
ATST AO• ATST AO requirements• Design Concepts
– wavefront sensor – DM– WFS optics
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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)
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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 and high frequency content in object (Poyneer 2003)
•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 preferred.
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Progress in steps
• Low- Order AO: 24 subapertures @ 1.2-1.5 kHz• High-Order AO: 76 subsparture @ 2.5 kHz
• Next: ATST- AO: order 1000 subapertures, >2kHz
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The NSO low-order AO system
Wavefront Sensor
WFS camera
DM 97
Video , AO corrected
Collimator/Camera lens
Dyson IF24 subapertures
Correlating SH-WFS
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Disk Center
Intensity & Magentogram:
6302 A
Exp: 18 sec
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FeI 5576A line: h~200km
Intensity Map & Velocity Map
Dark: downflow
Bright: upflow
First direct measurements of flows in magnetic flux tubes
Exposure: 30sec
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Large variations in Strehl on short time scales
•Lack of consistent time sequences
•Interpretation of spectral, polarimetric data becomes difficult High order AO
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HO-AO – 76 subapertures high Strehl for median r0
maintains reasonably high Strehl as seeing fluctuates
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High order AO WFS geometry
Pupil image & lenslet
d=7.5 cm subaperture – pushing it for granulation
Subaperture images 2-d x-correlations Camera arrangement
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Parallel processing using DSPs
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
See K. Richards for details
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Intelligent 2.5kfps CMOS AO camera
Poster by K. Richards
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DSP WFS&Reconstructor
Mostly off-the-shelf parts
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Performance
• Detailed performance characterization in progress: Strehl > 0.7
• Update rate: 2500 Hz• Servo delay:
– 400 μsec readout + 250 μsec processing = 650 μsec– Bandwidth: ~130 Hz (0dB cross-over error attenuation)
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DLSP
UBF
WFS
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First light Dec. 2002
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High order AO:
Digitized real-time video
Seeing: mediocre&highly variable
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High Order AO + UBF:
FeI 5434 wing intensity
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FeI 5434 bisector velocity (dark = downflow)
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Summary
• The high order solar AO operational DST• Closed-loop bandwidth: 130 Hz • Diffraction limited imaging over long periods of
time• High Strehl ratios • First Scientific results – MHD confirm
fundamental model predictions• Successful stepping stone towards ATST AO!
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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 V
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ATST AO PERFORMANCE
Fitting error & Bandwidth error only
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Adaptive Optics for the ATSTNIR (1.6 micron)
High Strehls should be fairly easy to achieve!
The HO-AO system just developed would do reasonably well
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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 average over several isoplanatic patches
• Bandwidth: fG ~ v/r0 ; σ2 ~ (fG/fs) 5/3
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1232 Subapertures 1313 Actuators
10 cm subaperture
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Hammerhead vs. Tiger Sharc
• 80 MHz Clock• 2 - 32bit float MAC per
clock• 160 MAC per second• 2 subapertures per DSP
• 300 MHz Clock (500Mhz)• 8 - 16bit int MAC per
clock• 2400 MAC per second• >15 times as fast!• 20 subapertures per
DSP
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Host Computer
LinkPort
toRS422
Monitor
Keyboard
SMARTINTERFACE
CameraTo
DSPs
SortsPixelsInto
Subapertures
CAMERA
800x800
32 ports
40 MHz
2000 fps
Tip/Tilt Mirror
Deformable Mirror
D/A
64 DSPs – 300MHz2400 16bit MACs per second
NetworkRemote ControlData Collection
Off-load fixed aberrations
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SH-WFS Camera
Need: ~ 8002 pixel camera
> 2000 fps
Custom Camera: CCD or CMOS or Hybrid
• CCD: 32+ parallel readouts @ 40 MHz
•Contacting vendors:
•E2V (doable but $$$)
•1kx1K running at 1kHz exist (in contact with vendors/developers)
• Design Contract with one or more vendors soon
Alternative (maybe not): split optically (e.g., prisms). Alignment? Stability?
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DM
• A number of ~1000 actuator systems are in operation
• “Off-the-Shelf” item at Xinetics, Inc.• Baseline design requires 5mm actuator spacing• New control electronics, 20 channels on 3U
board, < $100/per channel. Availability: end of 2003
• Big Issue: Thermal Control! (Nathan Dalrymple)– ~900W/m2 (200mm pupil, R=90%)– Air-cooled or liquid cooled
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Optics
• Integrated AO• Where do(es) the wavefront sensor(s) go?
– Close to instrument(s) preferred– Right after DM
• Uncommon path issues, air path to Coude lab
• Other Drivers/Issues:– Interaction with instrumentation, scanning, modulator,
analyzer– Complexity due to multiple instrument setup
requirement
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DM
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Reconstruction
• Modal Reconstruction• Simple Zonal Approach won’t work because of
rotation between WFS and DM• Or: Rotate WFS• 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
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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• Status: under development, collaboration with
Gemini AO folks (J.P. Veran) and CfAO and ONERA
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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
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Reconstructed image
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MCAO
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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
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MCAO
•Large subaperture FOV(60+ arcsec)
•3 ROIs in FOV (~10x10 arcsec)
3 “guide stars”
•Enough real estate on device•Read-out at sufficiently high frame rates