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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