Adaptive Optics

AO Team

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

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)

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.

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

The NSO low-order AO system

Wavefront Sensor

WFS camera

DM 97

Video , AO corrected

Collimator/Camera lens

Dyson IF24 subapertures

Correlating SH-WFS

Disk Center

Intensity & Magentogram:

6302 A

Exp: 18 sec

FeI 5576A line: h~200km

Intensity Map & Velocity Map

Dark: downflow

Bright: upflow

First direct measurements of flows in magnetic flux tubes

Exposure: 30sec

Large variations in Strehl on short time scales

•Lack of consistent time sequences

•Interpretation of spectral, polarimetric data becomes difficult High order AO

HO-AO – 76 subapertures high Strehl for median r0

maintains reasonably high Strehl as seeing fluctuates

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

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

Intelligent 2.5kfps CMOS AO camera

Poster by K. Richards

DSP WFS&Reconstructor

Mostly off-the-shelf parts

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)

DLSP

UBF

WFS

First light Dec. 2002

High order AO:

Digitized real-time video

Seeing: mediocre&highly variable

High Order AO + UBF:

FeI 5434 wing intensity

FeI 5434 bisector velocity (dark = downflow)

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!

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.

SRD: 99% of flux within 0.”3

Nordlund, Stein Keller simulations

Scatter Plots: Stokes V

ATST AO PERFORMANCE

Fitting error & Bandwidth error only

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

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

1232 Subapertures 1313 Actuators

10 cm subaperture

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

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

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?

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

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

DM

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

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

PSF MTF

Estimation of long exposure PSF from wavefront sensor statistics.

Implement as standard feature!

Low-order AO1.5sec exposure

Reconstructed image

MCAO

Sum of 11 one sec. exposures

Destretched before averaged

Long exposure w/AO at DST

Fair Seeing

High altitude seeing

Sum of 11

No destretch

Long exposure w/AO at DST

Good seeing

Good high altitude conditions

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