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Adaptive Optics

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

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Adaptive Optics

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  1. Adaptive Optics AO Team

  2. 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

  3. 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)

  4. 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.

  5. 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

  6. The NSO low-order AO system Dyson IF 24 subapertures Correlating SH-WFS Collimator/Camera lens Video , AO corrected Wavefront Sensor DM 97 WFS camera

  7. Disk Center Intensity & Magentogram: 6302 A Exp: 18 sec

  8. FeI 5576A line: h~200km Intensity Map & Velocity Map Dark: downflow Bright: upflow First direct measurements of flows in magnetic flux tubes Exposure: 30sec

  9. Large variations in Strehl on short time scales • Lack of consistent time sequences • Interpretation of spectral, polarimetric data becomes difficult  High order AO

  10. HO-AO – 76 subapertures high Strehl for median r0maintains reasonably high Strehl as seeing fluctuates

  11. 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

  12. Parallel processing using DSPs See K. Richards for details

  13. Intelligent 2.5kfps CMOS AO camera Poster by K. Richards

  14. DSP WFS&Reconstructor Mostly off-the-shelf parts

  15. 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)

  16. WFS DLSP UBF

  17. First light Dec. 2002

  18. High order AO: Digitized real-time video Seeing: mediocre&highly variable

  19. High Order AO + UBF: FeI 5434 wing intensity

  20. FeI 5434 bisector velocity (dark = downflow)

  21. 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!

  22. 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.

  23. SRD: 99% of flux within 0.”3

  24. Nordlund, Stein Keller simulations

  25. Scatter Plots: Stokes V

  26. ATST AO PERFORMANCE Fitting error & Bandwidth error only

  27. Adaptive Optics for the ATST NIR (1.6 micron) High Strehls should be fairly easy to achieve! The HO-AO system just developed would do reasonably well

  28. 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

  29. 10 cm subaperture 1232 Subapertures 1313 Actuators

  30. 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 Hammerhead vs. Tiger Sharc

  31. CAMERA 800x800 32 ports 40 MHz 2000 fps SMART INTERFACE Camera To DSPs Sorts Pixels Into Subapertures 64 DSPs – 300MHz 2400 16bit MACs per second Link Port to RS422 Deformable Mirror Tip/Tilt Mirror Monitor D/A Keyboard Network Remote Control Data Collection Off-load fixed aberrations Host Computer

  32. SH-WFS Camera • Need: ~ 8002 pixel camera • > 2000fps • 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?

  33. 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

  34. 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

  35. DM

  36. 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

  37. 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

  38. Estimation of long exposure PSF from wavefront sensor statistics. Implement as standard feature! PSF MTF

  39. Low-order AO1.5sec exposure

  40. Reconstructed image

  41. MCAO

  42. Long exposure w/AO at DST Fair Seeing High altitude seeing Sum of 11 one sec. exposures Destretched before averaged

  43. Long exposure w/AO at DST Good seeing Good high altitude conditions Sum of 11 No destretch

  44. 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

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