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A Short Introduction to Adaptive Optics Presentation for NGAO Controls Team

A Short Introduction to Adaptive Optics Presentation for NGAO Controls Team. Erik Johansson August 28, 2008. Overview. Why we need AO The basics of AO Intro to wavefront sensing Intro to tip-tilt correction Intro to higher-order wavefront correction LGS vs NGS AO Limitations of AO

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A Short Introduction to Adaptive Optics Presentation for NGAO Controls Team

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  1. A Short Introduction to Adaptive OpticsPresentation for NGAO Controls Team Erik Johansson August 28, 2008

  2. Overview • Why we need AO • The basics of AO • Intro to wavefront sensing • Intro to tip-tilt correction • Intro to higher-order wavefront correction • LGS vs NGS AO • Limitations of AO • How NGAO will differ form our current AO system • Q&A

  3. Why do we need AO? Short exposure images of the stars Gamma Perseus and Alpha Orionis (Betelgeuse) demonstrate the effects of atmospheric turbulence

  4. Without atmosphere, the telescope forms a perfect “diffraction-limited” spot in the focal plane Light from distant star Telescope aperture Focal Plane Image Spot size = 2.44 l/D

  5. The atmosphere acts like many lenses of size r0 to create moving “speckles” in the image Light from distant star Atmosphere (lens size = r0) Telescope aperture Focal Plane Freeze the speckles by using short exposures < ~0.1 sec r0 is characteristic size of the atmosphere Number of speckles ~ (D/r0)2 First characterized by Fried in 1966 What is D/r0 for Keck? Image Spot size = 2.44 l/r0

  6. A broad optical bandwidth smears thespeckles out in a radial fashion Narrowband Broadband (Credit C. Neyman, AMOS)

  7. 1) Amplitude 2) Intensity Details of diffraction from circular aperture First zero at r = 1.22  / D FWHM  / D

  8. Stars at Galactic Center Ghez: Keck laser guide star AO Diffraction pattern from hexagonal Keck telescope

  9. What is a spatial frequency? A sheet with a sinusoidal “wave” which can vary in frequency (wavelength) and orientation (direction) A spatial frequency also has phase: its peaks and valleys have some kind of reference to a known point in the image

  10. How does the atmosphere affect system performance? Telescope OTF Seeing Limited TF Tip-Tilt Compensated TF For D/r0 = 15 Normalized Spatial Frequency

  11. The Basics of AO

  12. How does AO work? AO corrects distorted wavefronts in real time to compensate for blurring effects of the atmosphere

  13. What do AO and flying potato chips have in common?

  14. Intro to Wavefront Sensing

  15. How do we measure wavefronts? Detectors cannot measure the phase of the light, only the intensity.

  16. Intro to Tip-Tilt Correction

  17. Mirror Disturbance Vector Tip-Tilt Mirror Controller Tip-Tilt Sensor Control law Servo Closed-loop Mirror Positioning Controller (CLMP) Rotation (UTT only) Mirror Position Commands (arc-sec) Residual Tip-Tilt Error (arc-sec) S S Telemetry Recorder (TRS) Angular Offset (DT Ctrl Offset) Control law Parameters Loop cmd Mirror Offset Variable Rotation Angle Tip-tilt correction

  18. Closed-Loop Mirror Positioning Controller Atmospheric Tip-Tilt Controller Arc-sec to actuator space conversion PID Servo Digital to Analog Converter High voltage Amplifier Mirror Actuators Mirror Position Commands (arc-sec) High voltage Actuator Signals Bridge Sensors Strain Gauge Outputs Current Mirror Position Conversion Matrix Servo Parameters Closed Loop Mirror Positioning

  19. Intro to Wavefront Reconstruction and Correction

  20. Residual WF error RSS Residual WF Error Tip-tilt error WFS focus error Centroids Subap intensity Actuator vector DM focus Telemetry Recorder (TRS) Raw frames Wave Front Processor (WFP) DM HW IF Deformable Mirror (DM) Wave Front Sensor (WFS) Camera Background Compensation Flat Field Compensation Pixel threshold Centroid Computation Matrix- Vector Multiply Control law Servo WFS HW IF Tip-tilt Controllers (DTT/UTT) Tip-tilt error WFS parameters Background image Flat field Intensity threshold Centroid gain Centroid origin Reconstruction Matrix Control law Parameters Loop command Actuator map DM origin WFC main data flow

  21. How a deformable mirror works (idealization) BEFORE AFTER Deformable Mirror Incoming Wave with Aberration Corrected Wavefront

  22. Deformable Mirror for real wavefronts

  23. Most deformable mirrors today have thin glass face-sheets Glass face-sheet Light Cables leading to mirror’s power supply (where voltage is applied) PZT or PMN actuators: get longer and shorter as voltage is changed Anti-reflection coating

  24. (paper coasters) Front view of Xinetics DM (Keck) 349 degrees of freedom; ~250 in use at any one time

  25. NGS vs LGS AO

  26. NGS AO Control Light from Telescope Telescope pointing offload TTM Tip/tilt NGS IR transmissive dichroic Science Camera DM Offload focus to telescope beamSplitter Wavefront Controller WFS Flux Rot & pupil angle When TT closed NGS Reconstructor Centroid Origins

  27. LGS AO Control Light from Telescope Telescope pointing offload TTM Tip/tilt LGS NGS IR transmissive dichroic Science Camera DM Offload focus to telescope Sodium transmissive dichroic STRAP Wavefront Controller Lenslets WFS LBWFS TSS x,y,z stage Focus Laser Orientation Spot size & flux Rot & pupil angle When DM closed Optimized centroids offsets LGS Reconstructor Tip/tilt to Laser Laser TT mirror Laser pointing offload

  28. Limitations of AO • Isoplanatism • Tip-Tilt Isoplanatism • Focus isoplanatism • Sky coverage • WFS sensitivity • TT sensor sensitivity • Imaging wavelength • Controller bandwidth • Error budgets, and more…

  29. Anisoplanatism: how does AO image degrade as you move farther from guide star? credit: R. Dekany, Caltech • Composite J, H, K band image, 30 second exposure in each band • Field of view is 40”x40” (at 0.04 arc sec/pixel) • On-axis K-band Strehl ~ 40%, falling to 25% at field corner

  30. AO image of sun in visible light: 11 second exposure Fair Seeing Poor high altitude conditions From T. Rimmele

  31. AO image of sun in visible light: 11 second exposure Good seeing Good high altitude conditions From T. Rimmele

  32. Focus Anisoplanatism: The laser doesn’t sample all the turbulence

  33. Additional slides from Claire Max’s UCSC Class

  34. NGWFC Results

  35. Successes: Old vs. new Some of the best images of a 7th magnitude star taken with the old WFC (left) and the NGWFC (right). The images have K-band Strehls of 58% and 66% respectively. Strehl record: 71% at K-band Limiting magnitude: R=16

  36. NGS performance exceeds expectations Requirement was to meet or exceed 30% Strehl for 14th magnitude guide star in good seeing (r0 ≥ 20 cm). 60+% Strehl for R=14 guide star Strehl record: 71% at K-band Limiting magnitude: R=16

  37. LGS performance has improved as well • LGS AO results during especially good seeing. • Best performance increased from 44% to 51% Strehl in K. • Limiting magnitude R=19

  38. Improved performance on Brown Dwarfs J-band image of a brown dwarf binary pair with separation of 80 mas (Michael Liu, 26 March 2007).

  39. Best LGS AO images of the galactic center K-band image of the Galactic Center in LGS AO (left) and NGS AO (right). Credit: Andrea Ghez, Jessica Lu.

  40. Extended Objects • J, H and K’ color composite o Uranus (left). The inset on the top left is an enlarged image of Miranda at K’. • H and K’ color composite of Neptune (middle) • K’ image of Titan (right).

  41. Uranus ring crossing The rings of Uranus as observed with the Keck AO system since 2004. Optically-thick rings like  disappear due to inter-particle shadowing; optically-thin rings like  brighten. Credit: Imke de Pater.

  42. Improved NGS/LGS crossover point • We are now able to use NGS in observing scenarios where we used LGS before and get better performance LGS perf NGS perf

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