A Short Introduction to Adaptive Optics Presentation 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 Light from distant star Telescope aperture Focal Plane Image Spot size = 2.44 /D Without atmosphere, the telescope forms a perfect “diffraction-limited” spot in the focal plane

5 Light from distant star Telescope aperture Focal Plane Image Atmosphere (lens size = r 0 ) Spot size = 2.44 /r 0 Freeze the speckles by using short exposures < ~0.1 sec r 0 is characteristic size of the atmosphere Number of speckles ~ (D/r 0 ) 2 First characterized by Fried in 1966 What is D/r 0 for Keck? The atmosphere acts like many lenses of size r0 to create moving “speckles” in the image

6 NarrowbandBroadband (Credit C. Neyman, AMOS) A broad optical bandwidth smears the speckles out in a radial fashion

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

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

10 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 What is a spatial frequency?

11 Telescope OTF Seeing Limited TF Tip-Tilt Compensated TF For D/r 0 = 15 How does the atmosphere affect system performance? Normalized Spatial Frequency

The Basics of AO

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

14 What do AO and flying potato chips have in common?

Intro to Wavefront Sensing

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

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Intro to Tip-Tilt Correction

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

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

Intro to Wavefront Reconstruction and Correction

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

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

26 Deformable Mirror for real wavefronts

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

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

NGS vs LGS AO

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

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

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

37 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 credit: R. Dekany, Caltech Anisoplanatism: how does AO image degrade as you move farther from guide star?

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

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

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

Additional slides from Claire Max’s UCSC Class

NGWFC Results

43 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

44 NGS performance exceeds expectations Requirement was to meet or exceed 30% Strehl for 14 th magnitude guide star in good seeing (r 0 ≥ 20 cm). 60+% Strehl for R=14 guide star Strehl record: 71% at K-band Limiting magnitude: R=16

45 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

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

47 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.

48 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).

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

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