Adaptive Optics and its Applications Lecture 1

1. Adaptive Optics and its ApplicationsLecture 1 Neptune Claire Max UC Santa Cruz January 5, 2006

2. Outline of lecture • Introductions, goals of this course • How the course will work • Homework for next week • Overview of adaptive optics for astronomy Please remind me to stop for a break at 2:45 pm!

3. Introductions: who are we? • Via video: • AEOS, Maui: Ben Wheeler • Indiana U. Optometry School: Weihua Gao, Yan Zhang • JPL: Ian Crossfield • Keck Observatory: Eric Johansson, Roger Sumner • UCLA: Tuan Do, Jessica Lu, Jon Mauerhon, Emily Rice, Shelley Wright • UC Irvine: Lianqi Wang • UCSC Mt. Hamilton: Bryant Grigsby • In the CfAO conference room at UCSC

4. Videoconference techniques • Please identify yourself when you speak • “This is Mary Smith from Santa Cruz” • Report technical problems to the UCOP contact person (see your email) • Microphones are quite sensitive • Do not to rustle papers in front of them • Mute your microphone if you are making side-comments, sneezes, eating lunch, whatever

5. Goals of this course • To understand the main concepts behind adaptive optics systems • To understand how to do astronomical observations with AO • Planning, reducing, and interpreting data (your own data, but just as importantly other people’s data) • Some of this will apply to AO for vision science as well • Get acquainted with AO components in the Lab. • Delve into engineering details if you are interested. • Brief introduction to non-astronomical applications • I hope to interest a few of you in learning more AO, and doing research in the field

6. How the course will work • Website: http://www.ucolick.org/~max/289C • Lectures will be on web after each class • (Hopefully before class) • Textbooks • Course components • Homework

7. Required Textbooks • Reader containing key articles and excerpts for this class. Available at Slug Books Coop, next to 7-11 Store. http://www.slugbooks.com/ • We can arrange to buy copies on behalf of folks in video land • PDF versions will be available on restricted website • Field Guide to Adaptive Optics by Robert K. Tyson and Benjamin W. Frazier, SPIE Press. Available from Bay Tree Bookstore.

8. Course components • Lectures • Reading assignments • Homework problems • Student group projects (presentations in class) • Laboratory exercises • Field trip to Lick Observatory • Web discussions (perhaps) • Final exam

9. Next Week • Next Tuesday I will be at the American Astronomical Society meeting in Washington DC • Instead of regular lecture class, there will be a tour of the Laboratory for Adaptive Optics • Meet here in CfAO Conference Room at 2 pm • Next regular class is Thursday January 12th

10. Homework for Thursday January 12 • Read Syllabus carefully (on web) • Do Homework # 1: “Tell me about yourself” • Reading: in Reader • Chapter 1 of Roggeman (pages 59-66) • Excerpt from Hardy’s Chapter 2 (pages 5-15) • Don’t sweat the details -- goal is to get a broad overview on where adaptive optics came from

11. Why is adaptive optics needed? Turbulence in earth’s atmosphere makes stars twinkle More importantly, turbulence spreads out light; makes it a blob rather than a point Even the largest ground-based astronomical telescopes have no better resolution than an 8" telescope!

12. Images of a bright star, Arcturus Speckles (each is at diffraction limit of telescope) Lick Observatory, 1 m telescope  ~ 1 arc sec  ~ l / D Long exposure image Short exposure image Image with adaptive optics

13. Turbulence changes rapidly with time Image is spread out into speckles Centroid jumps around (image motion) “Speckle images”: sequence of short snapshots of a star, taken at Lick Observatory using the IRCAL infra-red camera

14. Turbulence arises in several places tropopause 10-12 km wind flow over dome boundary layer ~ 1 km Heat sources w/in dome stratosphere

15. Atmospheric perturbations cause distorted wavefronts Rays not parallel Index of refraction variations Plane Wave Distorted Wavefront

16. Optical consequences of turbulence  blur • Temperature fluctuations in small patches of air cause changes in index of refraction (like many little lenses) • Light rays are refracted many times (by small amounts) • When they reach telescope they are no longer parallel • Hence rays can’t be focused to a point:  Point focus Light rays affected by turbulence Parallel light rays

17. Imaging through a perfect telescope With no turbulence, FWHM is diffraction limit of telescope,  ~l / D Example: l / D = 0.02 arc sec for l = 1 mm, D = 10 m With turbulence, image size gets much larger (typically 0.5 - 2 arc sec) FWHM ~l/D 1.22 l/D in units of l/D Point Spread Function (PSF): intensity profile from point source

18. Characterize turbulence strength by quantity r0 Wavefront of light r0 “Fried’s parameter” • “Coherence Length” r0 : distance over which optical phase distortion has mean square value of 1 rad2 (r0 ~ 15 - 30 cm at good observing sites) • Easy to remember: r0 = 10cm  FWHM = 1” at l = 0.5m Primary mirror of telescope

19. Effect of turbulence on image size • If telescope diameter D >> r0 , image size of a point source is l / r0 >> l / D • r0 is diameter of the circular pupil for which the diffraction limited image and the seeing limited image have the same angular resolution. • r0  10 inches at a good site. So any telescope larger than this has no better spatial resolution! l / D “seeing disk” l / r0

20. How does adaptive optics help?(cartoon approximation) Measure details of blurring from “guide star” near the object you want to observe Calculate (on a computer) the shape to apply to deformable mirror to correct blurring Light from both guide star and astronomical object is reflected from deformable mirror; distortions are removed

21. Infra-red images of a star, from Lick Observatory adaptive optics system With adaptive optics No adaptive optics Note: “colors” (blue, red, yellow, white) indicate increasing intensity

22. When AO system performs well, more energy in core When AO system is stressed (poor seeing), halo contains larger fraction of energy (diameter ~ r0) Ratio between core and halo varies during night AO produces point spread functions with a “core” and “halo” Definition of “Strehl”: Ratio of peak intensity to that of “perfect” optical system Intensity x

23. Adaptive optics increases peak intensity of a point source Lick Observatory No AO With AO Intensity With AO No AO

24. Schematic of adaptive optics system Feedback loop: next cycle corrects the (small) errors of the last cycle

25. How to measure turbulent distortions (one method among many) Shack-Hartmann wavefront sensor

26. Shack-Hartmann wavefront sensor measures local “tilt” of wavefront • Divide pupil into subapertures of size ~ r0 • Number of subapertures  (D / r0)2 • Lenslet in each subaperture focuses incoming light to a spot on the wavefront sensor’s CCD detector • Deviation of spot position from a perfectly square grid measures shape of incoming wavefront • Wavefront reconstructor computer uses positions of spots to calculate voltages to send to deformable mirror

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

28. Real deformable mirrors have continuous surfaces • In practice, a small deformable mirror with a thin bendable face sheet is used • Placed after the main telescope mirror

29. Deformable Mirror for real wavefronts

30. 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

31. Deformable mirrors come in many sizes • Range from 13 to > 900 actuators (degrees of freedom) About 12” A couple of inches Xinetics

32. New developments: tiny deformable mirrors • Potential for less cost per degree of freedom • Liquid crystal devices • Voltage applied to back of each pixel changes index of refraction locally • MEMS devices (micro-electro-mechanical systems)

33. If there’s no close-by “real” star, create one with a laser • Use a laser beam to create artificial “star” at altitude of 100 km in atmosphere

34. Laser is operating at Lick Observatory, being commissioned at Keck Keck Observatory Lick Observatory

35. Galactic Center with Keck laser guide star Keck laser guide star AO Best natural guide star AO

36. Adaptive Optics World Tour

37. Adaptive Optics World Tour (2nd try)

38. Steady growth in AO astronomy publications since 1995

39. Citations for AO papers are equal to astrophysics average

40. Astronomical observatories with AO on 3-5 m telescopes • ESO 3.6 m telescope, Chile • Canada France Hawaii • William Herschel Telescope, Canary Islands • Mt. Wilson, CA • Lick Observatory, CA • Mt. Palomar, CA • Calar Alto, Spain

41. Adaptive optics system is usually behind main telescope mirror • Example: AO system at Lick Observatory’s 3 m telescope Support for main telescope mirror Adaptive optics package below main mirror

42. Lick adaptive optics system at 3m Shane Telescope DM Off-axis parabola mirror IRCAL infra-red camera Wavefront sensor

45. Palomar adaptive optics system AO system is in Cassegrain cage 200” Hale telescope

46. Adaptive optics makes it possible to find faint companions around bright stars Two images from Palomar of a brown dwarf companion to GL 105 200” telescope Credit: David Golimowski

47. The new generation: adaptive optics on 8-10 m telescopes Summit of Mauna Kea volcano in Hawaii: Subaru 2 Kecks Gemini North And at other places: MMT, VLT, LBT, Gemini South

48. The Keck Telescope Adaptive optics lives here

49. Keck Telescope’s primary mirror consists of 36 hexagonal segments Person! Nasmyth platform

50. Neptune in infra-red light (1.65 microns) With Keck adaptive optics Without adaptive optics 2.3 arc sec May 24, 1999 June 27, 1999