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

2. Please remind me to stop for a break at 2:45 pm!
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 )
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:
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.
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
Lick Observatory, 1 m telescope
 ~ 1 arc sec
 ~ l / D
Speckles (each is at diffraction limit of telescope)
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
stratosphere
tropopause
10-12 km
wind flow over dome
boundary layer
~ 1 km
Heat sources w/in dome

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

16. Optical consequences of turbulence
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 
blur
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 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”
Primary mirror of telescope
“Coherence Length” r0 : distance over which optical phase distortion has mean square value of 1 rad (r0 ~ cm at good observing sites)
Easy to remember: r0 = 10cm  FWHM = 1” at l = 0.5m

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
No adaptive optics
With adaptive optics
Note: “colors” (blue, red, yellow, white) indicate increasing intensity

22. 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
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

23. Adaptive optics increases peak intensity of a point source
Lick Observatory
No AO
With AO
Intensity
No AO
With 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
Incoming Wave with Aberration
Deformable Mirror
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
So now that we know the wavefront, or aberrations of the eye we are measuring we can shape a mirror appropriately so that upon reflection, the wavefront is rendered parallel. The appropriate shape of the mirror to compensate for each aberration is simply the same shape as the wavefront with half the amplitude.
Recall that the aberrations of each eye are different so each eye requires a unique correction.

30. Most deformable mirrors today have thin glass face-sheets
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 TelescopeDM
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
Nasmyth
platform
Person!

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

51. (Two different dates and times)
Neptune at 1.6 m: Keck AO exceeds resolution of Hubble Space Telescope
HST - NICMOS Keck AO
~2 arc sec
2.4 meter telescope
10 meter telescope
(Two different dates and times)

52. Uranus with Hubble Space Telescope and Keck AO
L. Sromovsky
Keck AO, IR
HST, Visible
Lesson: Keck in near IR has same resolution as Hubble in visible

53. Uranus with Hubble Space Telescope and Keck AO
de Pater
HST, Visible
Keck AO, IR
Lesson: Keck in near IR has same resolution as Hubble in visible

54. European Southern Observatory: 4 8-m Telescopes in Chile

55. Hubble Space Telescope WFPC2,  = 800 nm
VLT NAOS AO first light
Cluster NGC 3603: IR AO on 8m ground-based telescope achieves same resolution as HST at 1/3 the wavelength
Hubble Space Telescope WFPC2,  = 800 nm
NAOS AO on VLT
 = 2.3 microns

56. Some frontiers of adaptive optics
Current systems (natural and laser guide stars):
How can we measure the Point Spread Function while we observe?
How accurate can we make our photometry? astrometry?
What methods will allow us to do high-precision spectroscopy with AO?
Future systems:
Can we push new AO systems to achieve very high contrast ratios, to detect planets around nearby stars?
How can we achieve a wider AO field of view?
How can we do AO for visible light (replace Hubble on the ground)?
How can we do laser guide star AO on future 30-m telescopes?

57. Frontiers in AO technology
New kinds of deformable mirrors with > degree of freedom
Wavefront sensors that can deal with this many degrees of freedom
Innovative control algorithms
“Tomographic wavefront reconstuction” using multiple laser guide stars
New approaches to doing visible-light AO

58. Adaptive optics at UCSC
Center for Adaptive Optics
This building is headquarters
NSF Science and Technology Center
AO for astronomy and for looking into the living human eye
11 other universities are members, as well as national labs and observatories
Laboratory for Adaptive Optics
Funded by the Gordon and Betty Moore Foundation
Two labs in Thimann
Experiments on “Extreme AO” to search for planets, and on AO for Extremely Large Telescopes

59. Enjoy!