1. “Twinkle, Twinkle Little Star”: An Introduction to Adaptive Optics
Mt. Hamilton Visitor’s Night
July 28, 2001

2. Turbulence in the atmosphere limits the performance of astronomical telescopes
Turbulence is the reason why stars twinkle
More important for astronomy, turbulence spreads out the light from a star; makes it a blob rather than a point
Even the largest ground-based astronomical telescopes
have no better resolution than an 8" backyard telescope!

3. Images of a bright star, Arcturus
Lick Observatory, 1 m telescope
Long exposure
image
Short exposure
“Perfect” image: diffraction limit of telescope
Distant stars should resemble “points,”
if it weren’t for turbulence in Earth’s atmosphere

4. Turbulence changes rapidly with time
Sequence of very short snapshots of a star.
Movie is much slower than "real time."

5. How to correct for atmospheric blurring
Light from both guide star and astronomical object is reflected from deformable mirror; distortions are removed
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

6. Basic idea of AO Wavefront corrector Aberrated wavefront Corrected
sensor
Wavefront
control
computer

7. Adaptive optics in action
Lick Observatory adaptive optics system
Star without adaptive optics
Star with adaptive optics

9. The Deformable Mirror
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.

10. Deformable mirrors come in many shapes and sizes
Today: mirrors from Xinetics. From 13 to 900 actuators (degrees of freedom); inches in diameter.
Xinetics Inc.Devens, MA
Future: very small mirrors (MEMS, LCDs); very large
mirrors (replace secondary mirror of the telescope)

11. 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 under main mirror

12. What does a “real” adaptive optics system look like?
Light from telescope
Wavefront sensor
Deformable mirror
Infra-red camera

13. If there is no nearby star, make your own “star” using a laser
Implementation
Concept
Lick Obs.

14. Laser in 120-inch dome

15. Laser guide star adaptive optics at Lick Observatory
Uncorrected image of a star
Laser Guide Star correction of a star: Strehl = 0.6
Ircal1129.fits RX J /20/00 2:04 Ks V=15 K=~ s S=0.6 LGS

16. AO at the Keck 10 m Telescope
Adaptive optics
lives here

17. 9th magnitude star imaged in infrared light (1.6 mm)
Adaptive optics on 10-m Keck II Telescope: Factor of 10 increase in spatial resolution
9th magnitude star imaged in infrared light (1.6 mm)
Without AO
Without AO
width = 0.34 arc sec
With AO
width = arc sec

20. Neptune in Infrared Light
Without adaptive optics
With Keck adaptive optics
2.3 arc sec
May 24, 1999
June 27, 1999
l = 1.65 microns

21. Neptune: Ground-based AO vs. Voyager Spacecraft
Infrared: Keck adaptive optics, 2000
Visible: Voyager 2 fly-by, 1989
Circumferential bands
Compact southern features

22. Saturn’s moon Titan: Shrouded by haze as seen by Hubble Space Telescope
Image at 0.85 microns
Hints of surface detail
Limb
Brightening due to haze

23. Titan at Keck: with and without adaptive optics
Titan without adaptive optics
Titan with adaptive optics
wavelength 1.65 mm
February 26-27, 1999

24. Uranus as seen by Hubble Space Telescope and Keck AO
false-color image
(1.1, 1.6, 1.9 mm)
Keck adaptive optics image (2.1 mm)

25. Keck AO Can See the Faintest Rings Discovered by Voyager
Voyager: 4 groups of rings
Keck AO: outer e ring plus inner groups (individual rings unresolved) e
d g 
b a
4 5 6

26. A volcano erupting on Io:
Jupiter's largest moon
Infrared image (2 microns)
Volcano erupting on limb
1 arc sec

27. Io with adaptive optics sees most of the
volcanic features seen by Galileo
Keck AO: three IR "colors"
Galileo: visible CCD camera
Same volcanoes
Same volcanoes

28. Other Uses for AO High-speed communications with laser beams
Cheaper and lighter telescopes in space
High-powered lasers for fusion power
Vision science research

29. Why Correct the Eye’s Optics?
Perfect Eye
Aberrated Eye
So, why would want to improve the eye’s optics?
Two reasons
1: To improve human vision.
2. To improve images of the retina.
Both possibilities will give us a better understanding of the visual system but will have clinical and real-world applications as well.

30. psfs for changing pupil size
Visual Acuity Is Worse at Night When Pupils Dilate
1 mm
2 mm
3 mm
4 mm
pupil images
followed by
psfs for changing pupil size
5 mm
6 mm
7 mm
This slide illustrates both diffraction and monochromatic aberrations in the eye. Each image shows the calculated point spread function for a range of pupil sizes in the same eye. I’ll explain how we compute these patterns later.
The eye has fairly good central optics. For small pupils, aberrations are low. So, you might think that the problem is solved by going to a small pupil. But the problem is that smaller pupils give more diffraction. So for a small pupil, the point spread function is very round, but it is also quite large.
Diffraction theory tells us that larger pupils should give us smaller point spread functions but near the margins, aberrations start to dominate so that we can not take advantage of the higher numerical aperture offered by a larger pupil.
In this image, the optimal pupil size lies somewhere between 2 and 4 mm, depending on your criteria.
If you can correct for the aberrations, then the best image quality is through the largest possible pupil.

31. The Rochester Adaptive Optics Ophthalmoscope
This is a typical AO telescope that uses the star being imaged as the source for wavefront sensing.

32. Adaptive optics provides a clear improvement in retinal image quality
Wave Aberration
Point Spread
Function
Retinal Image
at 550nm
Retinal Image in
White Light
Before adaptive optics:
After adaptive optics:
1 deg YY
6.8 mm pupil

33. Williams, Roorda et al. (U Rochester) Resolve individual cones
Adaptive optics provides highest resolution images of living human retina
Williams, Roorda et al. (U Rochester)
With AO:
Resolve individual cones
Without AO

34. Looking Inside the Eye with AO

35. View of Lunar Eclipse

36. Retinal Imaging – Basic Science
Scale bar = 5 µm
First images of the trichromatic photoreceptor mosaic in the human eye
(Roorda and Williams, Nature, 1999)

38. Primary Mirrors: CELT vs. Keck

39. CELT and Stonehenge
Keck

40. CELT in PacBell Park

41. Keep your eye on Adaptive Optics!

42. How to measure turbulent distortions (one method among many)

43. Applications and Results