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“Twinkle, Twinkle Little Star”:An Introduction to Adaptive Optics
Mt. Hamilton Visitor’s Night
July 28, 2001
Turbulence in the atmosphere limits the performance of astronomical telescopes
Even the largest ground-based astronomical telescopes have no better resolution than an 8" backyard telescope!Even the largest ground-based astronomical telescopes have no better resolution than an 8" backyard telescope!
• 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
Distant stars should resemble “points,”
if it weren’t for turbulence in Earth’s atmosphere
Images of a bright star, Arcturus
Lick Observatory, 1 m telescope
Long exposureimage
Short exposureimage
“Perfect” image: diffraction limit
of telescope
Turbulence changes rapidly with time
Sequence of very short snapshots of a star.Movie is much slower than "real time."
Sequence of very short snapshots of a star.Movie is much slower than "real time."
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
How to correct for atmospheric blurring
Basic idea of AO
Aberratedwavefront
Wavefront
sensor
Wavefront control
computer
Corrected
wavefront
Wavefrontcorrector
Adaptive optics in action
Star with adaptive opticsStar without adaptive optics
Lick Observatory adaptive optics system
The Deformable MirrorThe Deformable Mirror
Deformable mirrors come in many shapes and sizes
• Today: mirrors from Xinetics. From 13 to 900 actuators (degrees of freedom); 3 - 15 inches in diameter.
• Future: very small mirrors (MEMS, LCDs); very large mirrors (replace secondary mirror of the telescope)
Xinetics Inc.Devens, MA
Adaptive optics system is usually behind main telescope mirror
• Example: AO system at Lick Observatory’s 3 m telescope
Support for maintelescope mirror
Adaptive optics package under main mirror
What does a “real” adaptive optics system look like?
Wavefront sensor
Infra-red camera
Deformable mirror
Light from telescope
If there is no nearby star, make your own “star” using a laser
ConceptImplementation
Lick Obs.
Laser in 120-inch dome
Laser guide star adaptive optics at Lick Observatory
Laser Guide Star correction of a star: Strehl = 0.6
Uncorrected image of a star
Ircal1129.fits RX J0258.3+1947 10/20/00 2:04 Ks V=15 K=~13.32 20s S=0.6 LGS
AO at the Keck 10 m Telescope
Adaptive optics lives here
Adaptive optics on 10-m Keck II Telescope: Factor of 10 increase in spatial resolution
Without AOwidth = 0.34 arc sec
Without AO
With AOwidth = 0.039 arc sec
9th magnitude star imaged in infrared light (1.6 m)
Neptune in Infrared Light
Without adaptive optics With Keck adaptive optics
June 27, 1999
2.3
arc
sec
May 24, 1999 = 1.65 microns
Neptune:Ground-based AO vs. Voyager Spacecraft
Infrared: Keck adaptive optics, 2000 Visible: Voyager 2 fly-by, 1989
Circumferential bands
Compact southern features
Saturn’s moon Titan: Shrouded by hazeas seen by Hubble Space Telescope
Limb Brightening due to haze
Hints of surface detail
Image at 0.85 microns
Titan at Keck: with and without adaptive optics
Titan with adaptive optics Titan without adaptive optics
Typical @ wavelength 1.65 m February 26-27, 1999
Uranus as seen by Hubble Space Telescope and Keck AO
Hubble Space Telescopefalse-color image (1.1, 1.6, 1.9 m)
Keck adaptive optics image (2.1 m)
Keck AO Can See the Faintest RingsDiscovered by Voyager
Voyager: 4 groups of rings
4 5 6
Keck AO: outer ring plus 3 inner groups
(individual rings unresolved)
Infrared image (2 microns)
1 a
rc se
c
A volcano erupting on Io: Jupiter's largest moon
Volcano erupting on limb
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
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
Perfect Eye
Aberrated Eye
Why Correct the Eye’s Why Correct the Eye’s Optics?Optics?
pupil images
followed by
psfs for changing pupil size
Visual Acuity Is Worse at Night When Pupils DilateVisual Acuity Is Worse at Night When Pupils Dilate
1 mm 2 mm 3 mm 4 mm
5 mm 6 mm 7 mm
The Rochester Adaptive Optics OphthalmoscopeThe Rochester Adaptive Optics Ophthalmoscope
Adaptive optics provides a clear improvement in retinal image quality
Wave Aberration Point Spread Function
Retinal Image at 550nm
Retinal Image inWhite Light
6.8 mm pupil
Before adaptive optics:
After adaptive optics: 1 deg
YY
Adaptive optics provides highest resolution images of living human retina
Without AOWith AO:
Resolve individual cones
Williams, Roorda et al. (U Rochester)
JW right eye1 deg field
1 deg eccentricity
PhotoreceptorsPhotoreceptorsCapillariesCapillaries
10 arcmin(48.6 )m
Looking Inside the Eye with AO
View of Lunar EclipseView of Lunar Eclipse
Retinal Imaging – Basic Science
First images of the trichromatic photoreceptor mosaic in the human eye(Roorda and Williams, Nature, 1999)
Scale bar = 5 µmScale bar = 5 µm
5 arc min
human (JW)human (JW) human (AN)human (AN) macaquemacaque
L 75.8%M 20%S 4.2%
L/M = 3.79
L 50.6%M 44.2%S 5.2%
L/M = 1.14
L 53.4%M 38%S 8.6%
L/M = 1.40
Primary Mirrors: CELT vs. Keck
CELT Keck
CELT and Stonehenge
Keck
CELT in PacBell Park
How to measure turbulent distortions(one method among many)
Applications and ResultsApplications and Results