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Adaptive Optics for Astronomy
Kathy Cooksey
AO Basics
• Photons
– Travel in straight lines
• Wavefront
– Line perpendicular to all photons’ paths
• Atmospheric turbulence
– Due to temperature differences
– Acts like many lenses
– Distorts wavefront
• AO System
– Corrects wavefront
– Makes it linear
Photons Travel in
Straight Lines
Wavefronts
Atmospheric Turbulence
Atmospheric Turbulence
Lick Observatory, 1 m telescope
Long exposure
image
Short exposure
image
“Perfect” image:
diffraction limit of
telescope
Distant stars should resemble “points” if it
were not for turbulence in Earth’s atmosphere
Speckle Images
• Turbulence changes
rapidly with time
• Sequence of short
snapshots of star
• Much slower than real
time
Applied Optics Group (Imperial College), Herschel 4.2-m Telescope
AO Straighten Wavefront
BEFORE
Incoming,
distorted
wavefront
(“aberrated”)
DEFORMABLE
MIRROR
AFTER
Corrected
wavefront
AO in Action
Lick Observatory adaptive optics system
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None decompressor
are needed to see this picture.
Star without adaptive optics
Star with adaptive optics
AO Specifics: Correcting for
Atmosphere and Improving
Images
Even the largest ground-based astronomical
telescopes have no better resolution than an 8”
backyard telescope!
Basic AO Process
(a) Measure details of
blurring from “guide
star” near object you
want to observe
(b) Calculate shape to
apply to deformable
mirror to correct
blurring
(c) Light from both
guide star and
astronomical object is
reflected from
deformable mirror
Schematic of AO System
Gemini: AO in “Action”
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Sorenson Video decompressor
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How to Measure Distortion
Shack-Hartmann Wavefront Sensor
…you will see this again
Ground-based AO Complements
Space Telescopes
Advantages of AO on 8-10 m
ground-based telescopes
• Four times better spatial
resolution in infrared
• Better faint-object sensitivity
at wavelengths > 2 microns
• Outstanding infrared
spectroscopy
– Higher spectral & spatial
resolution
Advantages of 2.4 m Hubble
Space Telescope
• Full wavelength coverage,
from UV to visible to nearinfrared light
• Can “see” virtually whole sky
• More precise brightness
measurements
• Very sensitive spectroscopy
for faint objects in infrared
– Lower spectral & spatial
resolution
Beautiful AO Images
Satellites for the Small
• Adaptive Optics has opened up study of
smaller bodies of solar system
Double Asteroid 90 Antiope
Merine et al. Keck
Eugenia and its moon
Merine et al. CFHT
Neptune at 1.65 microns
With Keck adaptive optics
2.3 arc sec
Without adaptive optics
May 24, 1999
June 27, 1999
Neptune Movie
• AO allows us to
monitor weather on
outer planets
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Institute for Astronomy (University of Hawaii) CFHT
Titan Occults Two Stars
• Occultation is when planet or moon passes
in front of star
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Original
Titan “subtracted”
Lightbridges on Sun
• Lightbridges discovered
with AO
• Those shown are ~5000
km in length
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Sharmer et al. Swedish Solar Vacuum Telescope
– Golden Gate ~2 km
• Believed to be “normal”
solar granulation that
penetrates strongly
magnetic sunspot umbras
AO Reveals Faint Companions to
Bright Stars
Mike Brown (CalTech)
Galactic Center
UCLA Galactic Center Group
Evidence for Black Hole at
Center of Milky Way
• Black hole is revealed
by presence of fast
moving stars at small
radii
• Stellar orbits in central
parsec, 1995-2006
NGC 6934 from Gemini North
• Adaptive Optics allows us to discern
separate stars in crowded cores of globular
clusters
Gemini Obs., NSF, & U. Hawaii IfA
Summary of Astronomical AO
• Remove effect of atmospheric turbulence
– “Twinkle” of stars
• Must “sense” blurring of star
– Either real or laser “star”
• Computers calculate how to correct light
– Send this signal to deformable mirror
• Resulting performance can equal or exceed
Hubble Space Telescope in some areas
• Astronomers use AO to study asteroids, moons,
planets, stars, and galaxies
More AO Tidbits
Titan’s Surface at Keck
Without AO
With AO
Typical at 1.65 μm
At 1.581 μm (surface window)
Surface Reflectivity
AO image
Model image of atmosphere
Surface albedo map
Model inputs:
Haze optical depth
Optical properties of haze
particles (varies with depth)
Model outputs:
Image of atmosphere
Atmospheric
properties:
Haze optical
depth, variation
with altitude
AO Image Sequence of 216
Kleopatra
• Movie of the asteroid Kleopatra, observed
during seven-hour period with CFHT AO
System
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None decompressor
are needed to see this picture.
Merine et al. CFHT
Extra-Solar Planetary System
Science with AO
• Dust disks as signatures of planetary systems
• Close-up views of forming planetary systems
• Detection and characterization of planets
eXtreme Adaptive Optics Planet
Imager
• XAOPI project (in
progress)
• System at Keck
observatory
• First images of extrasolar planets