Adaptive Optics for Astronomy

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Adaptive Optics for Astronomy

• Kathy Cooksey

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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

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Photons Travel in Straight Lines
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Wavefronts
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Atmospheric Turbulence
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Atmospheric Turbulence
Distant stars should resemble points if it were
not for turbulence in Earths atmosphere
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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
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AO Straighten Wavefront
BEFORE
AFTER
Incoming, distorted wavefront (aberrated)
DEFORMABLE MIRROR
Corrected wavefront
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AO in Action
Lick Observatory adaptive optics system
Star without adaptive optics
Star with adaptive optics
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AO Specifics Correcting for Atmosphere and
Improving Images

• Even the largest ground-based astronomical
  telescopes have no better resolution than an 8
  backyard telescope!

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Basic AO Process
(b) Calculate shape to apply to deformable mirror
to correct blurring
(a) Measure details of blurring from guide star
near object you want to observe
(c) Light from both guide star and astronomical
object is reflected from deformable mirror
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Schematic of AO System
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Gemini AO in Action
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How to Measure Distortion

• Shack-Hartmann Wavefront Sensor

you will see this again
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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 gt
  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
  near-infrared light
• Can see virtually whole sky
• More precise brightness measurements
• Very sensitive spectroscopy for faint objects in
  infrared
• Lower spectral spatial resolution

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Beautiful AO Images
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Satellites for the Small

• Adaptive Optics has opened up study of smaller
  bodies of solar system

Double Asteroid 90 Antiope
Eugenia and its moon
Merine et al. CFHT
Merine et al. Keck
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Neptune at 1.65 microns
With Keck adaptive optics
Without adaptive optics
2.3 arc sec
May 24, 1999
June 27, 1999
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Neptune Movie

• AO allows us to monitor weather on outer planets

Institute for Astronomy (University of Hawaii)
CFHT
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Titan Occults Two Stars

• Occultation is when planet or moon passes in
  front of star

Original
Titan subtracted
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Lightbridges on Sun

• Lightbridges discovered with AO
• Those shown are 5000 km in length
• Golden Gate 2 km
• Believed to be normal solar granulation that
  penetrates strongly magnetic sunspot umbras

Sharmer et al. Swedish Solar Vacuum Telescope
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AO Reveals Faint Companions to Bright Stars
Mike Brown (CalTech)
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Galactic Center
UCLA Galactic Center Group
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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

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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
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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

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More AO Tidbits
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Titans Surface at Keck
With AO At 1.581 µm (surface window)
Without AO Typical at 1.65 µm
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Surface Reflectivity
AO image
Model image of atmosphere
Surface albedo map
Atmospheric properties Haze optical depth,
variation with altitude
Model inputs Haze optical depth Optical
properties of haze particles (varies with
depth) Model outputs Image of atmosphere
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AO Image Sequence of 216 Kleopatra

• Movie of the asteroid Kleopatra, observed during
  seven-hour period with CFHT AO System

Merine et al. CFHT
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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

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eXtreme Adaptive Optics Planet Imager

• XAOPI project (in progress)
• System at Keck observatory
• First images of extra-solar planets