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 exposureimage

Short exposureimage

“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 AFTER

Incoming, distorted

wavefront (“aberrated”)

DEFORMABLE MIRROR

Corrected wavefront

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AO in ActionLick Observatory adaptive optics system

Star without adaptive optics Star with adaptive optics

AO Specifics: Correcting for Atmosphere and Improving

ImagesEven 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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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 near-infrared 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 Eugenia and its moon

Merine et al. CFHTMerine et al. Keck

Neptune at 1.65 micronsWithout adaptive optics With Keck adaptive optics

June 27, 1999

2.3

arc

sec

May 24, 1999

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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Sharmer et al. Swedish Solar Vacuum Telescope

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

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 Typical at 1.65 μm

With AOAt 1.581 μm (surface window)

Surface Reflectivity

Model inputs:Haze optical depthOptical properties of haze particles (varies with depth)Model outputs: Image of atmosphere

Atmospheric properties:

Haze optical depth, variation with altitude

AO image

Surface albedo map

Model image of atmosphere

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

eXtreme Adaptive Optics Planet Imager

• XAOPI project (in progress)

• System at Keck observatory

• First images of extra-solar planets