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Introduction to Adaptive OpticsIntroduction to Adaptive Optics
Antonin Bouchez(with lots of help from Claire Max)2004 Observatory Short Course
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OutlineOutline
•• Why do we need adaptive optics?Why do we need adaptive optics?
•• How is it supposed to work?How is it supposed to work?
•• How does it really work?How does it really work?
•• Astronomy with adaptive optics.Astronomy with adaptive optics.
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Why do we need adaptive Why do we need adaptive optics?optics?
Turbulence in earth’s atmosphere makes stars twinkle
More importantly, turbulence spreads out light; makes it a blob rather than a point
Even the largest ground-based astronomical telescopes have no better resolution than an 8" telescope!
Even the largest ground-based astronomical telescopes have no better resolution than an 8" telescope!
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Optical consequences of Optical consequences of turbulenceturbulence
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Optical consequences of Optical consequences of turbulenceturbulence
•• Temperature fluctuations in small patches of air cause changes iTemperature fluctuations in small patches of air cause changes in n index of refraction (like many little lenses)index of refraction (like many little lenses)
•• Light rays are refracted many times (by small amounts) Light rays are refracted many times (by small amounts)
•• When they reach telescope they are no longer parallelWhen they reach telescope they are no longer parallel
•• Hence rays can’t be focused to a point:Hence rays can’t be focused to a point:
} Point focus } blur
Light rays affected by turbulenceParallel light rays
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Kolmogorov Kolmogorov turbulence cartoonturbulence cartoon
Outer scale L0
ground
Inner scale l0
hνconvection
solar
hν
Wind shear
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Turbulence arises in several Turbulence arises in several placesplaces
stratosphere
tropopause
Heat sources w/in dome
boundary layer~ 1 km
10-12 km
wind flow over dome
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Short exposures through the Short exposures through the atmosphereatmosphere
What a star really looks like through a large (6 m) telescope.
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Long exposures through the Long exposures through the atmosphereatmosphere
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Light rays and the Light rays and the wavefrontwavefront
}Point focus } blur
Parallel light rays Light rays affected by turbulence
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Light rays and the Light rays and the wavefrontwavefront
Parallel light rays Light rays affected by turbulence
} blur}Point focus
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Characterize turbulence Characterize turbulence strength by quantity rstrength by quantity r00
Wavefront of light
r0 ““Fried’s Fried’s parameter”parameter”
Primary mirror of telescope
•• “Coherence Length” r“Coherence Length” r0 0 : distance over which optical phase : distance over which optical phase distortion has mean square value of 1 raddistortion has mean square value of 1 rad22
•• rr00 ~ 15 ~ 15 -- 30 cm on Mauna Kea30 cm on Mauna Kea
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Imaging through a telescopeImaging through a telescope
FWHM ~λ/D
in units of λ/D
1.22 λ/D
With no turbulence, FWHM is With no turbulence, FWHM is diffraction limit of telescope, diffraction limit of telescope, ϑϑ~~ λλ / D/ D
Example: Example:
λλ / D = 0.02 arc sec for / D = 0.02 arc sec for λλ = 1 = 1 µµm, D = 10 mm, D = 10 m
Point Spread Function (PSF): Point Spread Function (PSF): intensity profile from point sourceintensity profile from point source
10m 0.02"
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Imaging through a telescopeImaging through a telescope
FWHM ~λ/r0
in units of λ/D
With turbulence, FWHM is With turbulence, FWHM is diffraction limit of regions of diffraction limit of regions of coherent phase, coherent phase, ϑϑ ~~ λλ / r/ r00
Example: Example:
λλ / r/ r00 = 0.80 arc sec for = 0.80 arc sec for λλ = 1 = 1 µµm, rm, r00 = 25 cm= 25 cm
Point Spread Function (PSF): Point Spread Function (PSF): intensity profile from point sourceintensity profile from point source
10m 0.80"
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How does adaptive optics help?How does adaptive optics help?(cartoon approximation)(cartoon approximation)
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
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Simplified AO system diagramSimplified AO system diagram
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How a deformable mirror How a deformable mirror works (idealization)works (idealization)
BEFORE AFTER
Deformable Mirror
Incoming Wave with Aberration
Corrected Wavefront
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Most deformable mirrors Most deformable mirrors today have thin glass facetoday have thin glass face--
sheetssheetsGlass face-sheet
Light Cables leading to mirror’s power supply (where
voltage is applied)
PZT or PMN actuators: get longer and shorter as voltage is changed
Anti-reflection coating
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Simplified AO system diagramSimplified AO system diagram
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How to measure turbulent distortions How to measure turbulent distortions (one method among many)(one method among many)
Shack-Hartmann wavefront sensor
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Simplified AO system diagramSimplified AO system diagram
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A real AO system: Keck 1 & 2A real AO system: Keck 1 & 2
Tip-tilt mirror
Deformable mirrorWavefront sensor
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Keck II Left Keck II Left Nasmyth Nasmyth PlatformPlatform
ElevationRing
Enclosure withroof removed
ElectronicsRacks
Adaptive Optics Bench
NasmythPlatform
Science cameras
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The adaptive optics benchThe adaptive optics benchIR Dichroic
Tip/tiltMirror
DeformableMirror
To wavefrontsensor
To near-infraredcamera
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Deformable mirrorDeformable mirrorFront view Rear view
15cm
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WavefrontWavefront SensorSensor
AOA Camera
Sodium dichroic/beamsplitterField Steering Mirrors (2 gimbals)
Wavefront Sensor Optics: field stop, pupil relay, lenslet, reducer optics
Camera Focus
Wavefront Sensor Focus
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6 AO systems on Mauna Kea!6 AO systems on Mauna Kea!
Summit of Mauna Kea volcano in Hawaii:
Subaru
2 Kecks
Gemini North
CFHT
UH 88"
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An adaptive optics system An adaptive optics system in action: Keck 2in action: Keck 2
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Neptune in infraNeptune in infra--red lightred light(1.65 microns)(1.65 microns)
With Keck adaptive opticsWithout adaptive optics
2.3
arc
sec
May 24, 1999 June 27, 1999
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Adaptive optics in astronomyAdaptive optics in astronomy
•• Planetary sciencePlanetary science–– Volcanoes on IoVolcanoes on Io–– Methane storms on TitanMethane storms on Titan
•• Dense star fieldsDense star fields–– Black hole at the center of our galaxyBlack hole at the center of our galaxy–– Star population in globular clustersStar population in globular clusters
•• HighHigh--contrast imagingcontrast imaging–– Faint material around bright stars (disks of dust, etc.)Faint material around bright stars (disks of dust, etc.)–– ExtraExtra--solar planets and supersolar planets and super--planetsplanets
•• Studying very distant galaxies.Studying very distant galaxies.
Occultation of a binary star by TitanOccultation of a binary star by Titan
Hubble Space Telescope image
Occultation of a binary star by TitanOccultation of a binary star by Titan
• Images taken with the Palomar Observatory 200" AO system.• One image taken every 0.843 seconds - 4700 images total.• Titan's atmosphere refracts the starlight, forming multiple images of each star!• Result: winds in Titan's stratosphere are very strong: ~250 m/sin a jet-stream type pattern.
Orbits of stars around the black hole at the Orbits of stars around the black hole at the center of our galaxycenter of our galaxy
Result: Black hole at center of the galaxy has a mass of 2.6 million suns.
MidMid--infrared flares from the black hole at infrared flares from the black hole at the center of our galaxythe center of our galaxy
Result: Direct detection of heat released by material falling on the accretion disk surrounding the black hole.
Searching for planets around Epsilon Searching for planets around Epsilon EridaniEridani
Searching for planets around Epsilon Searching for planets around Epsilon EridaniEridani
Result: They're all background stars!
Studying the star populations ofStudying the star populations ofgalaxies at z~0.5galaxies at z~0.5
Result: Galaxies were brighter than today, but about the same size.
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Sodium laser Sodium laser guidestarsguidestars
Light from Na layer at ~ 100 km
Rayleigh scattered light
Max. altitude of Rayleigh ~ 35 km
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Keck 2 laserKeck 2 laser
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First images with KeckFirst images with Kecklaserlaser--guidestar guidestar adaptive opticsadaptive optics
HK Tau B hidden behind an edge-on disk of dust.
HK Tau A (106 yr old T-tauri star)
