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Introduction to Adaptive Optics. Antonin Bouchez (with lots of help from Claire Max) 2004 Observatory Short Course. Outline. Why do we need adaptive optics? How is it supposed to work? How does it really work? Astronomy with adaptive optics. Why do we need adaptive optics?. - PowerPoint PPT Presentation
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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?
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!
Turbulence in earth’s atmosphere makes stars twinkle
More importantly, turbulence spreads out light; makes it a blob rather than a point
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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 in Temperature fluctuations in small patches of air cause changes in 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:
Parallel light rays Light rays affected by turbulence
blur Point focus
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Kolmogorov turbulence cartoonKolmogorov turbulence cartoon
Outer scale L0
ground
Inner scale l0
hconvection
solar
h
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 wavefrontLight rays and the wavefront
Parallel light rays Light rays affected by turbulence
blurPoint focus
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Light rays and the wavefrontLight rays and the wavefront
Parallel light rays Light rays affected by turbulence
blurPoint focus
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Characterize turbulence Characterize turbulence strength by quantity rstrength by quantity r00
• “ “Coherence Length” rCoherence 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 - 30 cm on Mauna Kea ~ 15 - 30 cm on Mauna Kea
Primary mirror of telescope
r0 ““Fried’s parameter”Fried’s parameter”
Wavefront of light
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Imaging through a telescopeImaging through a telescope
With no turbulence, FWHM is With no turbulence, FWHM is diffraction limit of telescope, diffraction limit of telescope, ~~ / D / D
Example: Example:
/ D = 0.02 arc sec for / D = 0.02 arc sec for
= 1 = 1 m, D = 10 mm, D = 10 m
FWHM ~/D
in units of /D
1.22 /D
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
With turbulence, FWHM is With turbulence, FWHM is diffraction limit of regions of diffraction limit of regions of coherent phase, coherent phase, ~ ~ / r / r00
Example: Example:
/ r/ r00 = 0.80 arc sec for = 0.80 arc sec for
= 1 = 1 m, rm, r00 = 25 cm = 25 cm
FWHM ~/r0
in units of /D
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
Incoming Wave with Aberration
Deformable Mirror
Corrected Wavefront
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Most deformable mirrors Most deformable mirrors today have thin glass face-today have thin glass face-
sheetssheets
Anti-reflection coating
Glass face-sheet
PZT or PMN actuators: get longer and shorter as voltage is changed
Cables leading to mirror’s power supply (where
voltage is applied)
Light
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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 Nasmyth PlatformKeck II Left Nasmyth Platform
ElevationRing
Enclosure withroof removed
ElectronicsRacks
Adaptive Optics Bench
NasmythPlatform
Science cameras
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The adaptive optics benchThe adaptive optics bench
Tip/tiltMirror
DeformableMirror
To wavefrontsensor
IR Dichroic
To near-infraredcamera
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Deformable mirrorDeformable mirrorFront view Rear view
15cm
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Wavefront SensorWavefront Sensor
AOA Camera
Sodium dichroic/beamsplitterField Steering Mirrors (2 gimbals)
Camera Focus
Wavefront Sensor Focus
Wavefront Sensor Optics: field stop, pupil relay, lenslet, reducer optics
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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 infra-red lightNeptune in infra-red light(1.65 microns)(1.65 microns)
Without adaptive opticsWith Keck
adaptive optics
June 27, 1999
2.3
arc
sec
May 24, 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
• High-contrast imagingHigh-contrast imaging– Faint material around bright stars (disks of dust, etc.)Faint material around bright stars (disks of dust, etc.)– Extra-solar planets and super-planetsExtra-solar planets and super-planets
• 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/s in 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.
Mid-infrared flares from the black hole at Mid-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 EridaniSearching for planets around Epsilon Eridani
Searching for planets around Epsilon EridaniSearching for planets around Epsilon Eridani
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 guidestarsSodium laser guidestars
Rayleigh scattered light
Light from Na layer at ~ 100 km
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 Kecklaser-guidestar adaptive opticslaser-guidestar adaptive optics
HK Tau B hidden behind an edge-on disk of dust.
HK Tau A (106 yr old T-tauri star)
