Adaptive Optics - Princeton Universitygk/AST542/ruobing.pdfWhy is adaptive optics needed? Turbulence...
Transcript of Adaptive Optics - Princeton Universitygk/AST542/ruobing.pdfWhy is adaptive optics needed? Turbulence...
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Adaptive Optics
Ruobing Dong
Large portion is adopted from Claire Max’s lecture in UCSC 03/30/2011
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Why is adaptive optics needed?
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
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Plane Wave
Distorted Wavefront
Atmospheric perturbations cause distorted wavefronts
Index of refraction variations
Rays not parallel
each small parallel beam makes a diffraction limited spot on the image plane
Patten changes at time scale of ~10ms
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Characterize turbulence strength by quantity r0
• “Coherence Length” r0 : Diameter of the circular pupil for which the diffraction limited image and the seeing limited image have the same angular resolution. (r0 ~ 15 - 30 cm at good observing sites
• Pupil larger than r0, images are seeing dominated.
Primary mirror of telescope
r0
Wavefront of light
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A much larger telescope, aperture contains many r0
Real time sequence from a small telescope with aperture the size of r0
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Schematic of adaptive optics system
Feedback loop: next cycle
corrects the (small) errors of
the last cycle
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How does adaptive optics help? (cartoon approximation)
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Adaptive optics increases peak intensity of a point source
Lick Observatory"
No AO" With AO"
No AO" With AO"
Intensity"
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AO produces point spread functions with a “core” and “halo”
When AO system performs well, more energy in core
When AO system is stressed (poor seeing), halo contains larger fraction of energy (diameter ~ r0)
Ratio between core and halo varies during night
Inte
nsity"
x"
Definition of “Strehl”: Ratio of peak intensity to that of “perfect” optical system"
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How to measure turbulent distortions (one method among many)
Shack-Hartmann wavefront sensor
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Shack-Hartmann wavefront sensor measures local “tilt” of wavefront
Divide pupil into subapertures of size ~ r0
Number of subapertures (D / r0)2
Lenslet in each subaperture focuses incoming light to a spot on the wavefront sensor’s CCD detector
Deviation of spot position from a perfectly square grid measures shape of incoming wavefront
Wavefront reconstructor computer uses positions of spots to calculate voltages to send to deformable mirror
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PZT or PMN actuators: get longer and shorter as voltage is changed
Cables leading to mirror’s power supply (where voltage is applied)
Light
BEFORE AFTER
Incoming Wave with Aberration
Deformable Mirror Corrected Wavefront
actuators
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If there’s no close-by “real” star, create one with a laser (Laser guide star)
Use a laser beam to create artificial “star” at altitude of 100 km in atmosphere
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Keck Observatory
Laser guide stars are operating at Lick, Keck, Gemini North, VLT, Subaru Obsy’s
Lick Observatory
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Keck laser guide star AO Best natural guide star AO
Galactic Center with Keck laser guide star
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Adaptive optics makes it possible to find faint companions around bright stars
Two images from Palomar of a brown dwarf companion to GL 105
200” telescope
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Neptune in infra-red light (1.65 microns)
Without adaptive optics With Keck adaptive optics"
2.3
arc
sec"
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Frontiers in AO technology
• New kinds of deformable mirrors with > 5000 degrees of freedom
• Wavefront sensors that can deal with this many degrees of freedom
• Innovative control algorithms
• “Tomographic wavefront reconstuction” using multiple laser guide stars
• New approaches to doing visible-light AO
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Thank you !