HST - AO - CoronographyOutline (2 classes)Slide 3Observing with a telescopeWhy not in real images?Atmosphere: a phase screenAtmospheric turbulenceAtmospheric specklesAtmosphere: basic propertiesAtmosphere: basic principlesAtmosphere: OTFWhy do we care?So what do we do?Space-based imagingSpeckle interferometry: fakeSpeckle interferometry: realSpeckle interferometryAdaptive optics: principlesSlide 19Adaptive opticsHST/AO/speckle interferometryAdaptive optics: qualitySlide 23Slide 24Slide 25Adaptive optics: anisoplatenismAdaptive optics: Zernike modesAdaptive optics: scalingSlide 29Slide 30WFS: Shack-HartmannSlide 32WFS: curvatureSlide 34WFS: curvature vs. SHDeformable mirrors (I)Deformable mirrors (II)Deformable mirrors (III)Deformable mirrors (IV)Adaptive optics: performancesSlide 41Adaptive optics: limitationsWFS: typical regimeWFS: using IR lightAdaptive optics: laser guide starAdaptive optics: laser guide starSlide 47Slide 48Slide 49Adaptive optics: next generationSlide 51Slide 52AO: different context (I)AO: different context (II)Slide 55HST/AO imaging dataSlide 57HST PSF fittingAdaptive optics PSFAO PSF: variabilitySlide 61Slide 62Slide 63Slide 64Slide 65Slide 66Slide 67Improving PSF subtractionSlide 69Slide 70Slide 71Slide 72Alternative: use dependenceUse wavelength dependenceSDI at VLT/NACOSlide 76Slide 77Can we improve even more?Slide 79Beyond imaging…A quick bibliographyHST - AO - CoronographyHST - AO - CoronographyExoplanets and circumstellar disks:Past and future scienceG. Duchêne (Obs. Grenoble)HST/AO/coronography: disks and planetsOutline (2 classes)Outline (2 classes)AO: why and how?AO: data processingExoplanets: current observationsCoronography: why and how?Disks: interpreting imagesThe big picture (interferometry, ELTs)HST/AO/coronography: disks and planetsAdaptive Optics :Why and How?HST/AO/coronography: disks and planetsObserving with a telescopeObserving with a telescopeBack to basics: Fourier opticsTelescope = circular pupil (usually)Image of a point source = Airy functionResolution: /DNeed for larger and larger telescopes!HST/AO/coronography: disks and planetsWhy not in real images?Why not in real images?Airy rings not seen in direct ground-based imagesAtmospheric turbulence!Motion of air patches with different refractive indexSeeing = time-averaged image of a point sourceVoigt profile (Gauss+Lorentz)HST/AO/coronography: disks and planetsAtmosphere: a phase screenAtmosphere: a phase screenRefraction distortion of incoming (planar) wavefrontSlope of wavefront image shiftHST/AO/coronography: disks and planetsAtmospheric turbulenceAtmospheric turbulenceStrongly layered structure:Ground layerA few km altitudeHST/AO/coronography: disks and planetsAtmospheric specklesAtmospheric specklesA single short exposure (0.137s)Many diffraction-limited “speckles”Lots of motion with timeTurbulence is very fastAverage = seeing haloObvious problem for interferometry!HST/AO/coronography: disks and planetsAtmosphere: basic propertiesAtmosphere: basic propertiesSome key parameters of turbulence:r0 = coherence lengtht0 = coherence time0 = anisoplatic angleTypically r0 = 10-20cm in Vt0 = few ms0 = few arcsecHST/AO/coronography: disks and planetsAtmosphere: basic principlesAtmosphere: basic principlesStrong dependence on wavelength:r0 6/5r0 = 15cm at 0.5 m (~0.7’’ seeing)r0 = 90cm at 2.2 mr0 = 5.5m at 10 mLarge telescopes are diffraction-limited in the mid-IRHST/AO/coronography: disks and planetsAtmosphere: OTFAtmosphere: OTFIn practice, atmosphere acts a low-pass filter in long integrationsShort integrationLong integrationHST/AO/coronography: disks and planetsWhy do we care?Why do we care?Examples of seeing-limited science:From Solar System to distant galaxies!Most disks are 1’’ acrossNeed to beat the atmosphere!HST/AO/coronography: disks and planetsSo what do we do?So what do we do?Place telescope above atmosphereHubble Space TelescopeDo very fast imaging to freeze the atmospheric turbulenceSpeckle interferometryCorrecting the atmosphere turbulenceAdaptive opticsHST/AO/coronography: disks and planetsSpace-based imagingSpace-based imagingProblem solved (!!)Advantage:Long-term stability of instrumentsNot necessarily the easiest thing to do:Very costly‘Small’ telescopes (2.5m for HST)Instrument not easily repairable (ACS )HST/AO/coronography: disks and planetsSpeckle interferometry: fakeSpeckle interferometry: fakeNeed to obtain images very fast!Remember t0 = few msPost-processing:Frame selection: ~5% with adominant single speckleShift-and-add all framesMaintains diffraction limit!HST/AO/coronography: disks and planetsSpeckle interferometry: realSpeckle interferometry: realMuch better: Fourier Transform each frame and add all interferogramsSpatial shift phase shiftAll speckles contribute!HST/AO/coronography: disks and planetsSpeckle interferometrySpeckle interferometryAdvantages:Recovers highest spatial frequencyCan resolve down to /2DLimitations:Low contrast (3-5 mag)Very low efficiency –5’ for 30s at Keck!!HST/AO/coronography: disks and planetsAdaptive optics: principlesAdaptive optics: principlesReal-time correction of turbulence!Deformable mirrorWavefront sensorFast computerMilitary: 1972!Astronomy: 1989…HST/AO/coronography: disks and planetsAdaptive optics: principlesAdaptive optics: principlesIncomingwavefrontCorrectedwavefrontDeformablemirrorAirypattern!SpecklecloudHST/AO/coronography: disks and planetsAdaptive opticsAdaptive opticsAdvantages:Obtain long exposures! Can be high quality at long wavelengthLimitations:Technical/computing challengesNeed a bright reference (guide) starNo (little) visible light correctionHST/AO/coronography: disks and planetsHST/AO/speckle interferometryHST/AO/speckle interferometryVisiblewavelengthsHigh contrastimagesHighest spatial resolutionSpaceTelescopesGround-basedAdaptive opticsGround-basedSpeckle interferometrySciencegoalHST/AO/coronography: disks and planetsAdaptive optics: qualityAdaptive optics: qualityAdaptive optics correction is always partial (limited bandwidth)Not as perfectas HST imagesProblem for highcontrast imagingDiffraction-limitedSeeing-limitedHST/AO/coronography: disks and planetsAdaptive
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