Ay 20 - Fall 2004Lecture 2• Telescopes and basicoptics• Atmospheric turbulenceand adaptive optics (AO)• Radio telescopes andinterferometry• Space observatories,high-energy astronomy• Surveys, archives, data-rich astronomy, andVirtual Observatory (VO)Note:This printout is missingmany pictures shown inthe class, in order tokeep the file sizereasonably smallBasic Optics: RefractionIndex of refraction:n (l) = c / v (l)Snell’s law: n1 sin q1 = n2 sin q2If sin q2 = 1, then we have a total internal reflection forq1 > sin-1 (n2/n1) ; e.g., in optical fiberse.g., nair ≈ 1.0003,nwater ≈ 1.33,nglass ~ 1.5, etc.Index of Refraction of the AirCauchy’s approximate formula:nair = 1.000287566 + (1.158102 œ 10 -9 m / l) 2 + O(l) 4 ! ~ 5 œ 10 -6 in visible lightThus, Dl/l ~ 3 œ 10 -4 in visible light ~ 1 - 3 ÅBeware of the air vs. vacuum wavelengths in spectroscopy!Traditionally, wavelengths ≥ 3000 (2800?) Å are given as airvalues, and lower than that as vacuum values. Sigh.Lenses and Refractive OpticsNo longer used for professional telescopes,but still widely used within instrumentsFocal lengthFocal planeInverted imagesLensmaker’s FormulaUsing the Snell’s law, it canbe shown that1/f = (n-1) (1/R1 + 1/R2)(aka the “lens power”)where: f = focal lengthR1, R2 = curvature radii of thetwo lens surfacesNote that for a sphericalmirror, f = R/2Magnification and Image ScaleM = F / f y = f tan q ≈ f qscale: dq/dy = 1 / fChromaticAberrationCured by lensmultiplets …Or by reflectiveoptics!Because the speedof light in anymedium is f(l)Reflecting TelescopesMirror figures: always conic sections, mostly paraboloid,sometimes hyperboloid (Cassegrain secondary, Ritchie-Chretenboth primary and sec.), rarely sphere (Schmidt, Maksutov).Gregorian vs. CassegrainPalomar Hale 200-inch TelescopeKeckSchmidt Telescopes:offering a large FOV(popular for sky surveys)AberrationsChromaticAchromaticImage Deterioration(spherical aberation,coma, astigmatism)Image Distortion(Petzval field curvature,pincushion, barreldistortion)Sphericalaberration:Spherical Aberration: The HST SagaComa and AstigmatismPincushion and Barrel DistortionPetzval Field CurvatureMagnification varies asa function of off-axisdistanceFocal “plane” isactually sphericalModern Telescope Mirror Designs• Lightweight honeycomb structures• Thin meniscus (+ active optics)• Segmented (all segments parts of the same conicsurface); e.g., the Kecks, CELT/TMT• Multiple (each mirror/segment a separatetelescope, sharing the focus); e.g., HET, SALT• Liquid, spinningThe critical issues:– Surface errors (should be < l/10)– Active figure support (weight, thermal)– Thermal equilibrium (figure, seeing)The History ofTelescopesLargetelescopeprojects1950-2020HaleKeck1Keck2MMTHETGemini (x2)VLT (x4)Magellan….othersLBT (x2)GTCCELTHSTSIRTFNGST19491990199520002005201020152020Telescope Site Selection• Site selection is critically important– Number of good nights and atmospheric quality determine theamount and the quality of the science done• Site selection issues and problems– Atmospheric (seeing, transparency, AO issues, wind …)– Logistical (ease and cost of construction and operation)– Political/sociological (availability, security, staffing, etc.)– Geological (earthquakes, volcanos)Historically, site selection was dominated by the seeing limitedvisible, conveniece (e.g., within a driving distance), and small orsubjective measurements. Nowadays the action is in the IR andAO, and the whole world is a stage.The Best Known Sites: Mauna Kea, Canarias, Northern Chile,Southern California + Baja, Namibia, Antarctica, + a few …Seeing measurementtelescopes at CerroTololo (CTIO) Ÿ° Typical seeingdistributionAnd its successor:James Webb SpaceTelescope (JWST)Telescopes in SpaceHubble SpaceTelescope: only 2.4-m,but location, location,location!Diffraction-Limited Imaging(an ideal telescope)The Airy function~ a Fourier transformof the actual opentelescope apertureIn reality, it tends to be morecomplex, due to the mirrorgeometry, etc.Diffraction-Limited ImagingWith no turbulence,FWHM is diffractionlimit of telescope:q [radians] ≈ l / DExample: l/D = 0.02 arc sec for l =500 nm, D = 10 mFWHM ~l/Din units of l/D1.22 l/DWith turbulence, image size (“seeing”) gets much larger,typically ~ 0.5 - 2 arcsec. In order to restore the intrinsicangular resolution, we need Adaptive Optics (AO)Turbulence arises in several placesstratospheretropopauseHeat sources within domeboundary layer~ 1 kmwind flow over dome10-12 kmImages of a bright starLick Observatory, 1 m telescopeLongexposureimageShortexposureimageImage withadaptiveoptics,(nearly)diffractionlimitedJ ~ 1 arc secJ ~ 0.1 arcsec ~ l / DSpeckles (each is atdiffraction limit oftelescope)Schematic of adaptive optics systemFeedbackloop: nextcyclecorrectsthe (small)errors ofthe lastcycleAtmosphericturbulenceBut you needa bright starvery close toyour target (afew arcsec) inorder tocompute thecorrectionDeformable mirrorIf there’s no close-by bright star,create one with a laser!Use a laser beam to create anartificial “star” at altitude of ~ 100km (Na layer, Na D doublet)Keck AO System PerformanceSingle Dish (the bigger the better) …The Green Bank Telescope (GBT), D = 100 mArecibo, D = 300 m … and InterferometersThey achieve the angular resolution corresponding to thelargest baseline between the elements (dishes), but thecollecting area is just the sum …How a Radio Telescope WorksProblems With Single Dishes1. Poor resolution!2. Sidelobes pick up scattered radiation, interferenceHow Interferometer WorksSignals from independent, separated receivers arecoherently combined (correlated). What is measured isthe amplitude of correlated signal as a function of aspatial baseline, i.e.,angular frequencyon the sky. This isa Fourier transformof the actual intensityimage on the sky.… how interferometer works …Signals from individualelements are delayedelectronically, in orderto simulate a flatwavefront, for slightlydifferent arrivaldirections - thusmapping a field ofview.Very Long Baseline Interferometry (VLBI)• Antennas very far apart (~ Earth size)H Resolution very high: milli-arcsec• Record signals on tape, correlate later• Now VLBA(rray)The Future of Radio AstronomySquare KilometerArray (SKA)ALMAX-Ray telescopes:Grazing incidence mirrorsWhy? So that theprojected interatomicseparations are << lDetecting Ultra-High Energy Cosmic RaysCGRO/COMPTELSky Surveys,