TelescopesandOptics SurveyofAstronomyA110 TelescopesandOptics • Goals – Howdotelescopeswork? – Howdoastronomersutilize telescopes? – Whydowemoveintospace? • PrincipalsofOptics – Refraction Figure6-1 • Lighttravelsslowerindensematerials. • c=3x108 ms-1 (vacuum) • c=2x108 ms-1 (glass) • Lightpassingfromonemediumtothe next(e.g.airtoglass)canchangethe directionofthelight. • Thisiscausedbythechangeinthe velocityoflight. TelescopesandOptics SurveyofAstronomyA110 • RefractionandLenses Figure6-2,6-4 – Refractionenablesalenstofocuslight • Lightincidentonlensisrefractedbyan angleα. • Lightleavingthelensisrefractedbyan angle-α. • Curvedlensescanfocusordisperselight. – Lenses • Convexlensesfocusparallelraysoflight toacommonfocalpoint. • Concavelensesdisperseparallelraysof light. • Focallength ofalensisthedistancefrom thelenstothepointwherethelightfroma parallelbeamisbroughttoafocus. • Focalplane istheplaneontowhichan extendedimagewillbebroughttoa commonfocus. – LightfromGalaxiesandStarsis Parallel. TelescopesandOptics SurveyofAstronomyA110 • RefractingTelescope – ASingleLens • Aswithacameraoryoureyeasinglelens willbringanimageintofocusatthefocal plane(filmoryourretina). • Toviewtheimage(notapictureofthe image)yourequireasecondlens(or projectontoascreen). – DoubleLensSystem Figure6-5 • Secondlensmagnifiestheimage. • Objectivelens:largerlens,largerfocal length,formstheimage. • Eyepiecelens:smallerlens,smallerfocal length,magnifiestheimage. angulardiameterthrougheyepiece Magnification= angulardiameterbyeye focallengthoftheobjectivelens = focallengthoftheeyepiecelens TelescopesandOptics SurveyofAstronomyA110 • RefractingTelescope – Magnificationandlightgathering • Mostimportantaspectofatelescopeisthe amountoflightitcancollect. Lightgatheringpower∝areaoflens ∝diameter 2 • Morelight:fainterobject(e.g.eye’spupil) • Limitonmagnificationistheatmosphere. – Example:RefractingTelescope • Objectivefocallength=120cm • Eyepiecefocallength=4cm 120cm Magnification= =30(30x) 4cm • Magnificationpoweriswrittenas30x • Ifaneyepieceof2cmfocallengthisused themagnificationis60x. • Shorterthefocallengthoftheeyepiece (orlongerthefocallengthoftheobjective lens)themoremagnification. TelescopesandOptics SurveyofAstronomyA110 • Aberrations – Chromaticaberration Figure6-7 • Therefractionoflightbyalensdepends onitswavelength. • Differentwavelengthsarebroughtto focusatdifferentfocalpoints. • Onlyonewavelengthwillbeinfocusanda coloredhalowillresult. • Combininglayersofglasscanresolvethis. – SphericalAberration Figure6-13 • Asphericalmirrordoesnotbringlightto acommonfocusatthesamepoint(a curvedfocalplane).Thisresultsinafuzzy image. • Parabolicmirrorssolvethisproblembut atthecostoffieldofview. • Ifthemirrordoesnothaveacommon focallengthatallpointsthelightisnot broughttofocusatacommonfocalplane (e.g.HubbleSpaceTelescope). TelescopesandOptics SurveyofAstronomyA110 • Refractingvs ReflectingTelescopes – Earlytelescopes(<1900s) • Refractingtelescopesusing2lenses(e.g. Galileo). • Largerthelensthefaintertheobjectswe canview. • Largelensesrequirelargefocallengths (shortfocallengthsarehardtomake). • Largedefectfreelensesarehardto manufacture- largestlensmade(Yerkes) is102cm(19.5mfocallength). • Lensarenotveryefficient. – NewerTelescopes(>1900s) • Usemirrorsinplaceoflenses. • Largestmirrorsare8minsize. • “Easy”toconstructshortfocallengths. • Mirrorsareveryefficient(99%). • Easytosupport. TelescopesandOptics SurveyofAstronomyA110 • ReflectingTelescopes Figures6-9,6-10 – ReflectingSurface • Lightincident toaflatreflectingsurface atanangleα totheperpendicularis reflected atanangleα. – AReflectingCurvedSurface • Aconcavereflectingsurfacewillbring lighttoacommonfocus. • Focallength ofamirroristhedistance fromthemirrortothepointwherethe lightfromaparallelbeamisbroughttoa focus. • Theobjectivemirroriscalledaprimary mirror. • Nochromaticaberration. • Lightisfocusedinfromofthemirror- needto“pickoff”theimage. • Pickoffmirroriscalledthesecondary. TelescopesandOptics SurveyofAstronomyA110 • TypesofReflectingTelescopes Figure6-11 – Newtonian • Beamispickedoffbya45o flatmirror. • Earliestreflectingtelescopedesign. – PrimeFocus • Detectorisplacedwithinthebarrelofthe telescope. • Limitsthenumberofreflections. – Cassegrain • Lightisreflectedbackdownbyaconcave secondarymirror. • Lightpassesthroughtheprimary. • Mostcommondesign- shorttube. – Coude • LightReflectsoffthesecondary. • Lightispickedoffbyatertiarymirror. andreflectedtothepivotpoint(Nasmyth) ofthetelescope. TelescopesandOptics SurveyofAstronomyA110 • ReflectingTelescope – ImageScale • Atelescope'sfocallengthdeterminesthe scaleofanimageformed. – Imagebrightness • Thetelescope'sf-valueorfocalratio(i.e., focallengthdividedbydiameter) determinesimagebrightness. – Obscurationduetosecondary • Secondarymirrorblockssomeofthelight fromreachingthemirror. • Secondarydoesnot blockpartofthe image(lightfromasourcecomesfromall angles). TelescopesandOptics SurveyofAstronomyA110 • Howwellcanwesee? – Imagequalityislimitedbythe atmosphere(e.g.twinklingstars) • Theatmosphereisturbulent. • Lightpassingthroughtheatmospheregets refractedandthepathsofphotonsarenot thesame. • Thesizeofapointsourceduetothe blurringoftheatmosphereiscalledthe seeingdisk(0.5- 1arcsec). – Telescopesizelimitsresolution • Howwellwecanseparatetwoclose sources(angularresolution)dependson telescopesize. λ θ =2.5x105 D θ :diffractionlimit(arcseconds) λ :wavelength(m) D :primarymirrordiameter(m) • Longerwavelength→ worseimages. • Biggertelescopes→ betterimages. TelescopesandOptics SurveyofAstronomyA110 – Example:(1mtelescopeat500nm) λ θ = 2.5x105 D 600x10-9 =2.5x105 1 = 0.15arcsec • Evenwitha1metertheatmospherelimits howwellwecanresolveobjects. 10mtelescope θ = 0.015arcsec – Adaptiveoptics • Wecancorrectfortheturbulenceby deformingtheprimary/secondary mirrors. • Equivalenttocorrectingthewavefront of thelight. TelescopesandOptics SurveyofAstronomyA110 • Detectors – Photographicplates • Usedforimagingfrom1900sto1980s. • Lowsensitivity(2%efficiency). • Non-linearreaction(exposureT intensity). • Widefield(5degrees). – ChargeCoupledDevices(CCDs) • Usedinimagingfrom1980s. • Highsensitivity(70-90%). • Linearrelationbetweenphotonsand signal. • Similartodigitalcameras(runat-90oC). • Largefields(1degreemosaics). TelescopesandOptics SurveyofAstronomyA110 • NewWavelengths – RadioTelescopes • Stars,galaxiesandgasemitatradio frequencies- synchrotronradiation. • Easiertobuild- surfaceoftelescopedoes notneedtobeasaccurate(1/10thofa wavelength). • Dishesmadeofwireandmetal. • Resolutionpoorer. λ 0.20 θ = 2.5x105 = 2.5x105 D 10 =1.4degrees • Useinterferometry toimproveresolution. • Resolutiondeterminedbythelargest distancebetweentelescopes(i.e.their separation). • Observethroughcloud,daytime,rain. TelescopesandOptics SurveyofAstronomyA110 – Space-basedObservatories Figure6-27 • Transparencyoftheatmosphereisnota problem(ultravioletandfar-infrared). • Atmosphericturbulenceisnotaproblem (diffractionlimitedimages). • Skybackgroundislower. • NASA’sGreatObservatoriesProgram HubbleSpaceTelescope(UV,optical,andIR)1990. GammaRayObservatory[Compton]1991. AdvancedX-RayAstronomicalFacility[Chandra]1999. SpaceInfraredTelescopeFacility[SIRTF]2003..