Astronomical Telescopes and Our Place in the Universeweb.hep.uiuc.edu/home/jjt/SEE telescope...
Transcript of Astronomical Telescopes and Our Place in the Universeweb.hep.uiuc.edu/home/jjt/SEE telescope...
Astronomical Telescopes and Our Place in the Universe Abriefhistoryoftelescopes,discussingtheircontribu5ontoourunderstanding
oftheuniverseandabriefdescrip5onofplansforthenext20-30years.
JonThalerUniversityofIllinoisPhysicsDepartment
AstronomyDepartmentSpring2016
•Inthebeginning.•Howtelescopeswork.•Scienceenabledbytelescopes.•Theevolu5onoftelescopetechnology.° Mirrorsvslenses° Clockdrives° Photography° Computersandelectronics° Telescopesinspace° Adap5veop5cs
•Thenearfuture(i.e.,20-30years).° DarkenergyanddarkmaPer° Exoplanets(exolife?)
Iwillonlydiscussop5caltelescopes.Noradio,X-ray,gammaray,cosmicray,orneutrinoinstruments.
Outline
Iwillnottalkaboutgravita5onalwaves,butcananswerques5onsattheend,ifyouwant.
Anop5csindustrydevelopedinthelate16thcentury,ledbytheDutch.Spectacles(onelensinfrontofeacheye)wereusedasreadingaids.Theinven5onofthetwo-lenscombina5onisabitobscure,butHansLippershey’sbinocularswereshowntotheDutcharmyin1608.Thiswasini5allytreatedasamilitarysecret,buttheknowledgespreadquickly.GalileoheardaboutitinMay1609andimmediatelymadeatelescopewithamagnifica5onofx3.Thecompoundmicroscopewasinventedataboutthesame5mebyHansandZachariasJansen(alsoDutch).(Youshouldaskforabiologytalk.)Thequalityoftelescopeswaslimitedbytheabilitytomakesurfaceswithaknowncurvature,andbythenonuniformop5calproper5esoftheglass.
In the Beginning
Singlelens(spectaclesorhand-heldmagnifier)Whatdoesmagnifica4onmean?Foratelescopetheimportantthingistheangularsizeoftheimage(howbigitappearstobe),comparedtothatoftheobject.Here,magnifica5on~2:
f
WordsWords
How Do Lenses Work?
Iamstressinganglesforareason:Telescopesaregreatatmeasuringangles,butterribleatmeasuringdistances.Thisproblemhasplaguedastronomyuptothepresentday.Determiningthedistancetoanobjectrequiresotherinforma5on.Istheorangewordsmallerthantheredone,ormerelyfartheraway?
objectimage
f thelens’focallength.
f
WhichPlanetIsFartherAway?
Brightness Can Be as Misleading as Size
f1andf2arethelenses’focallengths.Theobjectandimagearebothat“infinity”.Themagnifica5on,misthera5of1/f2.Theimageisupright.That’simportantforterrestrialviewing,butnotforastronomy.Galileowasnotaprofessionalop5cian.Heusedexis5ngdesigns.
How do telescopes work (1)? Twolenses(atleast)areneededtomanipulateimagesofdistantobjects.ThisisGalileo’sfirsttelescope(1609).ThedesignwascopiedfromLippershey.
Galileo’sfirsttelescopehadamagnifica5onofabout3.Theonesheusedtodohisscience(1610)had10-30.
Ihavetworeproduc5onsofG’s1610telescopes.Theyareveryhardtouse.
f1Distancestoimageandobjectareverylarge.
f2
Diverginglens
Objec5ve Eyepiece
ConverginglensAbout2’long.
Objec5ve
Eyepiece
object
image
How Did the Telescope Help?
Themagnifica5onimprovedhisabilitytoseefinedetail,about305mesbePerthanTychoBrahe(thebestnaked-eyeastronomer).The5cmapertureenabledobserva5onoffainterobjects(byaboutafactorof300)thanhecouldseewithunaidedeyes(2-3mm).HewasabletoseeJupiter’smoons.Oneundesirablefeature:Galileantelescopeshaveaverysmallfieldofview.Galileocouldseeabout1/3thediameteroftheMoon.Thisisnotgoodifyouwanttosurveythesky,ordon’tknowexactlywheretolook.Imaginehowtediousitwastomakethesedrawings.
DrawingsinSidereusNuncius(StarryMessenger)
ThemoonsofJupiter
What Galileo Saw MoonsofJupiterTheEarthisnottheonlycenterofmo5on.
PhasesofVenusVenusdoesnotshinebyitsownlight.ItgoesaroundtheSun,nottheEarth.
Craters&mountainsontheMoonTheMoonissimilartotheEarth.Theheavenlyrealmisnotperfect.
SunspotsTheSunisalsoimperfect,andisspinning.
This(mul5plecentersormo5on)wasthefinalproofthatCopernicuswasright.TheEarthisnotthecenteroftheuniverse.
PtolemaicCosmology:
Epicycle
Phasesanddistancetellusabouttheorbit.
Venus
MostastronomershadalreadyacceptedtheCopernicantheory.
TheCopernicantheorytellsustherela5vesizesoforbitsandobjectsintheSolarSystem,butnottheactualdistances.Telescopesonlymeasureangles,sohowtoobtainadistance?Thefirstmethods(Cassini&Richer,1672)usedparallax:Theangle,a,isverysmall,about25arcseconds(1%thediameteroftheMoon)Atitsclosestapproach,Marsis55millionkmfromEarth.
TheSolarsystemisverylarge!!Parallaxisusefultodistancesofafewthousandlight-years(s5llwithinourgalaxy).
The Size of the Solar System
Mars
a
Cassini,inParis
Richer,inCayenne(SouthAmerica)
7070km
Stellarparallaxwasnotobservableatthat5me.Starsareveryfaraway.
1arcsecond= 1
3600degree
TheGreeksknewthisinthe3rdcenturyBC,butdidn’thaveprecisenumbers.Telescopicaccuracyisrequired.
Animals(eveninsects)usemo5onparallax
toes5matedistances
The Speed of Light Between1671and1690,Cassini,Römer,&HuygensstudieddiscrepanciesintheorbitofIo.TheymeasuredthatwhentheEarthandJupiterareonoppositesidesoftheSun,theorbitlagsbyabout22minutes,comparedtowhentheyareonthesameside.TheyaPributedthistothe5meittakeslighttotravelacrossEarth’sorbit.Theiranswer,132,000mi/secwasabout30%smallerthanthecurrentlyacceptedvalue.
(Their5memeasurementswereincorrect.)
Sun
EarthEarth
Jupiter
Iosameside
oppositeside
Astronomyandphysicshaveacon5nuing,closerela5onship.
Acurvedmirrorcanalsomakeimages:
Advantages(comparedwithlenses): •Nochroma5caberra5on(rainboweffect).•Lighteratlargerdiameters.Massofalensdependsonvolume(R3). Largelenses(over1meter)sag.Massofamirrordependsonarea(R2).
Disadvantages(Earlymirrorsweremadeofmetal): •Reflec5vitywaspoor(66%,atbest)•Thesurfacecorrodedfairlyquickly.•Thefocusisinthepathoftheincominglight.
Likelenses,thesphericalshapeisnotop5mal.Onewantsaparabolicmirror.
Becauseofthedisadvantagesofmetal,lenseswerepreferredovermirrorsun5lsilveredglass(>90%reflec5vity)wasinventedin1856(bySteinheilandFoucault).
Mirrors
Yerkes40”Refractor(thelargest).It’sinWisconsin;youcanvisitit.
Amirrorcanbethin.Alensmustbethick.
Focus
Tosolvethe“lightpath”problem,anothermirrorwasadded:Newton’ssolu5on(1662):Cassegrain’ssolu5on(1672):MostlargemoderntelescopeshaveCassegraingeometry.Itputstheeyepiece(orcamera)attheboPom.
The First Reflecting Telescopes
Eyepiece
Eyepieceorcamera
Eyepiece
The Rotation of the Earth Ifyou’veeverusedatelescope,you’veno5cedtheannoyingfactthattheEarthrotates.Objectsmoveoutofthefieldofview.Fornaked-eyeobserva5on,thisismerelyanannoyance.Forlongexposureastrophotography,thisisadisaster.
Abouta30-minuteexposure.
Northpole
HowtocompensatefortheEarth’srota4on:MountthetelescopeonanaxlealignedwiththeEarth’saxis.RotatetheaxlecountertotheEarth’srota5on.Thiswillstabilizetheobjectinthevieldofview,enablingmorepreciseviewing.
The Equatorial Mount
Hale200”CassegrainreflectoronMt.Palomar(largestequatorialmount)
mirror
camera
Notelescope“tube”
Tothenorthpole
Counterweight
91cmCassegrain,atSapporo west
Throughoutthe18thcentury,maintainingpropertelescopepoin5ngwasverylaborintensive,withtheobserverhavingtoregularlyinstructanassistanttomovethetelescope.Fraunhofersolvedtheproblemproblemin1825bytheuseofaclockdrive.Automa5onimprovesobservingefficiency.FriedrichvonStruveusedthistelescopetosurvey120,000stars,including3,000doublestars,aboutfour5mesthepreviouslyknownnumber.BeginningtomapthestructureoftheMilkyWay.Accuratetelescopedrivesarenecessaryforlongexposurephotography(20yearslater).
Clock Drives
Dorpat9.5”refractor
ToNorthpole
Clockdriveweights
Mountthetelescopeontwoperpendicularaxes,onever5calandonehorizontal.Thisismechanicallymuchsimplerand,forlargetelescopes,morerobust.However,unliketheequatorialmount,trackingobjectsrequiresacomputer,becausethemotorspeedsareconstantlychanging.(It’satrigonometryproblem.)Everylargetelescopebuiltsince1970usesthismount.
The Altitude-Azimuth Mount
Asimplealtazimuthmount.ANewtonianreflector!
TheLSST:8.4m(underconstruc5on)Adifferentop5caldesign.Thecameraisuphere.
Ifwecanmeasurethe5mewhenaknownstarpassesthroughdueNorth(ordueSouth),wecancalculateourlongitude.Oneminute5meaccuracyleadsto¼°longitudeaccuracy(about15miles,dependingonlaytude).Thefirstsufficientlyaccurate(to5secondsduringatransatlan5cvoyage)marinechronometerwastestedbyJohnHarrisonin1761.Ofcourse,theposi5onsofthestarsontheskymustbeaccuratelymeasured.ThiswasoneoftheimportanttasksoftheGreenwichObservatory(andothers).
Navigation Aprac5calspin-offfrombasicscience.
What5medoeseachstarcrossthisline?
?
Theadventofphotographydrama5callyincreasedthecapabilityoftelescopes.Puyngacameraatthetelescope’sfocusproducesseveralbenefits:• Fainterobjectscanbeseen.Thehumaneyeintegrateslightforabout1/20second,so,lookingatafaintobjectlongerdoesnotsignificantlyimproveone’sabilitytoseeit.
• Acameraisusuallysmallerthataperson.Thesecondarymirrorisn’trequired.• Photographycanbeusedintheultravioletandinfrared.• Onecanstudyatomicspectra(whichrequirelongexposures).Moderncosmologywouldbeimpossiblewithoutphotographictechniques.JohnDrapertookadaguerreotypeoftheMoonin1840(a20minuteexposure).ThisimageoftheOrionNebulain1883(byAndrewCommon,anamateur!)wasthefirsttoshowstarstoofainttobeseenwiththehumaneye.ThisallowedmoredetailedstructureoftheMilkyWay(ourgalaxy)tobemapped.
Photography
In2003,theHubbleSpaceTelescope’sACScameratookaonemillionsecond(about11days)exposure.Thereareveryfewstarsinthisphoto,andabout10,000galaxies,thefaintestofwhichareaboutabillionthasbrightascanbeseenbyeye.WeareseeingthelightthattheyemiPedabout13billionyearsago.(becausetheyaresofaraway)
The Longest Photographic Exposure
TheHubbleUltraDeepField
Onecandeterminethechemicalcomposi5onofastronomicalobjectsbymeasuringthespectraofthelighttheyemit.
Onecanalsomeasurethespeedtowardorawayfromthetelescopebyobservingthespectral(Doppler)shiz.Theshiz(tothered)shownherecorrespondsto9%thespeedoflightawayfromtheobserver:Spectroscopyrequiresalotoflight:Longexposuresandlargetelescopes.
Incosmology,onetalksaboutredshiz,z,thefrac5onalchangeofwavelength.
Spectroscopy Incandescent
Mercury
Lithium
Cadmium
Stron4um
Barium
Calcium
Sodium
Helium
Hydrogen
Lab
Star
In1800,WilliamandCarolineHerscheldiscoveredthatsunlighthasconsiderableenergybeyondtheredendofthespectrum.Thisisinfraredradia5on.In1868,Janssen&Lockyearsawanunknownlineinthesolarspectrum,whichJanssenaPributedtoathenunknownchemicalelement.HeliumisrareonEarth(notfoundun5l1895),butis27%oftheSun.Spectralanalysis(requiresphotography)hasbecomeakeytoolinastronomy.Becausedifferentatomsemitdifferentcolors,wecandeterminethechemicalcomposi5onofeventhemostdistantobjects.Spectralanalysisenablestestsoftheconstancyofthephysicallawsastheuniverseevolves.Noevidenceofchangesofar.
Two Important Discoveries
Thermometerbecomeshot.
Thesimultaneousmeasurementsofspeed(usingspectroscopy)anddistance(usingstarsofknownbrightness)enabledthebeginningofmoderncosmology.
• Manynebulasturnedouttobe“islanduniverses”–othergalaxies(i.e.,outsidetheMilkyWay).Thiswasaconten5ousissueun5lthe1920’s(partlyasaresultofsomeincorrectmeasurements).
• Distantgalaxiesaremovingawayfromus.Thespeedispropor5onaltothedistance.Thisimpliesthattheuniverseisexpanding(apredic5onofgeneralrela5vity).Thisledtotheconceptofa“bigbang”(verycontroversialun5lthe1960’s).
Extragalactic Cosmology
Hubble’soriginaldata(1929).Hisdistancecalibra5onwasoffbyx1/7.
~6.5Mly
1000km/s
~2.8Gly
2007data(acompendium)
Hubble’sdataishere:
Becausethespeedoflightisaninvariant(everyonemeasuresthesamevalue),whenwelookatdistantobjectsweareseeingthemastheywereinthepast:1light-year =1year1billionlight-years =1billionyearsThatishowwemeasuretheevolu5onoftheuniverse.Extrapola5ngtheHubbleexpansionbackwardsin5me,welearnthatheuniverseisabout13.7billionyearsold.Lightcanonlyhavetraveled13.7billionlight-yearssincethebeginningoftheexpansion.Iwillnotgointothedetailsofthecosmological5me-line.That’sawholeothertalk.
An Important Cosmological Concept
Computersandelectronicsenableseveralbigadvances:• Biggertelescopes,usingal5tude-azimuthmounts. Followingthecircularpathofastarwithonlyhorizontalandver5calcontrolsislikedrawingacirclewithanEtch-a-Sketch.Computersaregoodatthat.
• BePerop5cs.Amirrororlenssurfacecanbemade(atsomecost)withanarbitraryshape.
• Digitalphotography,usingCCDs.° CCDsaremoresensi5vethanfilm,andeasiertocalibrate.
° ImagescanbetransmiPedtoscien5stsovertheinternet. SometelescopesinChilesendtheirdatatoNCSAforanalysis.° Digitalimagescanbeanalyzedmorequickly(bycomputer!).
• LargedatasetsTheLSSTcamerawillhave3gigapixels,andeachimagewillbe6gigabytes.In10years,itwilltakenearlyamillionpictures.Thefinaldatasetwillbe~100petabytes(100milliongigabytes).
Computers (1)
About3005mesmorethanyour10Megapixelcamera.Giga=billionTera=trillionPeta=quadrillion
• Robo5ctelescopes(nohumansinvolved!)Example:ThePROMPTproject(ledbyUNC),isusedforop5calfollow-upofgamma-raybursts(hugeexplosionsatthecentersofgalaxies).Theseburstsareveryshort,andafew-secondresponse5meisneeded.Theinternetisrequired.
° Space-basedastronomy.
Computers (2)
PROMPT,atCerroTololo,Chile
TheHubbleSpaceTelescope
Imagequalityiscrucialforexoplanetstudies.
Adap5veop5cs:LookingatstarsthroughtheatmosphereislikelookingatobjectsattheboPomofaswimmingpool.Ifwecanquicklymeasurethedistor5on(beforethedistor5onchanges),wecancorrectit.Thisrequires:• Powerfulcomputers(1000measurementspersecond).• Flexiblemirrors,supportedbycomputercontrolledfingers,tochangetheshapeofthemirrorasneeded.
Onecanachieveideal(diffrac5onlimited)resolu5on.
Computers (3) Undistortedincominglight
Inhomogeneousatmosphere
Distortedlight
• Mul5plemirrors.TheThirtyMeterTelescope(TMT,construc5onabouttobegin)willhavea30mmirrorthatconsistsof4921.5mhexagonalmirrors.Itisnotprac5caltoconsiderasingle30m(98z)pieceofglass.
Alignmentofthemirrorsegmentssotheyfunc5onasasingle“mirror”issimilartoadap5veop5cs.
MoreonTMTscienceinaminute...
Computers (4)
The Discovery of Dark Energy Darkenergywasdiscoveredin1998byobservingtheapparentluminosityof“typeIa”supernovasasafunc5onofdistance.Distancewasmeasuredusingthefactthatthesesupernovasare“standardcandles”(allaboutthesamebrightness).Theinverse-squarelawtellsusthedistance.
Thedatacamefromacombina5onofgroundandspace-basedtelescopes.Accuratemeasurementofthecosmologicalparametersrequireslotsofdata,accuratecalibra5on,andtheabilitytoseeintheinfrared.Theplotshowstheaccuracyofthedatawewillachievewith4000supernovasusingtheDarkEnergySurvey(databeingtakennow). Comparisonofsupernovabrightness
inauniversewithdarkenergytoauniversewithwithoutit.
0.1
z
Δm
Redshiz(ameasureofdistance)
About7billionyearsago
Astronomicalmagnitude(inversebrightness)
Darkenergydecreasesthebrightnessbyabout15%.
Measurethecurvature
A Supernova Video DatafromtheBerkeleySupernovaCosmologyProject.Thesupernovareachespeakbrightnessinabouttwoweeks,andfadesoverseveralmonths.
Telescopes in Space Theadvantagesofpuyngatelescopeinspace:• Spaceisalmostblack,theterrestrialskyisnot.Onecanseefainterobjects.Thesetwo“stars”arethesamebrightness:
• Theatmospherefuzzesoutimages.Thebestterrestrial“seeing”(withoutadap5veop5cs)isabout55mesworsethanHubble’simagequality.Here,thegroundimage(sametotalbrightness)is40%bigger.
• Nogravity.Nodistor5onofthemirrorshapewhenthetelescopepointsadifferentdirec5on.
• Theatmosphereabsorbsultravioletandinfrared(andisweatherdependent).Theabilitytoobserveinfraredisimportanttocosmology(duetotheredshiz).
ThegainfrombePerimagequalityinspaceispar5allyreducedbythefactthatterrestrialtelescopescanhavelargermirrors.Adap5veop5csalsohelpstomi5gateatmosphericeffects.
Space Ground
The Future
Verylargeterrestrialtelescopes(TMT,E-ELT).30-40meterdiameter.Theymustbemul5-mirror.Onecannotmakeasinglepieceofglassthatlarge.Adap5veop5csyieldsexquisiteresolu5onina5nyfieldofview.Newspacetelescopes(JamesWebbSpaceTelescope)Sixmeterdiameter.Mul5-mirrortofitinarocket.Operatesintheinfrared,wherespacehasabigadvantage.Seeolderobjects(higherredshiz).Lookforthefirststars.Moderatesize,wide-fieldtelescopes(LSST)Eightmeterdiameter.Fieldofview10xMoon’sdiameter.Good,butnotexquisiteresolu5on.Rapidimageacquisi5on.Designedforall-skysurveysandsearchesfornewphenomena,aswellasdarkmaPeranddarkenergystudies.
Very Large Terrestrial Telescopes TheThirty-MeterTelescope(TMT)willhaveasmallfieldofview(1/3thesizeoftheMoon,aboutthesameasGalileo’s),butwillhavediffrac5onlimited(bestpossible)imagequalityandmorethanamillion5mesthelightgatheringpower.
Planetaryimaging:Theangularresolu5on(withadap5veop5cs)willbeabout10-3arcsecond:• About1cmwhenlookingfromNYCtoLA.• About100,000kmwhenlookingatnearbystars.
That’sgoodenoughtoimagelargeplanets(super-Jupiters)andstudyplanetaryatmospheres.
Lookforlife!
30meters
492computercontrolledmirrorsegments.
21differentsurfaces.
human
Exoplanet Images Thesepicturesweretakenwith8-metertelescopes(oneinChileandoneinHawaii)withoutadap5veop5cs.Thepointis,wecanseeexoplanetsnow.We’llstudythemwiththenextgenera5onofverylargetelescopes.Bothstarsaresun-likeandareafewhundredlight-yearsaway.Allfourplanetsaresuper-Jupiters(8to705mesasmassive).Theyarequitefarfromthestar–comparabletoorfartherthanthedistancetoPluto.
Very Large Terrestrial Telescopes (2)
Studythefirststarsandgalaxies.• Thefirststarsappearedlessthanabillionyearsazerthebigbang,soaninfraredcameraisneeded(duetothelargeDopplershiz).
• Theyarealsoveryfaint,soalargetelescopeisneeded.Observa5onsmaystartin2024.
Space Telescopes
Sunshield.Tooperateintheinfrared,theen5retelescopemustbecold.
Anewspacetelescope:JamesWebb(JWST),viewingintheinfrared.Themirrormustbesegmentedtofoldupintotherocket.ManyofthesciencegoalsaresimilartotheTMT.Itwilllaunchin2018.TheEndoftheDarkAges:Thefirststarsandgalaxies.
StellarandPlanetaryEvolu5ontheOriginsofLife:Thephysicalandchemicalproper5esofsolarsystems(includingourown)andwherethebuildingblocksoflifemaybepresent.
6.5m
2.4m
Mirrormassisonly700kg!
Wide-Area Survey Telescopes
8.4m
Rapidall-skysurveys(LSST).Theyrequirealargefieldofview,andautomatedopera5on(anexposureevery17seconds).Surveytheskyevery2-3days.Thisenablesthestudyofraretransientphenomena.Someobjec5ves:• NearEarth(“killer”)asteroids.A90%completesurveyofeverything>100m.• MilkyWay&othergalaxystructure.• Transientphenomena(supernovas,gammaraybursts,ac5vegalac5cnuclei).• Cosmology:Supernovas,galaxyclusters,gravita5onallensing.Alarge(100petabyte!)publiclyaccessibledataset,foreveryone’suse.Underconstruc5onnow;itwillstartin2021.
Anf1.2camera!
Moon
LSST
G
Extraslides
Problems with Lenses Theindexofrefrac5onvarieswithcolor(chroma5caberra5on):Thelenssurfaceneedstobeahyperboloid,notasphere(sphericalaberra5on).Bothcanbecorrected,butnotwith17th–18thcenturytechnology.Botheffectsaresmallerwithlongfocallengths,leadingtoverylongtelescopes!!!
Hevelius’150’telescope(~1670)
Alensateachend