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