19 91ApJ. . .3 6 9L. .21B The AstrophysicalJournal,369:L21-L25,1991March10 © 1991.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. versities forResearchinAstronomy,Inc.undercontractwiththeNational Drive, Baltimore,MD21218. Aeronautics andSpaceAdministration.Postaladdress:3700SanMartin physics, UniversityofCalifornia,Santa Cruz,SantaCA95064. (FGSs), thatsimultaneouslyviewa28'diameterfield.A formance whencomparedtospecifications. is broughttoafocusabout40mmbehindtheoflight (Sis), togetherwiththreeinterferometricfineguidancesensors infrared. Itcarriesacomplementoffivescientificinstruments but isseriouslycompromisedinimagingandpointingper- choice offocalplaneyieldstheexpecteddiffractionlimited passing closetothesecondarybaffle.Theresultisthat no aberration. Lightstrikingneartheedgeofprimarymirror have clearlydemonstratedthatHSTsuffersfromspherical scope capableofimagingfrom115nmouttothethermal April 24.Thespacecrafthasperformedwellinmostrespects images. Thetoplevelspecificationwasthat70%oftheenergy tests andequipmentusedintheprimarymirrorfabrication scientific potentialisgiveninHall(1982). summary oftheexpectedcapabilitiesobservatoryandits results areoutlinedinthenextsection. optimum, focussettinggivesonlyabout16%.Theoptical be focusedinaO'.Tradius,butthepresent,andclose to 4 2 3 6 7 5 1 LowellObservatory,MarsHillRoad, Flagstaff,AZ86001. AlsoAstrophysicsDivision,SpaceScience DepartmentofESA. Wide-Field/PlanetaryCameraInvestigation DefinitionTeam. NOAO,P.O.Box26732,Tucson,AZ 85726. DepartmentofPhysics andAstronomy,BeloitCollege, Beloit,WI53511. UCO/LickObservatories,Board of StudiesinAstronomyandAstro- SpaceTelescopeScienceInstitute;operatedbytheAssociationofUni- The HubbleSpaceTelescope(HST)waslaunchedon1990 HST containsa2.4maperturef/24Ritchey-Chrétientele- Optical testsontheobservatoryandexaminationofground Image qualityisalsocriticallydependentontheper- © American Astronomical Society •Provided bythe NASAAstrophysics Data System diffraction-limited images.Amaximumofabout16%thelightfromapointsourceisconcentratedinO'.T compared toanexpectedlimitof14.5mag.Someareastheskyarethereforenotaccessibleinfinelock.In correctly. GroundtestresultsuncoveredbytheAllenCommissionagreewithderivedfromstudiesof radius, where70%wasexpected.Theimagesconsistofthiscore,surroundedbyacomplex4'.'0diameterinner halo thatcontainsmostofthelightandiscausedbyportionsprimarymirrorarenotfocusing thermal shocks. addition, thespacecraftundergoesseverepointingdisturbancesduringportionsoforbit,causedby the on-orbitimagery. through deconvolutiontechniques.SuchtechniquesrelyonthedetailedinformationaboutPSFthatis There isalossofabout2maginlimitingmagnitudeforpointsources. given here.HSTcansplithighercontrastfieldsintocomponentswhenphotometricaccuracyisnotimportant. Subject headings:analyticalmethods—imageprocessinginstrumentsnumerical The HubbleSpaceTelescopesuffersfromsignificantsphericalaberrationanddoesnotgivethepredicted The pointingperformanceisalsodegraded,withguidestarsforfinelockpresentlylimitedto13.5mag, Nevertheless, HSTrepresentsauniqueresourceforhigh-resolutionimagingoflow-contrastbrightobjects 1,23-45 Christopher J.Burrows,JonA.Holtzman,S.M.Faber,PierreY.Bely, THE IMAGINGPERFORMANCEOFHUBBLESPACETELESCOPE 1. INTRODUCTION photometry —skyphotographsultraviolet:general 13,67 Hashima Hasan,C.R.Lynds,andDanielSchroeder Received 1990October17;acceptedDecember7 ABSTRACT L21 formance ofthepointingcontrolsystem.Thespacecraftunder- goes twoperiodsoflarge-amplitudepointingexcursionsper periods, thespacecraftachievesastabilityofabout0'.'007rms, random loweramplitudedisturbances.Duringquiescent orbit, associatedwithsunriseandsunset,apparently performance bemaintainedfor24hr.Further,theFGSsare but thisfallsshortofthespecificationwhichrequiredthatsuch discussed in§3. not guidingwellonfaintstars,perhapsanindirectresultofthe spherical aberration.Consequently,thereisalossofskycover- on thescientificprogramandourconclusionsaregivenin§4. age athighGalacticlatitude,especiallyfortheWide-Field/ hyperbolic secondaryby4.9m(Schroeder1987;Burrows diameter f/2.3hyperbolicprimarymirror,separatedfrom the Planetary Camera(WFPC).Thepointingperformanceis ical aberrationandfieldcoma. degrees ofrigid-bodyfreedomtocollimatethetelescope.Three secondary mirrorsarechosentoyieldzerothird-orderspher- located attheinneredgesofFGSfieldsview.Theywere radial shearinginterferometerscalledwavefrontsensors are mation, buttheirperformance isdegradedbythespherical designed tomeasurethewavefrontandallowaccuratecolli- collimation effortshadtorely onanalysisoftheimageryfrom cient dynamicrange tocorrectthespherical aberration. abberation. Asaresultmost ofthediagnosis,focusing,and small adjustmentstothemirror figurebutdonothavesuffi- arranged intwoconcentriccircles. Theycanbeusedtomake the Sis.Behindprimary mirrorare24forceactuators 1990). Inthedesign,conicconstantsonprimary and A generaloutlineoftheeffectsdegradedperformance The OpticalTelescopeAssembly(OTA)containsa2.4 m The secondarymirrorcanbepreciselypositionedwith 6 2. INSTANTANEOUSIMAGINGPERFORMANCE 19 91ApJ. . .3 6 9L. .21B in thetelescopepupilplaneandthereforevignettebeam in illuminated, TexasInstruments800xdevices,with15//m because ofmisalignments. ation isdecenteredtypicallyby5%atthecenterof chip a field-dependentmannerasillustratedinFigure1.The pre- pixels. ThesecondaryobscurationsintheWFPCrelaysare not Camera (PC).TheCCDdetectorsarethinned,backside- WFPC. Imagesofcrowdedstarfieldsshowthattheobscur- pupil functioncausesfielddependenceonthePSFin the the f/12.9Wide-Field(WF)camera,andf/30Planetary sence ofsphericalaberrationtogetherwiththevariation the where afour-facetedpyramidcanberotatedtodividethefield into oneoftwopossiblesetsfourCassegraincamerarelays: secondary baffle,asillustratedinFigure1. the secondarymirrorspiders,primarypads,and by thesecondarymirrorbaffle.TheOTAentrancepupilis extra obscurationsintroducedbytheCassegrainobscurationandspidersup- therefore definedbyamaskattheedgeofprimarymirror, of theincominglight.Thecentralthirdbeamisobscured legged spidermount,whichcausesacross-shapedobscuration circular padsthatmaskcorrespondingareasinthepupil.The secondary mirrorassemblyismountedonaconventionalfour- supports, theprimarymirrorpads,andoutermask.Dashedlinesdefine obscurations intheOTAconsistingofsecondarymirrorbaffle,spider ports atthecenterofPCchip5foraproperlycenteredbeam.Thedottedlines and f/48modesdonotintroduceanyextrapupilobscurations. show theirpositionsontheOTAaxis(atcornerofchip).TheFOCf/96 L22 BURROWSETAL.Vol.369 The WFPC(Griffiths1990)reimagestheOTAfocalplane, The primarymirrorissupportedbythree15cmdiameter Fig. 1.—Thenominalexitpupilusedinthemodels.Solidlinesdefine © American Astronomical Society •Provided bythe NASAAstrophysics Data System 5 2 6 F96 120 8.6Aug15-10-1.16GRW+705824 4 PC6 230 37Aug15-10-1.16HD124063 F96 486 3.4Aug15-10-1.16GRW+705824 PCS 547 44Jun23+3422.70V=12.3near iCar 3 PC6 487 3.1Aug15-10-1.16HD124063 1 PCS547 44 Jun21-312-4.42V=12.3neariCar Case Camera(nm) (1990)Focus(waves)Target +V3 Axis o 0 Cameras, Filters,andFocusPositionsUsedinFigures3,4,5 Filter WidthDateZ 4 TABLE 1 mirror positioncorrespondstoachangeof11.09inZ. point. TheZcoefficientisthefocussettinginwavesrms at zonal mirrorsurfaceerrorsvisibleintheprelaunch 547 nmusedonthemodels.Achangeof1mminsecondary ondary mirror,relativetoanarbitraryconventional zero presents dataoncameras,filters,andfocuspositionsshown in fore maybeopticallycorrectedinfutureinstruments.Table 1 Figure 3.Thefocussettingismeasuredinmicronsatthe sec- strates thattheaberrationissimplycharacterized,andthere- between themodelsanddataisexcellent,whichdemon- differences indetailthecoresofimagescausedbypixel contains onlysphericalaberrationandfocusterms.Theouter registration andattitudejitter.Otherwise,theagreement interferograms arenotincludedinthemodels.Therealso rings visibleintheultravioletdata(cases4and6)causedby gral inthemodelsisoverpupilfunctionFigure1and tains theimagefromFigure2.TheFraunhoferdiffractioninte- (Plate LI)showsindetailoneoftheobservedpoint-spread set takenatthenominalfocalpositionwithWFPCand images clearlydemonstratethepresenceofsphericalaberra- or outeredgesarebrighter.Belowthedefocusedimagesa tion. Insteadofauniformlyilluminatedpupilimage,theinner images takenoneachsideofthenominalfocusposition.These columns showcorrespondingmodels.Thetoptworows functions (PSF).Thecorecomesfromaninnerzoneofthe FOC throughvisibleandultravioletfilters.Thethirdrowcon- images ofbrightstarsattwodifferentscales,whiletheright primary mirror.Thehalo,whichextendstoaradiusof2'.'0, 4 pads andthesecondaryspidersupports)areclearlyvisible. The shadowsofobscurationsinthepupil(theprimarymirror comes fromaberratedraystheremainderofpupil. “ tendrils.” Diffraction attheseobscurationscausethemtoappearas 4 ing inapparentlyrandomdirectionsfromtheplateau.Figure2 plateau containingmostoftheenergy,and“tendrils”extend- came withthefirstlightimageinWFcamera.The had atightcorecontaining~10%ofthelight,surrounding matic. coupled byrelayopticstoanEBSTVtube.Apartfromtheir filters, bothcamerasareallreflectiveandthereforeachro- camera. Thedetectorsarethree-stageimageintensifiers the astigmatismfromOTAandallowsfocusingof lowed byfilterwheelsandacylindricalfoldlenswhichcorrects consists ofanunobscuredoff-axistwo-mirrorCassegrain,fol- main camera/detectorcombinationsoperatingatf/48andf/96. Each camerareimagestheOTAfocalplane6'off-axisand operates primarilyintheultraviolet.TheFOCconsistsoftwo the imagesathighestresolutionsthatHSTcandeliverand other SIwithdirectimagingcapability.Itisdesignedtosample In Figure3(PlateL2),thelefttwocolumnsshowobserved An indicationofthesphericalaberrationinHSToptics The FaintObjectCamera(FOC)(Paresce1990)istheonly 19 91ApJ. . .3 6 9L. .21B Burrows etal.(see369,L22) © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 2.—HD124063observedwiththe PCthroughanarrowfiltercenteredat487nmthefocussettingused fortheSAO/EROSobservations PLATE LI 19 91ApJ. . .3 6 9L. .21B lmae cor 16 o eimagewings. Theobservedimagesarerenormalized toanintegralofunity,aftersubtracting amedianfilterestimateofthe backgroundlevel. Burrows etal.(see 369,L22) PLATE L2 r ^?°t* bottomfourcasesismagnifiedby a factorof8inordertobringoutdetails. Itissaturatedintheunmagnified view,designedtoshowtheprofile Fig. 3.Amontageshowing,horizontally, theobservedandpredictedimagesofpointsourcesinpresence of0.43waves(at633nm)sphericalaberration, © American Astronomical Society •Provided bythe NASAAstrophysics Data System No. 2, 1991 IMAGING PERFORMANCE OF HST L23 The model fits to the camera images show that HST suffers from about —0.43 waves rms of spherical aberration at 633 nm. The error corresponds to a 4.6 //m optical path length error for marginal rays at the paraxial focus, a T.'6 diameter circle of least confusion, a 2.3 /¿m surface error at the edge of the primary, or a change of the primary mirror conic constant from the specified -1.0022985 to about -1.014. The marginal focus is about 43 mm from the paraxial focus in the f/24 focal plane. The spherical aberration is known to originate in the OTA because it affects images in the FOC as well as the WFPC as demonstrated by the bottom two rows of Figure 3. It is also affecting the wavefront sensors. The major error is on the primary mirror because of the absence of significant field coma. A NASA commission (Allen et al. 1990) has uncovered an error of 1.31 mm in the placement of the field lens in the reflective null corrector used in the manufacture of the HST Fig. 5.—Log PSF curves for cases 3,4, 5, and 6 in Table 1. The zonal errors primary mirror. This error alone would lead to the primary that appear in the ultraviolet images are similar in both cameras. mirror having a conic constant consistent with the value of -1.0133 that they derive from ground test interferograms and a wavefront error of better than 1/14 wave rms. By this defini- an on-axis wavefront error in the OTA of —0.40 waves rms at tion, HST was expected to be diffraction-limited at visible 633 nm, which is 7% less than the value reported here on the wavelengths. basis of on-orbit measurements. A diffraction-limited telescope with the same central obscur- Figures 4 and 5 show the observed azimuthally averaged ation as HST would have 80% of the energy within O'.T radius PSFs and encircled energy for the four cases at nominal focus at 633 nm which includes half of the second bright Airy ring. in Figure 3. The slightly lower encircled energy curves for the The HST specification was 70%, after allowing for diffraction planetary camera compared to the FOC are due to a com- on the other obscurations in the pupil, microroughness scat- bination of the larger obscuration and additional vignetting tering, dust, and jitter. Instead, we obtain at best 16% at the caused by the internal misalignments at this field position and nominal focus setting 13 mm from the paraxial focus. This focus setting. Most of the energy is contained within the geo- focus position was selected for all the scientific observations metric image, but some will be scattered to still larger angles by reported in this issue and is close to the setting likely to be mirror microroughness and diffraction on the HST pupil. Pre- adopted for the General Observer program. launch predictions based on the primary mirror measurements Our model calculations show that HST has a full width at (Schroeder 1987; Burrows 1990) indicate that about 5% of the half-maximum (FWHM) of 0'.'078 instead of the expected 0"052 light will be scattered to angles larger than 3'.'0 at 633 nm, but at 633 nm. The FWHM is approximately proportional to this has not yet been measured. Instead, the encircled energy is wavelength in both cases. The encircled energy within a O'.T defined here to be 100% at 3"0 radius. radius is 15.9% for the FOC and at best 11.8% for the WFPC. The on-axis image was expected to have wavefront aberra- This difference is caused by the variable larger central obscur- tions totaling about 1/20 wave rms at 633 nm. The convention- ation in the WFPC. The Strehl ratio of HST was predicted to al definition of “diffraction-limited” is that the ratio of the be 0.9 at 633 nm. It is 0.10 and decreases to 0.035 at 250 nm. peak of the PSF to the theoretical limit in the absence of For comparison, excellent ground-based astronomical seeing aberrations (Strehl ratio) be greater than 0.8, corresponding to of 0'.'5 FWHM through the same aperture telescope would give a Strehl ratio of 0.012 and encircled energy of 0.081 in O'.T radius. 3. POINTING PERFORMANCE During observations, pointing information is provided by gyroscopes with periodic position updates supplied by two of the three FGS tracking guide stars. Control is obtained by varying the speed of reaction wheels. The FGSs have two oper- ational modes: coarse track based on image centroiding, and fine lock based on an interferometric system. Fine lock was to have provided an image stability of 0'.'007 rms. Coarse track was considered an intermediate step in the acquisition sequence and a degraded guiding mode (0'.'021 rms for V = 14.5) for nondemanding observations. The current performance of the guiding system is sum- marized in Figure 6. As indicated by the dotted line in the figure, coarse track has a variance inversely proportional to the count rate but equals the expected 0'.'021 rms only for a 12th mag guide star. In fine lock, tracking is essentially within spe- Fig. 4.—Encircled energy curves for FOC and PC images in the ultraviolet cifications during quiescent periods and is almost magnitude- and visible (cases 3,4,5, and 6 in Table 1). independent. However, fine lock cannot be reliably obtained

© American Astronomical Society • Provided by the NASA Astrophysics Data System 19 91ApJ. . .3 6 9L. .21B purposes, aredescribedinother Lettersinthisissue. acterization of thePSF.Mostofimaging projectshave Release Observations(EROS) takenforpublicinformation the twoimagingcameras.The results,alongwithseveralEarly- Assessment Observations(SAO), havesofarbeenobtainedfor capabilities oftheobservatory. Thesedata,calledScience decided toobtainrealisticscienceobservationstest the range ismaintained. loss offinelock,althoughcoarsetrackwithitslargerdynamic disturbances. Theday/nighttransitionsalmostalwaysproduce der oftheorbit.ThepointsinFigure6excludesuch lations (0"015-0"050)occurintermittentlyduringtheremain- and componentsof102speriods.Smalleramplitudeoscil- create lineofsightjitterwith150maspeak-to-peakamplitude orbital nightorviceversa,thermalshocksinthesolararrays For 2-3minutesassociatedwithpassagefromorbitaldayto ures duetoundetectedbinarystars. stars isneededor25%iftwopairsarerequiredtoavoidfail- Galactic latitudesgreaterthan30°ifonlyonepairofguide coverage infinelockfortheWFPCisreducedto70% instruments, becauserollingthespacecraftaboutanon-axis target doesnotaddmanynewcandidateguidestars.Sky high Galacticlatitude.Theeffectoflimitingtheguidestarsto mag withthe2/3aperturestopandwillalsoimprovecoarse will allowoperationinfinelockatthepredictedlimitof13.5 track. FGS optics.Itisexpectedthattuningofsystemparameters curve indicatesthatthecoarsetrackpointingvarianceisinverselyproportion- al tothenumberofdetectedphotons. telescope, orsecondaryeffectsofthesphericalaberrationin with thefullpupil,becauseofstillimperfectalignment quiescent periodsintheorbit.Therearefewpointsplottedfinelockabove 13.5 magismoreseverefortheWFPCthanother 14.5 magtogivean85%probabilityoffindingguidestarsat and coarsetrack(stars)asafunctionofstellarmagnitude,duringthemost V =13.5becausetheFGSgenerallyloseslockonfainterstars.Thedashed L24 The SAO/EROSresultsareconsistent withtheabovechar- When thesphericalaberrationwasdiscovered,it Jitter andlossoflockisinducedbyday/nighttransitions. The HSTguidingsystemwasdesignedtooperateonstarsof Fig. 6.—Trackingperformance:rmspointingerrorinfinelock(diamonds) 4. SCIENTIFICIMPACTANDPROSPECTS © American Astronomical Society •Provided bythe NASAAstrophysics Data System BURROWS ETAL. 26555. we willsubstantiallyrecoverthe expectedscientificcapabilities of theWFPC respect totheOTAcanbeimproved overthepresentcamera, are overcome,andtheinternalalignmentsofcameras with ment fittedwithcorrectiveoptics.Providedthattheprescrip- tion isdeterminedadequately,thepointingcontrolproblems by theendofScienceVerification(SV)period. many unansweredquestions.Mostoftheseshouldberesolved tion, internalopticalalignments,andscatteredlightthere are ground andbafflingefficiency,UVthroughput,targetacquisi- changes isstillunclear.Finally,inareassuchasskyback- well characterized,buttheeffectivenessofflightsoftware for futurescienceinstruments.Thepointingsystemisfairly tions, yetnotquitewellenoughtoprescribecorrectiveoptics understood wellenoughtopredicttheeffectsonmostobserva- a preliminarystageinmanyrespects.Theopticalproblemsare (Holtzman etal.1990).Itisclearthatthelossoffaintobjects tific programoftheobservatory. a severeblowtotheoverallimagingcapabilities,andscien- on faintstarswithintheextendedhaloofbrighter models, wecomputealossof2.0maginlimitingsensitivityfor of 2.5magonisolatedstarsandanevenmoresignificantloss a givenintegrationtime.Stellarphotometrytestsshowloss background limited.AttheSAO/EROSfocussettingfrom portional totherootmeansquareofnormalizedPSFwhen For pointsources,thelimitingintensityisinverselypro- objects inevitablyfailswhenthesignal-to-noiseratioislow. when workingonfaintobjects.Deconvolutionextended Charon system. tional lens,themassivestarsystemR136a,andPluto- close totheoriginalpoweroftelescope.SeveralLettersuse this abilityincludingsplittingthecomponentsofagravita- merely positionsandapproximatebrightnesses,thenthecore of thePSFallowsonetoresolvebinarystarsataseparation are knowntobepointsources.Ifthedesiredinformationis tion. Thesimplestexampleisastarfield,wherealltheobjects structure ofthesource,morecanbedoneinimageinterpreta- stable. conjunction withsuchdataappearstobeapromising approach, becausetheopticsaresimplycharacterized,and mined fromengineeringtelemetry.TheuseofmodelPSFsin function isobservedbytheFGSandgyroscanbedeter- of theinstantaneousPSFwithajitterblurringfunction.This data. Inanygivenimage,theobservedPSFisconvolution purposes willusuallysufferfromdifferentjittertothetarget from HST. deconvolved imagesthatapproachtheimagequalityexpected surface-brightness objects.SeveralLettersinthisissueshow volution alwaysamplifiesnoise,itismostsuccessfulonhigh- spatial detailfromtheoriginalimage.However,sincedecon- the PSF,imagedeconvolutiontechniquescanextractfine bright Hiiregions.Becauseofthesharpcoreandstability various galaxynuclei,thecoreofaglobularcluster,andseveral dealt withhigh-surface-brightnessobjectssuchasSaturn, This workwassponsoredinpart byNASAcontractNAS5- NASA planstoreplacetheWFPCwitha“clone”instru- Our understandingoftheobservatoryperformanceisstillat The shortcomingsoftheopticsbecomepainfullyapparent When thereissubstantialaprioriinformationaboutthe Direct referenceobservationsofthePSFfordeconvolution Vol. 369 19 91ApJ. . .3 6 9L. .21B No. 2,1991 Allen, L.,etal.1990,TheHubbleSpaceTelescopeOpticalSystemsFailure Griffiths, R.1990,WideFieldPlanetaryCameraInstrumentHandbook Holtzman, J.A.,etal.1991,Api,369,L35 Burrows, C.1990,OpticalTelescopeAssemblyHandbook(Baltimore:Space (Baltimore: SpaceTelescopeInstitute) Telescope ScienceInstitute) Report (Washington:NASA) © American Astronomical Society •Provided bythe NASAAstrophysics Data System IMAGING PERFORMANCEOFHST REFERENCES Hall, D.,ed.1982,TheSpaceTelescopeObservatory(Washington:NASA Paresce, F.1990,FaintObjectCameraInstrumentHandbook(Baltimore: Schroeder, D.1987,AstronomicalOptics(Orlando:AcademicPress) Scientific andTechnicalInformationBranch,1) Space TelescopeScienceInstitute) L25