哈勃望远镜成像技术

HarbertImagingTechnologyDesignedbyGuirongLiuCollectedbyHaihuaZhaoCorrectedbyYanjunZhangPrefaceSincelaunchingin1990,theHarbertSpaceTelescopehasbeendedicatedtotheworldinnumerablespectacularpicturesoftheuniverse.Thesepicturesarenotonlypublishedinnewspapers,magazines,almostalltextbookinastronomy,butalsointhealbumcovers,advertisements,T-Shirts,coffeecups,andmoviescenes.Thesepictureshavealsoarousedpublicinterestinastronomy,theresultingimpactonthesecularcultureisnotcomparabletoanyotherastronomicalinstrumentinmoderntimes.LetusuncoverthesecretofhowHubblecreateimages.AfterrepairBeforerepairTheeaglenebulaThefirstphotofromtheHubbletelescope

OnApril1,1995,theHubblespacetelescope'swidefieldandplanetarycamera2(WFPC2)takepicturesoftheeaglenebula.Justlikeordinarydigitalcamera,WFPC2alsousecharge-coupleddevice(CCD)ratherthanthefilmtorecordtheimage.CCDisacomposedofphotodetectorarray,oneofthesmallestunitiscalled"pixel".Itsroleisreceivelightsignalsandturnintoelectricalsignals.

WFPC2'sfieldofviewcontainsabout1600x(multipliedby)1600pixels,whichmakesitroughlyequivalenttoa2500000megapixeldigitalcamera.AlthoughthecolorofWFPC2'simageisnottrue,butthesceneitcanseeisclosertothenakedeyethancolorfilm.Whyarethetopright-handcornerofthepicturelessapiece?

WFPC2isactuallymadeupof4camera-threewidefieldplanetarycamera(WF)and1framecamera(PC1）Let'sglanceattheprocess

oftheHubblecreateimages.Differentatomsemitspecificradiationwhichhavespecificwavelengths,thushavingaspecialcolor.Thisallowsastronomerstotakepicturesofthelightemittedbysomekindofatom.ForthepictureofEagleNebula,thefilterusedinWFPC2makesitcantakethephotosofthoseemittedlightfromhydrogenatoms,sulfurionsandionizedoxygen,andthefourthfiltercanonlypassthelightwhichemittedfromthefixedstars.Firstofall,cosmicraysandothernoisewillappearinthepictures.we'llthinkofsomewaytoremovethembelow.

Secondly,thepicturewhichweseenowisonlyfromtheimageofPC1,thenwewillprocessingtheimageofthe3WFcamera.Afterthat,wejuestsawoneofthe4kindsofcolorsthateachcameracanshot.JustasthecoloroftheTVsetismadeupofred,greenandblue,thefinalpictureismadeupofdifferentcolorthatcombinationoffilters.Differentcolorsrepresentdifferentatoms.EachcameraintheWFPC2uses4piecesofthefiltertotaketwopicturestoeliminatecosmicraysinthepicture."Snow"isgeneratedbythecollisionofthecosmicraywiththeCCD,andthecosmicrayistheatomicnucleuswhosemovingspeedisclosetolightspeed.EachtimethecosmicrayhittheCCDwillleaveatracewhichwillinterferingwiththeimagewewanttoshoot.Fortunately,thecosmicraysarerandom,sotheyleavedifferentmarksontwodifferentpictures.Soaslongaswecomparewithtwodifferentphotoscanweremovetheseannoyinguniverse“graffiti”.Firstofall,findoutthecosmicraytraceswhichmerelyappearedinoneofthephotosandremovethemintwophotos.Thencombinethesetwopicturestoimprovethesignaltonoiseratioinimage.Ofcourseatthesametimetherearesomecorrectionworkwhichrelatedtheinstrumentitselfneedstobedonesuchasthe"undercurrent","flatfield"and"chargetransferefficiency.Althoughithasdonesomuchwork,buttherearestillsomecosmicrayanddefectsneedtobefurtherprocessedOnewaytolookforresidualcosmicraysandotherflawsistocomparethe4imagestakenwith4differentfilters.Anotherwayistocomparethecontouroftheimagewiththestarimage.Ifacertainprofileisnotthesameasstar,thenitispossibletheescapedcosmicraysorotherdefectswhichrequiredtobeclear.Theimageaboveshowstheremaininghighlightsofthewhitelabel,witharedbackgroundinordertomakeiteasiertoshowtheflaws.Cleanup

TheimageaboveisacompletelycleanupofthePC1photos.

Thisistheother3finishedpicturestakenbytheother3cameraswhichusedionizedoxygenfilter.Thesearethefinalpictures,andhasgonethroughthedjustmentofdirection.Joiningtogether

Similartolookingatthingsthroughamagnifyingglass,eachpicturehasvaryingdegreesofdistortion.Inordertoaccuratelysplicethese4imagestakenbydifferentcamerasintoanimage,itisnecessarytomeasureandcorrectthedistortion.Onceyouhavecompletedthisstep,4photoswillbeabletoproduceaperfectpictureofEaglenebulaundertheionizedoxygenfilter.stitching

Similartreatmentisalsomadeforthepicturestakenbyhydrogenatomsandsulfurions.Therearesubtledifferencesbetweenthem.Comparisonofthelightemittedbydifferentatomsnotonlyallowsustogetabeautifulpicture,butalsotellusalotofusefulphysicaldetails.

Hydrogenion

Sulfurion

Inorderto