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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
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