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REMOVINGHALOSFROMBRIGHTSTARSINO(III)FILTEREDIMAGESUSINGPIXINSIGHT
RonBrecher–astrodoc.ca
January8,2017
INTRODUCTION
Internalreflectionsintheopticaltraincanproducehalosaroundbrightstars,andsomehavefoundthatO(III)filtersareparticularlypronetothisproblem.Formysetup,O(III)istheonlyfilterthatgiveshalosaroundbrightstar.ThehalosmakeitchallengingtoblendtheO(III)datawithmyotherdata,andcanproducepaleorgreenishhalosaroundbrightstarsincolourimages.
ManymethodshavebeendevelopedforusingPixInsighttosubduehalosaroundbrightstars.Someinvolvemakingcomplexmasksand/orPixelMathexpressions.Othersrequireaccuratemeasurementsofthehalostomakemodelsthatcanbesubtractedtoreducethehalo.Thesemethodsworkverywell,buttheyrequiremultiplemeasurementsforeachstartoberepaired,mayrequiretheuseofmultiplePixelMathexpressionsandmasks,andtakequiteawhiletoactuallyperformonanimage.Thisarticlepresentsanalternativemethodthathasthefollowingbenefits:
- Nomeasuringrequired- Nomasksrequired- Repairshaloeseffectivelywithoutdamagingdetails- Quickandeasy
Inthisdemonstration,halosaroundbrightstarswillberepairedinanO(III)filteredimage.Twoimagesareused:theO(III)imagetoberepairedandahalo-freemasterfromanotherfilter,inthiscase,theGreenmaster.ItisrecommendedthatcopiesoftheGreenandO(III)mastersbesavedtopreservetheoriginaldata.
SUMMARYOFTHEMETHOD
HalosintheO(III)dataaresubduedbyblendingwithhalo-freedatafromtheGreenchannel.Therearethreepartstotheprocedure:
1.MatchGreen’sbrightnesstoO(III)2.MatchGreen’snoisetoO(III)3.SelectivelyblendO(III)withGreentoreducehalos
Thishaloreductionprocedureshouldbecompletedimmediatelyafterstackingorafterbackgroundmodelization.
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STEP-BY-STEPINSTRUCTIONS
1. MatchGreen’sBrightnesstoO(III)a) Aftercalibrating,registeringandstackingalldata,makeacopyoftheGreenmasterandO(III)
mastersothatyouroriginalmastersarepreserved.OpenbothcopiesandapplyanAutoStretchusingCTRL-AinWindowsorCMD-AonaMac.
b) ApplyLinearFittoGreen,usingO(III)astheReferenceImage.Todoso,dragthetriangletothegreenimage(seeFigure1).
Figure1:LinearFitisappliedtoGreenwithdefaultsettingsandO(III)astheReferenceImage.
Figure2:NoiseGeneratorisusedtoincreasethe
noiseintheGreentobesimilartoO(III).
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2. MatchGreen’sNoisetoO(III)a) MakeapreviewinalowsignalareaoftheO(III)image.Selectthepreviewbyclickingonitstab,and
thendragthetabtotheleftborderareaofGreen.ThiswillgiveamatchingpreviewintheGreenimage.SelectbothpreviewsandapplyanAutostretchtoeachofthem.
b) UseNoiseGeneratorontheGreenpreview,tryingdifferentsettings(AutoStretchaftereachtrial)untilyoufindonethatmakestheGreenpreviewlooksimilartotheO(III)preview.TrythesettingsshowninFigure2asastartingpoint.Whenyoufindthebestsettings,applytoGreen.
3. SelectivelyBlendO(III)WithGreentoReduceHalosa) OpentheCloneStamptoolandclickontheO(III)imagetomakeittheTargetImage.b) SelecttheGreenimagebyclickingonitstitlebar(atthetopoftheimage).c) PlacethecursorattheextremetopleftofGreen(coordinates0,0)andCTRL-clicktoselectthisas
thesourceimageandposition.Changetheradiusto1.d) ClickonthesamecoordinatesinO(III)(coordinates0,0).e) Changetheradiustoabout50-100soyoucanseeacircle,andthenadjusttobeslightlybiggerthan
thediameterofthehalos(Figure3).Setthesoftnesstobebetween0.1and0.2andtheOpacityaround0.5.
Figure3:SettheCloneStampRadiustoalittle
biggerthanthesizeofthehalos.
f) Clickonthehaloyouwanttoremove.• Ifresultistoodark,undoitbyclickingtandtryagainwithreducedopacity.• Ifresultisn’tdarkenough,tryclickingasecondtime,orundoitbyclickingtandtryagainwith
increasedopacity.• Notethatifnoiselooksveryunnatural,youshouldgobacktostep2(b)andimprovethenoise
matchingbeforecontinuing.g) Repeatstep(e)and(f)untilallthestarswithhaloshavebeencorrected,adjustingradius,softness
andopacityforeachindividualtargetstartogetthebestresult.Trytosettheradiusjustbigenoughtocorrectthehalo.
h) Whenallhaloshavebeencorrected,clickthegreencheckmarkatbottomleftoftheCloneStampprocesswindowtomakethechangespermanent(Figure4).