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).

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Figure4:O(III)masterbefore(top)andafterhaloreduction.Thebrightstarneartopcentrewasnotcorrected.

Onceyouaresatisfiedwiththeresult(Figure4),saveyourhalo-freeO(III)masterandit’sreadyforyoutouseinfurtherprocessing.