Week 10 - University of Virginia · Week 10 • Lab 3! – Photometric quality. Stamp out those bad...
Transcript of Week 10 - University of Virginia · Week 10 • Lab 3! – Photometric quality. Stamp out those bad...
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Week10
• Lab3!– Photometricquality.Stampoutthosebad
points.Finishit.• Lab4!
– Greatdata.EveningsessionsthisweekfocusonLab3wrap-upandLab4reducGons.
• Examsreadyforreturn• Readthebook!ChromeyChapters9
and10areexcellentreinforcement.– Don’tplanongoingtograduateschool
without“owning”thisinformaGon.• Finalproject–familiarizeyourselfwith
theavailableinstrumentsatAPO,browsethescheduleforproposalabstracts.– hVp://www.apo.nmsu.edu
http://www.skyandtelescope.com/astronomy-news/observing-news/another-impact-on-jupiter-032920161/
Topics: • CCD/Detector/Calibration wrapup.
• At-the-telescope techniques wrapup.
• Aperture photometry
• PSF fit photometry
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CCDOverscan
• SincethenumberofshiYsappliedtoaCCDduringthereadoutphaseisarbitraryitispossibleto– Underscan–readoutonlyafracGonofthepixels(foraCCDlargelyinone
corner).DoingsocouldreducereadoutGmebetweenframessubstanGally.– Overscan–clockmorepixelsthanareactuallythere.Doingsobrings
“empty”fakepixelstothereadoutamplifier.ThesefakepixelsareusefulfordeterminingGmedependentbiaschangessincetheyarereadatnearlythesameGmeasthepixelsinagivenrow.
Chromey
Electrical “pickup”
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InfraredArrays
• VisiblewavelengthCCDsbenefitfromthefactthattheprimaryintegratedcircuitconstrucGonmaterialissilicon,whichalsoservesasthedetectormaterial.
• Infrareddetectorsrequirematerialwithsmallerbandgap(InGaAs,InSB,HgCdTearepopular)whichmustbebondedtothereadoutelectronics(sGlloutofnecessitymadeofsilicon).
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InfraredArrays• Thisarchitecturenaturallyleadstodesigningreadoutcircuitswhich
havereadoutamplifiersunderEVERYpixel.– Thechargenevermoves.Pixelsare“addressed”bytheir(x,y)coordinates.– ThecollecGngimagecanbereadoutwithoutdisturbingit(non-destrucGve
readout).– VeryeasytoaddressmulGpleandrandomlydistributedsubapertures.– Easytoestablishguideareaswhiletherestofthechipintegrates.
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InfraredArrays• Astrophysicaladvantages
– SensiGvitytocoolerobjects.• Weinpeakisat500nminthevisiblefor6000Kobjects.
– TheSunand“hot”stars• Weinpeakisat2000nm(2microns)intheinfraredfor1500Kobjects.
– Formingstars,selfluminousyoungmassiveplanets,browndwarfs…
• Weinpeakisat100,000nm(100microns)inthefarinfraredfor30Kobjects.
– “Warm”dustingalaxies,masslosingstars
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InfraredArrays• Astrophysicaladvantages
– SensiGvitytocoolerobjects.• Weinpeakisat500nminthevisiblefor6000Kobjects.
– TheSunand“hot”stars• Weinpeakisat2000nm(2microns)intheinfraredfor1500Kobjects.
– Formingstars,selfluminousyoungmassiveplanets,browndwarfs…
• Weinpeakisat100,000nm(100microns)inthefarinfraredfor30Kobjects.
– “Warm”dustingalaxies,masslosingstars
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InfraredArrays
• Astrophysicaladvantages– SensiGvitytocoolerobjects.
• Weinpeakisat500nminthevisiblefor6000Kobjects.– TheSunand“hot”stars
• Weinpeakisat2000nm(2microns)intheinfraredfor1500Kobjects.– Formingstars,selfluminousyoungmassiveplanets,browndwarfs…
• Weinpeakisat100,000nm(100microns)inthefarinfraredfor30Kobjects.
– “Warm”dustingalaxies,masslosingstars
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InfraredArrays• Astrophysicaladvantages
– Increasingtransparencyofdustatlongerwavelengths.• Interstellargrainsaretypicallysub-microninsizeandgoodatblockingwavelengthsoflighttheirsizeorsmaller.Red/infraredgetsthrough,blueblocked.
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InfraredArrays• Astrophysicaladvantages
– Increasingtransparencyofdustatlongerwavelengths.• Interstellargrainsaretypicallysub-microninsizeandgoodatblockingwavelengthsoflighttheirsizeorsmaller.Red/infraredgetsthrough,blueblocked.
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Focusandf/#
• The“speed”ofanopGcalsystemdeterminesitssensiGvitytofocuschanges(specificallydefocusasafuncGonofdisplacementofthesensorfocalplanerelaGvetotheopGcstruefocalplane).– “Fast”systemshavelowf/#’s.“Slow”systemshavehighf/#’s.
• Thesizeofthedefocusedimageistheupstreamordownstreamdisplacementofthefocalplanedividedbythef/#.
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FocusinganOpGcalSystem
• Mosttelescopeshaveaphysicalknobtoturnoranelectroniccontrolofprimaryvs.secondaryseparaGontoalterthelocaGonofthefocalplane.
• InsimplesttermsfocusinginvolvesminimizingtheFWHMorRMSsizeofthefocalplaneimagesofpointsources.– Theimagesizeminimumcanbedictatedbymanyfactors
• Airydiscsize/diffracGonlimit• SystemopGcs• Camerapixelsize• Seeing
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FocusinganOpGcalSystem
• Mosttelescopeshaveaphysicalknobtoturnoranelectroniccontrolofprimaryvs.secondaryseparaGontoalterthelocaGonofthefocalplane.
• InsimplesttermsfocusinginvolvesminimizingtheFWHMorRMSsizeofthefocalplaneimagesofpointsources.– Theimagesizeminimumcanbedictatedbymanyfactors
• Airydiscsize/diffracGonlimit• SystemopGcs• Camerapixelsize• Seeing
Extreme defocus shows you the telescope entrance pupil.
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SeekingOpGmalFocus
• BecausefindingfocusinvolveslocaGngtheminimumofafuncGon,“eyeballing”focuscanbefrustraGngandGmeconsuming,especiallywithvariableseeing.
• BeVertotakeFWHMmeasurementsvs.encodedfocusvaluesystemaGcallyandfittheminimum.
– Or…“findthewings,splitthedifference”
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Focusvs.Temperature
• Metalscontractastemperaturedecreases.• Sinceametalstructureestablishestheprimary/secondaryseparaGon
inaCassegraintelescopeortheobjecGve/eyepieceseparaGoninaclassicalrefractoropGmalfocusistemperaturedependent.
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TrackingandAlignment
• AtelescopecannotnecessarilybeleYonitsowntotrackthestars– TheR.A.driveratecanbeslightlyoff– Thegeartraincanbeimperfect– Thepolaraxiscanbemisaligned
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Guiding/Autoguiding
• WithrealGmeimagery,eitherfromasub-regionofthe“science”arrayorfromaseparateguidingarray,trackingerrorscanbeminimizedwithsmallguidingcorrecGons(buVonpushes)appliedtothetelescopedrives.
http://www.celestron.com/browse-shop/astronomy/mounts-and-tripods/cge-pro-mount
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HandPaddles
• Manualcontroloftelescopesoccursbothatcontrolroomconsoles,oncomputerscreensand,inthedome,withamulG-buVonhandpaddle.
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HandPaddles
• Manualcontroloftelescopesoccursbothatcontrolroomconsoles,oncomputerscreensand,inthedome,withamulG-buVonhandpaddle.
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AperturePhotometry
• Asinglepixelinastar’simagecontainsfluxfromthestarandbackgroundfromtheskyandpotenGallyfromunwantedcelesGalsources.
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AperturePhotometry
• Measuretheskyinanannulussurroundingthestar,hopingthattheannulusmedian(notaverage!)fluxisrepresentaGveofthebackgroundunderlyingthetarget.
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AperturePhotometry
• Addupthefluxinthe(red)staraperturesubtracGngofftheesGmateoftheskyfluxfromthe(green)skyannulus.
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AperturePhotometry–Decisions,Decisions…
• Howbigshouldthestaraperturebe?– ToobigandyougetextraskyfluxandassociatedPoissonnose(nottomenGon
contaminaGngunwantedstars).– ToosmallandyoucaptureonlyafracGonofthestar’sflux.
• Whereshouldtheinnerandouterskyannulusradiibe?– InnerradiustooGghtandtheskyannuluswillcontainstellarfluxbiasingthe
background.– OuterradiustoolargeandthemeasuredskymaybeunrepresentaGveofwhat
underliesthetarget.
• Whattorejectandhow?– Theskyannulusintheexamplecontainsabrightstar.
• Usea“cookiecuVer”toremoveitfromtheaccounGng?• RejectitsfluxstaGsGcally,forexampleusingamedianratherthananaverage.
• RejectpixelslyingmorethanNstandarddeviaGonsabovethemeanintheskyannulus.
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EffectsofVaryingtheStarAperture• Theencircledfluxfromthestar
increaseswithincreasingprimaryapertureunGltheaperturecapturesmostalloftheflux.
• Aslongasthesameapertureisusedonthetargetandcomparisonstarsthemagnitudedifferencewillbecorrectevenifnotallofthefluxisintheaperture.
• InfactitmaybepreferabletouseanaperturethatthatisnotoverlylargetoavoidcontaminaGonfrombackgroundandotherstars.
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EffectsofEstablishingtheCentroid
• PhotometricresultsdependonthecenteroftheextracGonaperture.– SlightshiYswillchange,fracGonally,whatpixelsarein/outoftheaperture.
• Sincesincetheaperturesumusedtogetthestellar(aswellasbackgroundannulus)fluxusespixelsandfracGonalpixelsbasedonthecentroidtheresultswillchangeslightlywithslightshiYsincentroid.
Chromey p. 311
xcen =
x * (Image[x, y]− Background)y∑
x∑
(Image[x, y]−Background)y∑
x∑
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CentroidNoise• Inhigh-precisionphotometrycentroidnoisemaybe
thelimiGngfactor.– ArobustcentroidingalgorithmisessenGal.– Consideraseriesofmeasurmentswithsmallsub-pixel
centroidoffsets.
https://mirametrics.com/tech_note_centroid_noise.htm
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PSF-fitPhotometry
• IndensefieldsaperturephotometrymaybecomeimpracGcal.• Inevitablyaperturephotometryalsoaccommodatesmore
backgroundthandesirable.– Extractstellarfluxintheleanestmeanestfootprintpossible….
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PSF-fitPhotometry
• Thepeakcountsforastarcanbeagoodproxyforitsflux.– BeVeryet,thefullstellarprofile/shapeprovidesinformaGonthatcanyieldan
opGmalesGmateofthetotalfluxusingleastsquarefiongprocedureswherethecentroidandpeakarethefreeparameters.
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PSF-fitPhotometry
• Thepeakcountsforastarcanbeagoodproxyforitsflux.– Determinethestellarprofile(PSF)fromallofthestarsontheframe.– IntheabsenceofsignificantopGcaldistorGonsallstarsonanimagehavethe
sameshape.Fitthatshapetoeachstarimage.– Fitthebrightstarsfirstandsubtract,leavingacleanerimageonwhichtofit
thefainterstars.