Week 10 - University of Virginia · Week 10 • Lab 3! – Photometric quality. Stamp out those bad...
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Week 10 • Lab 3! – Photometric quality. Stamp out those bad points. Finish it. • Lab 4! – Great data. Evening sessions this week focus on Lab 3 wrap-up and Lab 4 reducGons. • Exams ready for return • Read the book! Chromey Chapters 9 and 10 are excellent reinforcement. – Don’t plan on going to graduate school without “owning” this informaGon. • Final project – familiarize yourself with the available instruments at APO, browse the schedule for proposal abstracts. – 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
Transcript of Week 10 - University of Virginia · Week 10 • Lab 3! – Photometric quality. Stamp out those bad...
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
CCDOverscan
• SincethenumberofshiYsappliedtoaCCDduringthereadoutphaseisarbitraryitispossibleto– Underscan–readoutonlyafracGonofthepixels(foraCCDlargelyinone
corner).DoingsocouldreducereadoutGmebetweenframessubstanGally.– Overscan–clockmorepixelsthanareactuallythere.Doingsobrings
“empty”fakepixelstothereadoutamplifier.ThesefakepixelsareusefulfordeterminingGmedependentbiaschangessincetheyarereadatnearlythesameGmeasthepixelsinagivenrow.
Chromey
Electrical “pickup”
InfraredArrays
• VisiblewavelengthCCDsbenefitfromthefactthattheprimaryintegratedcircuitconstrucGonmaterialissilicon,whichalsoservesasthedetectormaterial.
• Infrareddetectorsrequirematerialwithsmallerbandgap(InGaAs,InSB,HgCdTearepopular)whichmustbebondedtothereadoutelectronics(sGlloutofnecessitymadeofsilicon).
InfraredArrays• Thisarchitecturenaturallyleadstodesigningreadoutcircuitswhich
havereadoutamplifiersunderEVERYpixel.– Thechargenevermoves.Pixelsare“addressed”bytheir(x,y)coordinates.– ThecollecGngimagecanbereadoutwithoutdisturbingit(non-destrucGve
readout).– VeryeasytoaddressmulGpleandrandomlydistributedsubapertures.– Easytoestablishguideareaswhiletherestofthechipintegrates.
InfraredArrays• Astrophysicaladvantages
– SensiGvitytocoolerobjects.• Weinpeakisat500nminthevisiblefor6000Kobjects.
– TheSunand“hot”stars• Weinpeakisat2000nm(2microns)intheinfraredfor1500Kobjects.
– Formingstars,selfluminousyoungmassiveplanets,browndwarfs…
• Weinpeakisat100,000nm(100microns)inthefarinfraredfor30Kobjects.
– “Warm”dustingalaxies,masslosingstars
InfraredArrays• Astrophysicaladvantages
– SensiGvitytocoolerobjects.• Weinpeakisat500nminthevisiblefor6000Kobjects.
– TheSunand“hot”stars• Weinpeakisat2000nm(2microns)intheinfraredfor1500Kobjects.
– Formingstars,selfluminousyoungmassiveplanets,browndwarfs…
• Weinpeakisat100,000nm(100microns)inthefarinfraredfor30Kobjects.
– “Warm”dustingalaxies,masslosingstars
InfraredArrays
• Astrophysicaladvantages– SensiGvitytocoolerobjects.
• Weinpeakisat500nminthevisiblefor6000Kobjects.– TheSunand“hot”stars
• Weinpeakisat2000nm(2microns)intheinfraredfor1500Kobjects.– Formingstars,selfluminousyoungmassiveplanets,browndwarfs…
• Weinpeakisat100,000nm(100microns)inthefarinfraredfor30Kobjects.
– “Warm”dustingalaxies,masslosingstars
InfraredArrays• Astrophysicaladvantages
– Increasingtransparencyofdustatlongerwavelengths.• Interstellargrainsaretypicallysub-microninsizeandgoodatblockingwavelengthsoflighttheirsizeorsmaller.Red/infraredgetsthrough,blueblocked.
InfraredArrays• Astrophysicaladvantages
– Increasingtransparencyofdustatlongerwavelengths.• Interstellargrainsaretypicallysub-microninsizeandgoodatblockingwavelengthsoflighttheirsizeorsmaller.Red/infraredgetsthrough,blueblocked.
Focusandf/#
• The“speed”ofanopGcalsystemdeterminesitssensiGvitytofocuschanges(specificallydefocusasafuncGonofdisplacementofthesensorfocalplanerelaGvetotheopGcstruefocalplane).– “Fast”systemshavelowf/#’s.“Slow”systemshavehighf/#’s.
• Thesizeofthedefocusedimageistheupstreamordownstreamdisplacementofthefocalplanedividedbythef/#.
FocusinganOpGcalSystem
• Mosttelescopeshaveaphysicalknobtoturnoranelectroniccontrolofprimaryvs.secondaryseparaGontoalterthelocaGonofthefocalplane.
• InsimplesttermsfocusinginvolvesminimizingtheFWHMorRMSsizeofthefocalplaneimagesofpointsources.– Theimagesizeminimumcanbedictatedbymanyfactors
• Airydiscsize/diffracGonlimit• SystemopGcs• Camerapixelsize• Seeing
FocusinganOpGcalSystem
• Mosttelescopeshaveaphysicalknobtoturnoranelectroniccontrolofprimaryvs.secondaryseparaGontoalterthelocaGonofthefocalplane.
• InsimplesttermsfocusinginvolvesminimizingtheFWHMorRMSsizeofthefocalplaneimagesofpointsources.– Theimagesizeminimumcanbedictatedbymanyfactors
• Airydiscsize/diffracGonlimit• SystemopGcs• Camerapixelsize• Seeing
Extreme defocus shows you the telescope entrance pupil.
SeekingOpGmalFocus
• BecausefindingfocusinvolveslocaGngtheminimumofafuncGon,“eyeballing”focuscanbefrustraGngandGmeconsuming,especiallywithvariableseeing.
• BeVertotakeFWHMmeasurementsvs.encodedfocusvaluesystemaGcallyandfittheminimum.
– Or…“findthewings,splitthedifference”
Focusvs.Temperature
• Metalscontractastemperaturedecreases.• Sinceametalstructureestablishestheprimary/secondaryseparaGon
inaCassegraintelescopeortheobjecGve/eyepieceseparaGoninaclassicalrefractoropGmalfocusistemperaturedependent.
TrackingandAlignment
• AtelescopecannotnecessarilybeleYonitsowntotrackthestars– TheR.A.driveratecanbeslightlyoff– Thegeartraincanbeimperfect– Thepolaraxiscanbemisaligned
Guiding/Autoguiding
• WithrealGmeimagery,eitherfromasub-regionofthe“science”arrayorfromaseparateguidingarray,trackingerrorscanbeminimizedwithsmallguidingcorrecGons(buVonpushes)appliedtothetelescopedrives.
http://www.celestron.com/browse-shop/astronomy/mounts-and-tripods/cge-pro-mount
HandPaddles
• Manualcontroloftelescopesoccursbothatcontrolroomconsoles,oncomputerscreensand,inthedome,withamulG-buVonhandpaddle.
HandPaddles
• Manualcontroloftelescopesoccursbothatcontrolroomconsoles,oncomputerscreensand,inthedome,withamulG-buVonhandpaddle.
AperturePhotometry
• Asinglepixelinastar’simagecontainsfluxfromthestarandbackgroundfromtheskyandpotenGallyfromunwantedcelesGalsources.
AperturePhotometry
• Measuretheskyinanannulussurroundingthestar,hopingthattheannulusmedian(notaverage!)fluxisrepresentaGveofthebackgroundunderlyingthetarget.
AperturePhotometry
• Addupthefluxinthe(red)staraperturesubtracGngofftheesGmateoftheskyfluxfromthe(green)skyannulus.
AperturePhotometry–Decisions,Decisions…
• Howbigshouldthestaraperturebe?– ToobigandyougetextraskyfluxandassociatedPoissonnose(nottomenGon
contaminaGngunwantedstars).– ToosmallandyoucaptureonlyafracGonofthestar’sflux.
• Whereshouldtheinnerandouterskyannulusradiibe?– InnerradiustooGghtandtheskyannuluswillcontainstellarfluxbiasingthe
background.– OuterradiustoolargeandthemeasuredskymaybeunrepresentaGveofwhat
underliesthetarget.
• Whattorejectandhow?– Theskyannulusintheexamplecontainsabrightstar.
• Usea“cookiecuVer”toremoveitfromtheaccounGng?• RejectitsfluxstaGsGcally,forexampleusingamedianratherthananaverage.
• RejectpixelslyingmorethanNstandarddeviaGonsabovethemeanintheskyannulus.
EffectsofVaryingtheStarAperture• Theencircledfluxfromthestar
increaseswithincreasingprimaryapertureunGltheaperturecapturesmostalloftheflux.
• Aslongasthesameapertureisusedonthetargetandcomparisonstarsthemagnitudedifferencewillbecorrectevenifnotallofthefluxisintheaperture.
• InfactitmaybepreferabletouseanaperturethatthatisnotoverlylargetoavoidcontaminaGonfrombackgroundandotherstars.
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∑
CentroidNoise• Inhigh-precisionphotometrycentroidnoisemaybe
thelimiGngfactor.– ArobustcentroidingalgorithmisessenGal.– Consideraseriesofmeasurmentswithsmallsub-pixel
centroidoffsets.
https://mirametrics.com/tech_note_centroid_noise.htm
PSF-fitPhotometry
• IndensefieldsaperturephotometrymaybecomeimpracGcal.• Inevitablyaperturephotometryalsoaccommodatesmore
backgroundthandesirable.– Extractstellarfluxintheleanestmeanestfootprintpossible….
PSF-fitPhotometry
• Thepeakcountsforastarcanbeagoodproxyforitsflux.– BeVeryet,thefullstellarprofile/shapeprovidesinformaGonthatcanyieldan
opGmalesGmateofthetotalfluxusingleastsquarefiongprocedureswherethecentroidandpeakarethefreeparameters.
PSF-fitPhotometry
• Thepeakcountsforastarcanbeagoodproxyforitsflux.– Determinethestellarprofile(PSF)fromallofthestarsontheframe.– IntheabsenceofsignificantopGcaldistorGonsallstarsonanimagehavethe
sameshape.Fitthatshapetoeachstarimage.– Fitthebrightstarsfirstandsubtract,leavingacleanerimageonwhichtofit
thefainterstars.
