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.