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Low–Resolu+onOp+calSpectroscopy
TomMatheson
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Howtogetfromhere
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Tohere:
WhySpectroscopy?
NOAOGeminiDataReduc+onWorkshopJuly19,2010
• Composi+on/abundances• Velocity(radial,dispersion,rota+oncurves)• Temperature• Excita+onmechanisms• Density/pressure• InterveningmaVer
Outline
NOAOGeminiDataReduc+onWorkshopJuly19,2010
• Briefoverviewoftheobstaclesbetweenarawframeandthefinalproduct• Choosingtherightgra+ngintherightspectrographontherighttelescope• Thekindsofcalibra+onsyouneedandhowtoapplythem
ThingsIWon’tCover(butares+llimportant)
NOAOGeminiDataReduc+onWorkshopJuly19,2010
• Mul+‐object(seetheGMOSpresenta+ons)• Nod&Shuffle(seetheGMOSpresenta+ons)• Extendedobjects• High‐resolu+on• Infra‐red(SeeNIRI/NIFSpresenta+ons)• So_ware/IRAFparameters(SeeA User’s Guide to Reducing Slit spectra with IRAF,Massey,Valdes,&Barnes,1992,availableontheNOAOwebsite)
Problem#1:TheCCD
NOAOGeminiDataReduc+onWorkshopJuly19,2010
QuantumEfficiencyoftheDetector
Problem#1:TheCCD
NOAOGeminiDataReduc+onWorkshopJuly19,2010
FladieldandFringing
Problem#2:TheSky
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Night‐SkyEmissionLines
Problem#2:TheSky
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Con+nuousAbsorp+on
Problem#2:TheSky
NOAOGeminiDataReduc+onWorkshopJuly19,2010
MolecularAbsorp+on
Wallace,Hinkle,&Livingston1993,1998)
OtherPoten+alProblems
NOAOGeminiDataReduc+onWorkshopJuly19,2010
• Findingtherightgra+ngforyourproject• Biases,darks,overscan(CCD/electronicseffects)• Second‐orderlight• Parallac+cangle• Observingstandardstars(fluxandother)• Geinggoodwavelengthcalibra+onlamps• Extrac+onofthe1‐Dspectrum• Wavelengthcalibra+on• Fluxcalibra+on• Telluriccorrec+on
ThreeThingstoTakeAwayfromthisPresenta+on
NOAOGeminiDataReduc+onWorkshopJuly19,2010
1.Donoharm
Don’tcompromisethedataDotheminimumnecessaryforremovalofinstrumentaleffectsandcalibra+on
ThreeThingstoTakeAwayfromthisPresenta+on
NOAOGeminiDataReduc+onWorkshopJuly19,2010
2.Lookatthedata
Don’texpecteverythingtoworkAmisplacedbiasframeorsaturatedflatfieldcanleadtoproblemsthataredifficulttodiagnose
ThreeThingstoTakeAwayfromthisPresenta+on
NOAOGeminiDataReduc+onWorkshopJuly19,2010
3.Takeallthecalibra+onframesyouneedandthentakeallthecalibra+onframesyoudon’tthinkyouneed
PlanningtheObserva+on:Gra+ngs
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Therearetwobasicquan++estoconsider:
1:ResolvingPower= R ==Nm
2:WavelengthCoverage
Resolu+onafunc+onofdispersion,detectorpixelscale,slitwidth,and(possibly)seeingEssen+allytheabilitytodis+nguishnearbyfeatures
Limitedmainlybysizeofdetectoraswellasop+cs,telescopethroughput,anddetectorresponse
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Diffrac+onGra+ngHandbook,Palmer2005Newport/RichardsonGra+ngs
PlanningtheObserva+on:Gra+ngs
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Inprac+calterms,gra+ngsaredescribedwithafewnumbers:
1. Thenumberoflinespermm(e.g.,R400,B1200)HighernumbersmeanbeVerresolu+on
2. Theblazewavelength,essen+allythewavelengthwiththehighestefficiency,butothereffectscanchangethis,soyoushouldseekouttheefficiencycurve
3. DispersioninÅ/pixel4. Resolu+on,measuredwithsomeslitwidth
Gra+ngEfficiencyCurve
NOAOGeminiDataReduc+onWorkshopJuly19,2010
PlanningtheObserva+on:Gra+ngs
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Inprac+calterms,gra+ngsaredescribedwithafewnumbers:
1. Thenumberoflinespermm(e.g.,R400,B1200)HighernumbersmeanbeVerresolu+on
2. Theblazewavelength,essen+allythewavelengthwiththehighestefficiency,butothereffectscanchangethis,soyoushouldseekouttheefficiencycurve
3. DispersioninÅ/pixel4. Resolu+on,measuredwithsomeslitwidth
Second‐orderLight
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Bluelightinsecondorderoverlapsredlightinfirstorder
Second‐orderLight
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Useanorder‐sor+ngfilter(generallyiden+fiedwiththehalf‐throughputwavelength)
Tes+ngtheCCD:Biases
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Usebiasesandflatstodeterminegainandreadnoise(forthisandalotmoredetailaboutCCDs,seeSteveHowell’stalk,orhisbook,HandbookofCCDAstronomy)
Theisyourfriend.Doenoughbiasestogetabovethereadnoise
Formostmoderndetectors,thereisn’tmuchneedtosubtractabiasforspectroscopicframes.Aslongasthereisn’tapaVern,anyresidualpedestalinthebiaswillberemovedbyskysubtrac+on
Therealvalueisasatestofinstrumenthealth.
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Tes+ngtheCCD:Darks
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Aswithbiases,darkcurrentinmodernop+caldetectorsisn’tusuallyaseriousproblem.
Itcantakealotof+metogetenoughdarkstobewellabovethereadnoise.
Checkwiththeinstrumentscien+sttoseeifdarkcurrentisaconcern.
Tes+ngtheCCD:Trim&Overscan
NOAOGeminiDataReduc+onWorkshopJuly19,2010
ExaminetheCCD,findoutaboutsatura+onandnon‐linearity
DeterminetheusefulregionoftheCCD.IfpartsoftheCCDdon’thavecounts(orhavetoomany),thenthatwillplayhavocwithsta+s+csusedtoscaleothercalibra+ons,somakesureyouhaveawell‐definedregionoftheCCDtouse.
Lookattheoverscaninsomeofyourbiasesandflats.Theregiondefinedintheheaderso_enincludespor+onsthataren’tgood.Chooseasubsetoftheoverscanthatgivesyouanunbiasedlook
NOAOGeminiDataReduc+onWorkshopJuly19,2010
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Crosscutofflat
Chooseregionswithreasonableresponse
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Crosscutofflat
Chooseregionswithreasonableresponse
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Overscan
Chooseregionswithreasonableresponse
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Overscan
Chooseregionswithreasonableresponse
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Overscan
Chooseregionswithreasonableresponse
FlatFields
NOAOGeminiDataReduc+onWorkshopJuly19,2010
• Removepixel‐to‐pixelvaria+on
• Getenoughcounts,10Xobjectisagoodrule
• Youdon’twanttoimprintthecolor‐temperatureoftheflatlampontoyourdata,soyouneedtoremovetheoveralltrend
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Removeshapewithfit,typicallycubicsplineUselowestorderpossibletoremovesignatureofthelamp,nottheCCD
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Normalizedflat,fringings+llpresentUsethesamenormaliza+onforallflatsinoneconfigura+on
FlatFields
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Fringingiscausedbythelightfallingonthechipinterferingwithitselfwhenthechipdepthisonthesamescaleasthelight
Dependssensi+velyonwavelengthandchipposi+on,sodoredflatsattheposi+onofyourobject
FlatFieldScreen
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Internallampsareanothercommonop+on
Dependingonflexureandtheop+cs,thiscanalsobeeffec+vehVp://www.jca.umbc.edu/telescope/UsersGuides/TakingFlats.html
AtmosphericDispersion
NOAOGeminiDataReduc+onWorkshopJuly19,2010
hVp://www.kenrockwell.com/tech/2008‐01‐new.htm
AtmosphericDispersion
NOAOGeminiDataReduc+onWorkshopJuly19,2010
h"p://xkcd.com/766/
NOAOGeminiDataReduc+onWorkshopJuly19,2010
AtmosphericDispersion
NOAOGeminiDataReduc+onWorkshopJuly19,2010
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Eveninthered,dispersionlossescanbesignificant
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Lowerairmasscanhelp,buts+llaproblemintheblue
UseanADC
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Effec+ve,buts+llsomedispersionathighairmassSlightthroughputloss,possibledistor+ons
LRISADCdesign ODIADCunderconstruc+on
UsetheParallac+cAngle
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Theparallac+cangleistheposi+onangleontheskyatyourcurrentazimuthandeleva+onthatorientstheslitperpendiculartothehorizon,i.e.,alongthedispersiondirec+on.
SeeFilippenko1982,PASP,94,715
hVp://star‐www.st‐and.ac.uk/~fv/webnotes/chapter7.htm
Extrac+onfromthe2‐DFrame
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Defineaprofile,chooseabackgroundregiontoextract
NightSkyEmissionLines
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Extrac+onfromthe2‐DFrame
NOAOGeminiDataReduc+onWorkshopJuly19,2010
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Extrac+onfromthe2‐DFrame
NOAOGeminiDataReduc+onWorkshopJuly19,2010
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Makesureyouknowwhatyou’reextrac+ngMakesureyouknowwhatyou’resubtrac+ng
Extrac+onfromthe2‐DFrame
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Trace:Loca+ngthecentroidoverthedispersionaxisUsealow‐orderfit
Extrac+onfromthe2‐DFrame
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Standard:Sumoffluxinextrac+onwindow
Op+mal:Eachpixelinextrac+onwindowweightedbyitsflux,givesactualvariancees+mate(Horne1986,PASP,98,609)Cleanscosmicraystoo
WavelengthLamps
NOAOGeminiDataReduc+onWorkshopJuly19,2010
WavelengthSolu+on
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Iden+fycalibra+on‐lampemissionlines
Assignwavelengthstopixels,typicallyusingapolynomialfit
Useaslow‐orderafitaspossible
Telescope/instrumentflexuremayrequirelampsattheposi+onoftheobjectdependingontheprecisionyouneed−checkwiththeinstrumentscien+st
WavelengthSolu+on
NOAOGeminiDataReduc+onWorkshopJuly19,2010
YoualwayshaveanothersetoflineswithknownwavelengthsEvenifyouusecalibra+onlamps,useskyforzero‐pointcheck
StandardStars
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Thewaytotranslatecountsintofluxunits
Thingschangewithposi+onand+me,soyouwantastandardstarascloselymatchedtoobjectaspossible
StandardStars
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Oke&Gunn(1983)HD19445,HD84937,BD+262606,BD+174708
Oke(1974)Stone(1977)Feige34,BD+284211Masseyetal.(1988)Oke(1990)Massey&Gronwall(1990)Hamuyetal.(1994)Bessell(1999)
TablesofABmagnitudevs.wavelength,all+edbacktoVega
ABmagnitude
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Oke&Gunn,1983
Shouldbeaminussign!
StandardStars:Caveats
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Fluxisincoarsebinsando_enafewstepsremovedfromVega
Rela+vespectrophotometryisfeasible,whenallthecalibra+onsareavailable
Absolutespectrophotometryisdifficult,butyoucandopreVywellwithextraeffort
TelluricAbsorp+on
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Wallace,Hinkle,&Livingston1993,1998)
TelluricAbsorp+on
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Smoothspectrumstarwithlotsofcounts,matchedinairmassandresolu+on
TelluricAbsorp+on
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Canscalewithairmass0.6(Wade&Horne1988),butbesttomatchstandardtoairmassofobject
TelluricAbsorp+on
NOAOGeminiDataReduc+onWorkshopJuly19,2010
TheReducedSpectrum
NOAOGeminiDataReduc+onWorkshopJuly19,2010
Notthefinalproduct.Reduc+onisastep,notthegoal.
FinalReminder
NOAOGeminiDataReduc+onWorkshopJuly19,2010
• Donoharm• Lookatyourdata• Makesureyouhaveallthecalibra+ons