Low – Resoluon Opcal Spectroscopy · 2010. 8. 5. · Workshop July. 19, 2010. •Composion/abundances •Velocity. (radial, dispersion, rotaon. curves) •Temperature •Excitaon

Low–Resolu+onOp+calSpectroscopy

TomMatheson

NOAOGeminiDataReduc+onWorkshopJuly19,2010

Howtogetfromhere


Tohere:

WhySpectroscopy?


• Composi+on/abundances• Velocity(radial,dispersion,rota+oncurves)• Temperature• Excita+onmechanisms• Density/pressure• InterveningmaVer

Outline


• Briefoverviewoftheobstaclesbetweenarawframeandthefinalproduct• Choosingtherightgra+ngintherightspectrographontherighttelescope• Thekindsofcalibra+onsyouneedandhowtoapplythem

ThingsIWon’tCover(butares+llimportant)


• 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


QuantumEfficiencyoftheDetector

Problem#1:TheCCD


FladieldandFringing

Problem#2:TheSky


Night‐SkyEmissionLines

Problem#2:TheSky


Con+nuousAbsorp+on

Problem#2:TheSky


MolecularAbsorp+on

Wallace,Hinkle,&Livingston1993,1998)

OtherPoten+alProblems


• 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


1.Donoharm

Don’tcompromisethedataDotheminimumnecessaryforremovalofinstrumentaleffectsandcalibra+on



2.Lookatthedata

Don’texpecteverythingtoworkAmisplacedbiasframeorsaturatedflatfieldcanleadtoproblemsthataredifficulttodiagnose



3.Takeallthecalibra+onframesyouneedandthentakeallthecalibra+onframesyoudon’tthinkyouneed

PlanningtheObserva+on:Gra+ngs


Therearetwobasicquan++estoconsider:

1:ResolvingPower= R ==Nm

2:WavelengthCoverage

Resolu+onafunc+onofdispersion,detectorpixelscale,slitwidth,and(possibly)seeingEssen+allytheabilitytodis+nguishnearbyfeatures

Limitedmainlybysizeofdetectoraswellasop+cs,telescopethroughput,anddetectorresponse


Diffrac+onGra+ngHandbook,Palmer2005Newport/RichardsonGra+ngs



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




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


Bluelightinsecondorderoverlapsredlightinfirstorder

Second‐orderLight


Useanorder‐sor+ngfilter(generallyiden+fiedwiththehalf‐throughputwavelength)

Tes+ngtheCCD:Biases


Usebiasesandflatstodeterminegainandreadnoise(forthisandalotmoredetailaboutCCDs,seeSteveHowell’stalk,orhisbook,HandbookofCCDAstronomy)

Theisyourfriend.Doenoughbiasestogetabovethereadnoise

Formostmoderndetectors,thereisn’tmuchneedtosubtractabiasforspectroscopicframes.Aslongasthereisn’tapaVern,anyresidualpedestalinthebiaswillberemovedbyskysubtrac+on

Therealvalueisasatestofinstrumenthealth.


Tes+ngtheCCD:Darks


Aswithbiases,darkcurrentinmodernop+caldetectorsisn’tusuallyaseriousproblem.

Itcantakealotof+metogetenoughdarkstobewellabovethereadnoise.

Checkwiththeinstrumentscien+sttoseeifdarkcurrentisaconcern.

Tes+ngtheCCD:Trim&Overscan


ExaminetheCCD,findoutaboutsatura+onandnon‐linearity

DeterminetheusefulregionoftheCCD.IfpartsoftheCCDdon’thavecounts(orhavetoomany),thenthatwillplayhavocwithsta+s+csusedtoscaleothercalibra+ons,somakesureyouhaveawell‐definedregionoftheCCDtouse.

Lookattheoverscaninsomeofyourbiasesandflats.Theregiondefinedintheheaderso_enincludespor+onsthataren’tgood.Chooseasubsetoftheoverscanthatgivesyouanunbiasedlook



Crosscutofflat

Chooseregionswithreasonableresponse


Crosscutofflat



Overscan



Overscan



Overscan


FlatFields


• Removepixel‐to‐pixelvaria+on

• Getenoughcounts,10Xobjectisagoodrule

• Youdon’twanttoimprintthecolor‐temperatureoftheflatlampontoyourdata,soyouneedtoremovetheoveralltrend


Removeshapewithfit,typicallycubicsplineUselowestorderpossibletoremovesignatureofthelamp,nottheCCD


Normalizedflat,fringings+llpresentUsethesamenormaliza+onforallflatsinoneconfigura+on

FlatFields


Fringingiscausedbythelightfallingonthechipinterferingwithitselfwhenthechipdepthisonthesamescaleasthelight

Dependssensi+velyonwavelengthandchipposi+on,sodoredflatsattheposi+onofyourobject

FlatFieldScreen


Internallampsareanothercommonop+on

Dependingonflexureandtheop+cs,thiscanalsobeeffec+vehVp://www.jca.umbc.edu/telescope/UsersGuides/TakingFlats.html

AtmosphericDispersion


hVp://www.kenrockwell.com/tech/2008‐01‐new.htm



h"p://xkcd.com/766/





Eveninthered,dispersionlossescanbesignificant


Lowerairmasscanhelp,buts+llaproblemintheblue

UseanADC


Effec+ve,buts+llsomedispersionathighairmassSlightthroughputloss,possibledistor+ons

LRISADCdesign ODIADCunderconstruc+on

UsetheParallac+cAngle


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


Defineaprofile,chooseabackgroundregiontoextract

NightSkyEmissionLines








Makesureyouknowwhatyou’reextrac+ngMakesureyouknowwhatyou’resubtrac+ng



Trace:Loca+ngthecentroidoverthedispersionaxisUsealow‐orderfit



Standard:Sumoffluxinextrac+onwindow

Op+mal:Eachpixelinextrac+onwindowweightedbyitsflux,givesactualvariancees+mate(Horne1986,PASP,98,609)Cleanscosmicraystoo

WavelengthLamps


WavelengthSolu+on


Iden+fycalibra+on‐lampemissionlines

Assignwavelengthstopixels,typicallyusingapolynomialfit

Useaslow‐orderafitaspossible

Telescope/instrumentflexuremayrequirelampsattheposi+onoftheobjectdependingontheprecisionyouneed−checkwiththeinstrumentscien+st

WavelengthSolu+on


YoualwayshaveanothersetoflineswithknownwavelengthsEvenifyouusecalibra+onlamps,useskyforzero‐pointcheck

StandardStars


Thewaytotranslatecountsintofluxunits

Thingschangewithposi+onand+me,soyouwantastandardstarascloselymatchedtoobjectaspossible

StandardStars


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


Oke&Gunn,1983

Shouldbeaminussign!

StandardStars:Caveats


Fluxisincoarsebinsando_enafewstepsremovedfromVega

Rela+vespectrophotometryisfeasible,whenallthecalibra+onsareavailable

Absolutespectrophotometryisdifficult,butyoucandopreVywellwithextraeffort

TelluricAbsorp+on


Wallace,Hinkle,&Livingston1993,1998)

TelluricAbsorp+on


Smoothspectrumstarwithlotsofcounts,matchedinairmassandresolu+on

TelluricAbsorp+on


Canscalewithairmass0.6(Wade&Horne1988),butbesttomatchstandardtoairmassofobject

TelluricAbsorp+on


TheReducedSpectrum


Notthefinalproduct.Reduc+onisastep,notthegoal.

FinalReminder


• Donoharm• Lookatyourdata• Makesureyouhaveallthecalibra+ons

Documents

Low – Resoluon Opcal Spectroscopy · 2010. 8. 5. · Workshop July. 19, 2010. •Composion/abundances •Velocity. (radial, dispersion, rotaon. curves) •Temperature •Excitaon