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Content1. Broadband Filters 2. Plastic Filters 3. Colored Glass Filters 4. Interference Filters 5. UBVRI Photometry 6. Precision Photometry 7. Extreme Photometry
[email protected] AstronomicalObservingTechniques2017:Imaging 2
Imaging• 2Dfield:starcluster,galaxy,protoplanetarydisk• photometry• astrometry• filterstorestrictwavelengthrange
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BroadbandColorFilters• DyedgelaKn(KodakWraNen)
– advantages:thin,cheap,largesizes– disadvantages:limitedopKcalquality,heat-sensiKve
• Coloredglass(SchoN,Corning)– advantages:stable,rugged,hightransmission– disadvantages:limitedbandpasses,limitedsizes
• Interferencefilters– advantages:verynarrowfilters,almostarbitrarybandpassshapeandwavelength
– disadvantages:expensive,verylimitedsizes,temperature-sensiKve,humidity-sensiKve,aging
[email protected] AstronomicalObservingTechniques2017:Imaging 4
WraEenFilters
• coloredplasKcsheets• namedaUerFrederickWraNen,manufacturedbyKodakforabout100years
• recently:WraNen2• upto100mmby300mm• canbeusedforexperiments• improvedperformancewhenputbetweenglassplates
www.edmundopKcs.com/onlinecatalog/displayproduct.cfm?producKd=1326
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TypicalWraEenFilters
www.edmundopKcs.com/onlinecatalog/displayproduct.cfm?producKd=1326
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ColoredGlassFilters• absorbspecificwavelengths• madebyaddingsubstancestoglassduringglassmanufacturing
• typicallyusefulfrom200-1200nm– allglassesabsorbsignificantlybelow200nm
• importantmanufacturers:– SchoN– Corning– Hoya
• canalsobeusedfororderselecKoninspectroscopykeller@strw.leidenuniv.nl AstronomicalObservingTechniques2017:Imaging 7
SchoEColoredGlass• UG:Blackandblueglasses,UVtransmi_ng• BG:Blue,blue-green,andmulK-bandglasses• VG:Greenglass• GG:Nearlycolorlesstoyellowglasses,IRtransmi_ng• OG:Orangeglasses,IRtransmi_ng• RG:Redandblackglasses,IRtransmi_ng• NG:NeutraldensityglasseswithuniformaNenuaKoninthevisiblerange
• N-WG:ColorlessglasseswithdifferentcutoffsintheUV,transmi_nginthevisiblerangeandtheIR
• KG:VirtuallycolorlessglasseswithhightransmissioninthevisibleandeffecKveabsorpKonintheIR(heatprotecKonfilters)
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SchoEColoredGlass1
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SchoEColoredGlass2
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SchoEColoredGlass3
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SchoEColoredGlass4
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SchoEColoredGlass5
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UBVRI
• UBVbyJohnsonandMorgan(1953)• VRIJKLMNQ(infrared)byJohnson(1960)• (combinaKonsof)glassfilters• inventedtoclassifystarswithphotomulKpliers• zeropointofB-VandU-BcolorindicesdefinedtobezeroforA0Vstars
www.sbig.com/products/filters.htm
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(Old)UBVRI
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LimitaTonsofUBVRIPhotometry• limitedspectralresoluKon• effecKvecentralwavelengthchangeswithcolorofstar• star'smagnitudesandcolordependonthestar'scolor• short-wavelengthsideofUfilterextendsbelowatmospherictransmissioncutoff
• properKesofskydefinewidthofbandpass,notfilter• nocleanseparaKonofinformaKonfromdifferentfilters• differentdetectorshavedifferentsensiKviKes• today:BesselorCron/CousinsUBVRIwithCCDs
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UBVRI
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OtherFilterSystems• havelessoverlapand/orhighertransmission
• Johnsonsystemforstars
• Thuan-GunnfiltersforfaintgalaxyobservaKons
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OtherFilterSystems• SloanDigitalSkySurvey(SDSS)forfaintgalaxyclassificaKon
• StromgrenforbeNersensiKvitytostellarproperKes(metallicity,temperature,surfacegravity)
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Fabry-PerotEtalon• 2parallelplateswithadjustableplateseparaKon• highlyreflecKvecoaKngsoninsidesurfaces• periodictransmissionpeaksduetoconstrucKveinterferenceof
successivelyreflectedbeams• transmissionpeaklocaKonchangeslinearlywithplate
separaKon
ChristophU.Keller,[email protected] AstronomicalObservingTechniquesCrashCourse2016Part3 20
• intensityreflecKvityr(transmissiont=1-r)• transmissionasfuncKonofwavelengthk:m/2λ,m:orderofinterferometer,λ:wavelengthd:plateseparaKoni:angleofincidencen:indexofrefracKonofmaterialbetweenplates
• ReflecKveFinessethehigherthebeNer
• FreeSpectralRange:FSR=λ2/2nd• SpectralResoluKon:R=λ/Δλ=λF/FSR=mF,R≈100-100’000
Fabry-PerotEtalon
( ) ( )1
22
2
0 cos2sin141
1
−
⎥⎦
⎤⎢⎣
⎡−
+⎟⎠⎞⎜
⎝⎛
−= idk
rr
rrII π
rrF
−=1π
ChristophU.Keller,[email protected] AstronomicalObservingTechniquesCrashCourse2016Part3 21
ThinFilm
• layerwiththickness≤λ• extendsin2otherdim.>>λ• materialwithdifferentindexofrefracKononthicksubstrate
• maximumtransmissionwhenreflectedraysinterferedestrucKvely
• maximumreflecKvitywhenreflectedraysinterfereconstrucKvely
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InterferenceFilters• stackofthinfilms• canbetailoredtoalmostanyspecificaKons
• sensiKvetotemperature,humidity,angleofincidence
• tuneinwavelengthwithtemperature,angleofincidence
• spectralresoluKonR~3–1000
:
ns
n1, d1
n2, d2
nL, dL
n3, d3
nm
90º
!0
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Fabry-PerotTunableFilter• mainingredientisFabry-PerotwithtunableplateseparaKon
• stabilityofcavityspacingiscriKcal
• oUencombinetwoormoretunableelements
• alwaysneedinterferenceprefilter
www.arcetri.astro.it/science/solare/IBISphoto.jpeg
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Photometry• goal:determinefluxofanastronomicalobjectinwell-definedwavelengthrange
• problems:– seeing– exKncKon– skybackground– telescope,instrument,detector
• calibraKonwith(standard)stars• all-skyphotometry:compareobjectsalloverthesky• differenKalphotometry:compareobjectsonsameCCDexposure
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SkyoverLaPalma
Atmosphere• photometricnight=uniform,stableskycondiKons
• thincloudsandcirrusarehardtoseeatnight• bright,greyanddarkobservingKme• seeingdefinessizeofstellarimageforlargetelescopes
• image=convoluKonofsourceandPoint-SpreadFuncKon(PSF)
• [email protected] AstronomicalObservingTechniques2017:Imaging 27
AirMass• exKncKonduetoatmospherealongline-of-sight
• airmass=amountofaironelooksthrough
• atzenith(z=0):airmassX=1.0
• atzenithdistancez,airmassX≈secz=1/cosz
• X(z=60°)≈[email protected] AstronomicalObservingTechniques2017:Imaging 28
AirMass• absorpKoncoefficientK=2.5log(f(z)/f(z=0)),wheref(z)ismeasuredfluxasfuncKonofzenithdistancez
• KΔX=ΔmchangeinmagnitudeisabsorpKoncoefficientKmeschangeinairmass
• measuremagnitudefordifferentairmassesandextrapolatetozeroairmass(noatmosphere!)
• linearextrapolaKondoesnotworkintheinfraredduetosaturated/overlappingmolecularlines
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SimulatedNear-InfraredAbsorpTon
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fromTokunagaetal.2002,adsabs.harvard.edu/abs/2002PASP..114..180T
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DarkSkyinAlps
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BrightSkyintheNetherlands
PhotometricObservaTons(1)• observeobjectandstandardstarswithdifferentcolors
• observestandardstarsatlowandhighairmasstodeterminetheir(color-dependent)exKncKoncoefficients
• reduceimageswithbias,dark,flatfield• measurefluxeswithaperturephotometry• calculateinstrumentalmagnitudes:minst=-2.5log(fi/texp)
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PhotometricObservaTons(2)• calibrateinstrumentalmagnitudeforzeroairmass:m0=minst-Ksecz
• zeropointfromstandardstars:mzp=mstd-mstd,inst• removezeropoint:m=mzp+minst• absolutephotometry:determineactualmagnitudeswithcalibraKonsderivedfromstandardstars
• differenKalphotometry:doesnotrequirecalibraKons
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SkyBackground• useannulusaroundstar
• usemedianinsteadofmeantoneglectcosmicrays
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AperturePhotometry
• determinelocaKonofstarbyfi_ng2-DGaussianorMoffatPSFmodel
• determineandremoveskybackground
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AperturePhotometry(conTnued)• bestaperturesizedependsonbrightnessofstar• trade-offbetweencapturingmorestarlightandaddingnoisefromskybackground,detectoretc.
• plotresultasfuncKonofincreasingapertureandesKmateasymptoKcvalue
• forvariabilitymeasurements(e.g.exoplanettransit)withdifferenKalphotometry(standardstarinsameimage):– usefixedandidenKcalaperturesfortargetandreferencestar(s)
– opKmumapertureminimizesscaNerinconstantpartsofmtarget-mstd
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ColorCorrecTon• exKncKondependsstronglyonwavelength(color)
• measuredfluxthroughbroadfilterdependsonstellarspectrum
• addiKonalcorrecKon m=m0+Ksecz+k2CseczwhereCiscolorindex(e.g.C=B-V)
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ExtremePhotometry
keplerscience.arc.nasa.gov/PyKEprimerWalkthroughC.shtml90-daylong-cadenceKeplerSAPlightcurveofKIC3749404.keller@strw.leidenuniv.nl AstronomicalObservingTechniques2017:Imaging 39
SubtleInstrumentalEffects• ThermalchangesinopKcs,electronics• Smallguidingerrorscoupledwithsub-pixelgainvariaKons
• CotrendingBasisVectors(CBV):Instrumentalparametersthathavebeenshowntocorrelatewithphotometryofstandardstars
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16KeplerLightCurves
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keplerscience.arc.nasa.gov/images/PyKE/PyKEprimerFig4.png
CotrendingBasisVectors
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keplerscience.arc.nasa.gov/images/PyKE/PyKEprimerFig3.png
SubtracTonof2CBVs
keplerscience.arc.nasa.gov/PyKEprimerWalkthroughC.shtml90-daylong-cadenceKeplerSAPlightcurveofKIC3749404.keller@strw.leidenuniv.nl AstronomicalObservingTechniques2017:Imaging 43
SubtracTonof8CBVs
keplerscience.arc.nasa.gov/PyKEprimerWalkthroughC.shtml90-daylong-cadenceKeplerSAPlightcurveofKIC3749404.keller@strw.leidenuniv.nl AstronomicalObservingTechniques2017:Imaging 44
NoCBVFi]ngDuringEclipses
keplerscience.arc.nasa.gov/PyKEprimerWalkthroughC.shtml90-daylong-cadenceKeplerSAPlightcurveofKIC3749404.keller@strw.leidenuniv.nl AstronomicalObservingTechniques2017:Imaging 45