Astronomy3130–Spring2016–Lab5
SpectroscopyatMcCormickObservatorywiththe26½”ClarkRefractor
NowthatyouarefamiliarwiththeoperationofCCD’s,haveseenaresearchtelescopein
action(FanMountain),knowhowtoplanasetofobservationsonagivennight,andknow
thebasicsofhowtobuildaspectrograph,thislabwillcombinealloftheseelementsinthe
collectionandanalysisofstellarspectrausingtheMcCormicktelescope.Youwillprocess
thedatatogeneratespectrawithinterpretablescientificcontent.TheCCDwillbethe
sameonethatservedastheimagerforthepinholecameraexperimentinPrelab4.The
datasheetfortheCCDcanbefoundathttp://www.ccd.com/alta_f9000.html.
Aswithalllabs,especiallywiththisone,youneedtobepreparedatthetelescope.Doingso
meansreviewingtheclassnotesonspectroscopy,readingtheChromeyandBirney
ChaptersregardingspectroscopyaswellasChapter14oftheASTR3130/5110
ObservatoryHandbook(Sections1-6)toknowhowthespectrographitselfworks.Youwill
beusingthe50µmslitandthe600grooves/mmgratingforallobservations.The
spectrographwillbeconfiguredinadvanceforyou,andtheTAwillensurethatthegrating
isattheproperangletodispersefirstorderintheblazedirectionontothearraycovering
thefullvisiblespectrum.
YoushouldverifythatthecameralensisadjustedforbestfocusofthespectrumontheCCD.
Youshouldalsoadjusttheeyepiecesothatyourrelaxedeyeseestheslitinfocus.The
eyepieceslidesinandout.Onceyouhavetheeyepieceintherightplaceyoucanadjustthe
telescopefocustofocusthestarasseenintheeyepiece.Followingthistwostepprocedure
willensurethatthestarisinfocusintheplaneoftheslitandthatyoucancomfortably
viewtheslitandthefocusedstarsimultaneously.
Forthislabyourchallengeistoobserveagridofknownspectralstandardstarsspanningthe
mainsequenceandobtainaspectrumofa“mysterystar,”makingyourbestestimateofits
spectralclassificationbasedonyourcalibrationgrid.
Ifyouneedarefresheronspectralclassificationorifyouneedreferencespectrafor
comparisoninordertocheckyourresultsthefollowingcouldbeofinterestinadditionto
therelevantsectionsinyourtextbook(s):
•
Ryden and Peterson (Sections 5.6 and 14.2)
• CarrollandOstlie,AnIntroductiontoModernAstrophysicsorAnIntroductionto
ModernStellarAstrophysics(thefirstincludesallthecontentinthesecond).
• Turnsheketal.,AnAtlasofDigitalSpectraofCoolStars
• Morganetal.,RevisedMKSpectralAtlasforStarsEarlierthantheSun
• Jacobyetal.(1984,ApJS,56,257)
• Montesetal.(1999,ApJS,123,283)
• Jaschek&Jaschek,TheClassificationofStars
• Jaschek&Jaschek,TheBehaviorofChemicalElementsinStars
• Kaler,StarsandTheirSpectra
• https://www.cfa.harvard.edu/~pberlind/atlas/atframes.html
Mostofthebooksshouldbelocatedintheastronomylibrary(unlesstheyareoutonloan).
Thelibrarycanbeopenedusingyourastronomydepartmentkey.Pleasedonotremove
themfromthelibrary.
Atminimumobtainspectraforfivestarstoestablishyourcomparisongrid(notincluding
themysterystar,sosixtargetsminimumintotal).Yourbestresourceforselectingyour
targetsisapageonbrightspectroscopicstandardsmirroredonthecoursehomepage:
http://faculty.virginia.edu/skrutskie/ASTR3130/users.erols.com/njastro/faas/pages/star
cat.htm
Selectyourfive“standard”starstospanagoodrangeofspectralclassfromcoollateK/earlyMtohotBstars.Notethat3rdmagnitudestarsandbrightershouldproducehigh
SNRspectrainmodestamountsoftime(lessthanoneminuteexposures).Besurenotto
saturate!!Pickstarsthatarereadilyaccessibleduringthefirsthalfofthenight.Ifyouare
thecautioustypeyoumightselectand“early”and“late”candidateforsomeofthetargets
togiveyouflexibility.
Priortoexecutingthelabyoumustlayoutanobservingprogramtoobserveallofthestars
optimallygivenrestrictionsonairmassandtransittime(likethepreambletothelabsaid,
thislabisyouropportunitytoputeverythingtogetherandoperateasanefficientand
effectiveobserver).Bychoosingstarswisely(largelymeaningnottoofarsouth)youcan
engineeralotofflexibilityintoyourobservations.
TheMysteryStar:11:09:39.8+44:29:54.7(2000.0)magnitude3.0
Resistthetemptationtolookthisstarup…Shunitifyouaccidentallypickitasaspectral
standardandfindasubstitute.
Themagnitudesprovidedshouldbeaguidetoexposuretime.Youmaynotbeableto
estimateintegrationtimesapriorifromthesemagnitudesbutonceyouhaveobservedone
starandaresatisfiedwiththesignal-to-noiselevel(howareyougoingtoestimatethatin
realtime?)youshouldbeabletoscaletheintegrationtimefortheotherobjects.
Rememberthatsaturateddatacontainsnousefulinformation.Don’texposetoolong.
YourTAshouldbehelpfulinconvergingtoanoptimalintegrationtime.
Atthebeginningofthenightyoushouldacquirearandombrightstarontheslit(oryour
first“science”target)andadjusttheinstrumentrotationsothatthespectrumfallsas
preciselyaspossiblealongasingleroworcolumn.Doingsowillhelpwithspectral
extractionlateron.Notethatthespectrographisconfiguredsothatineachobservationof
anisolatedstaryouobtainaspectrumofthe“dark”skyandcontaminatingbackground(e.g.
mercurystreetlampemission).
Consider,for“fun”,thefollowingbonusspectraifyouhavethetimeortenacity:
1)TakeaspectrumofJupiterandinterpret.
2)TakeasspectrumoftheilluminatedportionoftheMoonandinterpret.
3)Chooseagiant(luminosityclassI,II,orIII)andcompareyourspectrumwithadwarf
(luminosityclassV).
4)Tryanemissionlineobjectsuchasaplanetarynebula.
Inadditiontothesestellarobservationsincludealong-exposure(howlongisuptoyouto
decide)spectrumofthenightskynearzenithandnearthehorizon(withthesame
exposuretime)lowonthehorizonoverCharlottesville.Don’tforgetcalibrationlamp
observationsasdescribedintheobservingmanual.Cantheybeobtainedinparallelwith
yourstellarobservations?Notethatthecomparisonlampbulbshavealimitedlifetime.
Pleaseturnthemoffwhennotneeded.
Don’tforgettheimportanceofbiasanddarkframesofappropriateexposuretimein
calibratingyourimagespriortoextractingspectra.Planyourrequiredcalibrations(dark,
bias,lamp)inadvance.
Workasagrouptoextractandwavelength-calibratespectraandtocarryoutspectral
classificationofthemysterytarget.TheTA’swillprovidetheusualiPythonguidance.Note
thatyouhavemadenoattempttomakeanoverallabsolutecalibrationoftheshapeofthe
spectrumsothattheoverallshapeofyourextractedspectrumwillnotmatchthecurvesin
thespectralatlas.Spectralclassificationdependsonthespectralabsorptionlines.Use
theseasyourguide.Aspartofyouranalysisidentifyelementsresponsibleforthemost
prominentabsorptionsinthevariousspectra.
Writeupthelabindividually,againcastingitasascientificpaper-thistimetodetermine
thespectralclassofthemysterystar(note“spectralclass”here,you’renotunder
obligationtodeterminetheluminosityclass(giantvs.dwarf)althoughyouarewelcometo
try).
Post-labactivity:Yourgroupmustarrangeatimewithoneoftheclassstafftovisitthe
fiber-fedechellespectrographintheinstrumentationlaboratory.Pointingthefiberoutof
thewindowindaylightwillprovidetheopportunitytoacquireahigh-resolutionspectrum
oftheSun.Appreciatethemulti-ordernatureoftheechellespectrograph.TheTA’swill
helpyouextractoneoftheordersfromthesolarspectrum.Theorderwillbetheone
containingthesodiumDlines.Havingreceivedthisdatayourjobistowavelength
calibratethisoneorderandmakeanestimateoftheresolution,R,ofthisspectrograph.
Identifylinesotherthansodiuminyourorderbyreferringtoahighresolutionsolaratlas
suchashttp://bass2000.obspm.fr/download/solar_spect.pdfandcomparetheextraction
withthesimulatedsolarspectrumprovidedathttp://bass2000.obspm.fr/solar_spect.php.
Workasagrouptoaccomplishthesetasksandthenincludetheresultsasanappendixto
yourindividuallabwriteup.
Questionsandtaskstomotivateyourlabwrite-up(notquestionstobeansweredserially):
Howwellsampledareyourobservedspectrabothinthespatialdirectionaswellasinthe
spectraldirection?
GiventhefocallengthoftheMcCormicktelescope,theinternalopticsofthespectrograph
andthepixelsizeoftheCCD(recallthereferencetothemanufacturer’sspecifications
above)determinethespatialextentofapixelperpendiculartothespectraldirection.
AccountforCCDbinningifyouwereinstructedtouseit.
Whydothespectratendto“flare”towardtheextremeends?
Whatistheresolvingpower,R,ofyourspectrographbasedoncalibrationlampsand/or
airglowlines.
Whichskylinesarenatural(originatingintheEarth’satmosphere–i.e.airglow)andwhich
areartificialbasedonyourskyobservationsatdifferentelevations?
Wavelengthcalibration–identifyspecificlamplinesasafunctionofpixelnumberanduse
atleastasecondorderfittoestablishaformulalinkingpixelnumbertowavelengthin
nanometers.
Canyouidentifytelluric(atmospheric)absorptionlinesinthespectralstandards?These
willbecommonfeaturesbetweenthedifferentspectraandnotonthespectralatlases.Can
youattributeanyofthesespectralfeaturestospecificatomsormolecules?
Afteryouhavecompletedspectralclassificationyoucanlookupthespectraltypesofthe
targetstocomparewithyourresults.Canyoudistinguishanydifferencebetweenthe
dwarfandgiantstars?Shouldyouexpecttobeabletodosobasedonyourresolution?
What do you regard as the greatest source of uncertainty in establishing your spectral
classifications relative to the “official” result.