texas tech university department of physics & astronomy

TexasTechUniversityDepartmentofPhysics&AstronomyAstronomy2401ObservationalAstronomy

OBSERVINGEXERCISE4-VARIABLESTARPHOTOMETRY

Objectives:Therearetwoprincipleobjectivesforthisobservingsession.(a) Toobtainatleast120minutesofCCDimagesofavariablestar.(b) TogainmoreexperienceinusingaCCDandcomputerizedtelescope.

Procedure:Fortonight’sobservinglab,youwillbeacquiringimagesofavariablestarforlateranalysis.Althoughthevariablestarhasashortperiod,youwillstillneedtogetatleast2hoursofimagesduringthelab,andlongerwouldbebetter.Sinceyouwillneedatleast20minutesatthestartofthelabtoobtainyourcalibrationframes,thiswillmeanthatyouwillhavetobeverypromptingettingyourtelescope/CCDcombinationoperational.Thevariablestaryouwillbeimagingwillbeoneofthoselistedbelow:

Star DYPeg CYAqr GPAnd BLCam V959OphBeforeyoustartimagingthestar,youneedtotakeanumberofimagesofthetwilightskyforflatfielding.Todothis,youneedtoaimthetelescopetowardsthewestwheretheskyisstillbrightwithtwilight.Movethetelescopetothisregionandtakea0.05–0.1secondtestimagedependingonhowbrighttheskyis.Onceyouhavethisimage,checkthepixelvaluenearthecenter.Itshouldbesomewherebetween10,000and35,000.Ifitisoutsidethisrange,youneedtoeitherincreaseordecreasetheexposuredependingonifitisaboveorbelowtherange.Alsocheckforanybrightstarsthatmightbevisibleintheimage.Ifthereareany,youwillneedtomovethetelescopesothattheyarenotinview.Onceyouareset,take20images,dependingonthelengthofyourexposures,andfollowthoseupwith10darkframesofequalexposure.Onceyouhavetakentheseimages,Take10darkframesofthesamedurationasyourimages.Makesureyounamethedarksdifferentlysothatyoucantellwhicharefortheflatfieldsandwhichareforthestarimages.Whileyouaretakingyourdarkframes,movethetelescopetoAltairandgetreadytosynchronizethetelescope.Oneyourdarksarefinished,synchronizethetelescopeandthenslewtoyourobject.

Use“TheSky”topointthetelescopeatyourstar.Tolocateyourstar,usethe“Find”icon,andthenclickon“Stellar”andthen“GCVS”.Thiswillbringupaconstellationlist.Scrolldownthelistuntilyoufindtheconstellationtowhichyourstarbelongs.e.g.;ifyouaretryingtofindavariablestarinAries,youwouldscrolldownuntilyoufindtheconstellation“Ari”.(TheofficialabbreviationforAries)Onceyouhavefoundyourconstellation,clickonthe“+”signbesideit.Thiswilldisplayalistofallthevariablestarsinthatconstellation.Scrolldownthislistuntilyoucometoyourstar,thenclick“Find”.Nowyouwillbeabletoslewthetelescopetothestarasperusual.UsethefinderchartincludedinthislabforyourselectedstarandmakesuretheTelescopeispointedatthecorrectfield.Thefinderchartsareat26’by26’andtheredboxcorrespondstothe16’by16’CCDFOVforthe12-inchwiththefocalreducerattached.OnceyouhavethestarcenteredintheCCDandyouhaveasuitableguidestarintheguider,starttakingimages.Sincethestarsaregenerallyquitebright,theexposuresrequiredwillberelativelyshort.Also,asfaraspossible,trytoensurethattherearenoverybrightstarsinthefieldwhenyouareimaging.Theapproximateexposuresforeachvariablestararegiveninthetablebelow.

Star DYPeg CYAqr GPAnd BLCam V959OphExposure 20seconds 20seconds 20seconds 30seconds 30seconds

Onceyouareready,starttakingimagesofthestar.DonotforgettomakesuretheAutoSaveboxischecked!Keeptakingimagesforatleast120minutes,oruntil10:45pm,whichevercomeslater.Whenyouarefinishedwiththedarkframes,disconnecttheCCDandclose“CCDSoft”and“TheSky”.Thencopytheimagesontoyourmemorystick.

FindingCharts