Observation Lab 1 (Beta Version) Getting around the sky...

Astronomy3130Spring2017ObservationLab1(BetaVersion)Gettingaroundtheskyandobservingwiththe6”doghousetelescopeAsanobserver,aprimaryskillisgettingyourtelescopepointedtoanappropriatetargetandcenteringitinyoureyepiece/instrument.Inthemodernworldthetelescopeandcontrolsystemtakescareofvirtuallyallofthisoverheadandyourtargetmagicallyappearsclosetothecenterofyourinstrumentfieldofview.Tosomeextentthislabisaboutgaininganappreciationforwhathappens“behindthecurtain”asyouassumetheroleofthetelescopecontrolcomputer.ThelabisalsoorientedtowardgivingyoufamiliaritywiththecelestialcoordinatesystemofRightAscensionandDeclinationandpositioninganequatorialmounttelescopeontheskyusingthatcoordinatesystem.Allactivitiesheremakeuseofthe6”refractingtelescopeinthe“doghouse”atMcCormickObservatory.TheTA(s)willbeyourguidetoproperuseofthefacility.Firstsomegeneralsactivities/tasks:1)Measure(onlyasaccuratelyasgreatcarefortheglasswillallow)theobjective(mainlens)diameterofthedoghousetelescope.Notouchingtheglass.Yes,itisa6”telescope,butisthatreallytheclearaperture?Inthisclass,ifyoumakeameasurementbesuretoattachawell-reasoned(ormeasured)uncertaintywhereappropriate.2)Measure,crudely,thefocallengthofthetelescopebymeasuringthedistancefromtheobjectivetotheeyepiece.3)Catalogthefocallengthsoftheeyepiecesintheeyepiecebox.Foreacheyepiececalculatethemagnificationgiventhefocallengthofthetelescope.(Basictrickforlater:Acquireyourtargetinthewidestfield(typicallylowestmagnification)eyepiecethatyouhave,centeritinthateyepiece,andthenputinhighermagnificationifyouwantit.)4)SetR.A.sothetelescopeisonthemeridian.Usetheleveltogetthetelescopepointedtothehorizonasexactlyasyoucan.Measurethedeclinationreportedonthedeclinationwheel(orusethedeclinationvernier/magnifierifyouaresoinclined).Fromthismeasurementyoushouldbeabletoderiveyourlatitudepresumingthetelescopeisproperlyinstalled.Makethemeasurementtoaprecisionof0.1degreeasbestyoucan.5)Usetheaboveprocessformeasuringofyourlatitudetoexplorethestatisticalnatureoftheobservation.Haveeachpartnerinyourlabgroupmakethemeasurement(re-levelingthetelescopeeachtime).Cyclethroughthegroupthreetimesforatotalof12-15measurements.Lookatthedistributionofanswers.Whatisthestandarddeviationoftheensembleofall12?Arethereindividualswhoseresultsstandoutasbiased?6)Identifythepolaraxisofthetelescope.IsitpointedattheNorthStarandthusparalleltotheearth'srotationaxis?7)Setthedeclinationofthetelescopeto0.ReleasetheRAclampandusethetelescopetotracethecelestialequatoracrossthesky.Whereisdueeast?west?a)Setthedeclinationto-23.5–tracethewinterSun'spathandrisingandsettingpointsin

thesameway.b)DothesameforthesummerSunat+23.5.8)Installthelowestpowereyepiece.Identifythefindertelescope.Scanaroundabitcomparingqualitativelytheviewinthefinderandinthemaineyepiece.Findabrightstarabitsouthofthecelestialequatorandneartransit(maybeRigel?).Try“eyeballing”thestarbyliningupthestarwiththeedgeofthetelescope.Canyougetthingscloseenoughwiththistechniquetoacquirethestarinthefinder?inthemaineyepiece?9)Setthehourangle/RAdialsothatitreadshourangle(readszerowhenthetelescopeisonthemeridian).FixthetelescopedeclinationtothedeclinationofBetelgeuse.CheckthesiderealtimeandcalculatethehourangleforBetelgeuse.Pointthetelescopetothishourangle.Wereyouclose?Usethe“setdecandscaninRA”techniquetoacquirethestarandcenteritinthefinder.10)Measurethefieldofviewofthefinder.CenterBetelgeuseinthefinderandreadthedeclination(whichshouldbeclosetothedeclinationofBetelgeuse).MovethetelescopeindeclinationonlyuntilBetelgeuseisattheedgeofthefinderfieldofview–measurethedeclination.Dothesamefortheotheredgeofthefieldofviewandusethedeclinationreadingstodeterminethesizeofthefinderfield.11)ThetablebelowcontainstheR.A.andDecforthe“hands”and“feet”ofOrion.YoushouldalreadybepointedtowardBetelgeuse.SettheadjustableR.A.wheeltoreadtheproperR.A.asopposedtohourangle.InturnpointbycoordinatestoeachoftheotherthreestarsreturningtoBetelgueselettingeachmemberofthegrouphaveatry(ortwobygoingthroughthesequencemorethanonce).Afterfirstsettingbycoordinatesforeachstar,notehowcloseyoucametocenteringthestarinthefinder.Fromtheenvelopeoftheseobservations,howgoodwasyourbasicabilitytousecoordinatestogetthestarcentered?Weresomemembersofyourgroupbetteratitthanothers?Ifso,whatweretheydoingtogetbetterresults?Considerthefollowingstepsasdefiningtwoexperimentsworthyofbeingwrittenupasascientificpaper(thestandardformatforpresentingyourobservationalconclusionsfromtheseandfutureobservationalexercises).Youcanincludethemorerotemeasurementsaboveasan“appendix”toyourpaper.Thosetwoexperimentsare:

1)Demonstratethateyepiecefieldofviewisinverselyproportionaltomagnification.2)Demonstratethattheeffectivedriftrateofstarsonthecelestialspheredecreasestowardthepolewiththecosineofthedeclination.

1)Selecttwoofthehigherpowereyepiecesandtestwhetherthefieldofviewisinverselyproportionaltothemagnificationbytimingthedriftofastaracrosstheeyepiecewiththeclockdriveturnedoff.CenterBetelgeuseinalowpowereyepieceandfocusthetelescope.Switchtooneofthetwohighpoweredeyepiecesofyourchoice.OffsetthetelescopeinRAsothatthestarisattheeasternedgeofthefieldofview.Turnofftheclockdriveandtimehowlongittakesthestartomovetothewesternedgeoftheeyepiece(itwilltaketwopeople,anobserverandatimer,

andsomecoordinationtomakethismeasurement).Lateronconvertthistimetoarcsecondsknowingthatthestarsshoulddriftatarateof15arseconds/second*cos(declination).Thedrifttimeshouldbeshortinahighpowereyepiecesoitshouldbepossibleforeachgroupmembertomake3-4observationsofthedrifttimeandgetsomestatisticalestimateoftheuncertainty.Repeateverythingforthesecondeyepieceandshowthatthefieldofviewscalesasexpected.2)Performthesamedriftexperiment,thistimeforthreestarsatdifferentdeclination–Betelgeuseandtwoothers(convenientlyyouselectedthosetwootherstarsaspartofyourpre-lab).Fortheseobservationsyouwilluseavideo-rateelectroniccameraanddisplayinplaceoftheeyepiece.Youmayneedtoacquireeachstarvisuallyinalowpowereyepiecefirsttogetthingscenteredbeforeinsertingtheelectronic“eyepiece”ifthefinderisnotwellenoughaligned.Sincethisexperimentislookingathowthingsscale(i.e.aproportionality)withcos(declination)youcangetawaywithnotknowingtheabsolutefieldofviewoftheelectronicimager.Youwillbetimingthepassageofastarbetweentwofiducialmarksonthescreenwiththetelescopedriveturnedoff.Itwillbetheratioofthesetimingsthatwillenableyoutoverifythecos(declination)effect.Ifyoudowanttocalibratethescale(andwhowouldn’t)youcanpointtheelectroniccameraattheTrapeziumCluster(ThetaOrionis)inOrionandusethefinderchartbelowandarulertoestablishthescreenscaleinarcsecondspermillimeter.IfyoucrankuptheintegrationtimeontheelectroniccamerayoumightgetanenhancedviewoftheOrionNebulaandpickupstarstoofainttoseeintheeyepiece.ThetablebelowidentifiesthefainterstarsintheTrapeziumfield(andcomesfromhttp://www.astropix.com/agds/samples/sample.html).Maketwomarksontheacetate(notthescreenitself!!!!)withthewhiteboardmarkerandtimehowlongittakeseachofyourthreetargetstodriftfromonemarktotheother(youmightwanttotakealookatthedriftratebeforemakingthemarkssothatyoucanselectaspacingthatisnottooshort(leadingtopoortiming)ortoolong(leadingtoaverylongnight)).Asbefore,multiplemeasurementsshouldpermityoutoplaceempiricaluncertaintiesonyourvalues.Star Name R.A. (J2000.0) Declination

Betelgeuse (α Orionis) 05:55:10.3 07:24:25

Bellatrix (γ Orionis) 05:25:07.9 06:20:59

Rigel (β Orionis) 05:14:32.3 -08:12:06

Saiph (κ Orionis) 05:47:45.4 -09:40:11

The video-rate electronic camera and its control paddle. The knob at the top of the paddle controls the exposure time. “6” on the “high” side is the shortest exposure with exposure time increasing as the dial rotates counterclockwise. If the frame is completely white, the integration time is too long – rotate the dial counterclockwise.

Looking into the imager you can see the light sensitive detector. Whatever scene the telescope objective casts onto this surface will appear on the TV display (if you are having trouble seeing the camera output on the monitor, press the “video” button on the display until the right mode comes up).