snr-qso with espadons: tests, impacts, and...
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SNR-QSOwithESPaDOnS:Tests,impacts,andfeasibilityClaireMoutou,January272016,updateAugust182016The scope of the document is to review the tests of the SNR-QSO mode withESPaDOnS (ESP) in January2016, to reviewapplicability for thismodeaswell asfollowuponitsperformanceonsubsequent2016data.
1. PreliminarynotesIn January 2016, the ESP control commandsweremodified in order to take intoaccount a goal SNR and to stop the exposure once the SNR calculated by theexposuremeter has achieved the requested goal. The exposuremeter SNR value iscalculatedfromthereceivedfluxpersecondandcalibratedintoSNRat730nm;thiscalibrationincludesanecessarycorrectionforthespectrograph(air)temperature,(seesec.5).Thetransmissionevolutionduetomirrordegradationwithtimefromaluminizationisnottakenintoaccount,asfortheETC.Notethatforexposuretimes<15s,theSNRfromtheexposuremeterislessaccurateand underestimates the real SNR –this should not be used directly to avoidsaturatingspectra.InPolarmode,eachexposureofasequenceistreatedindependentlyandthusthe4exposuretimeshavedifferentexposuretimes.Changingtheindividualexposuretimes in a polar sequence has an effect on the heliocentric correction of eachexposure,and thus,on the finalvelocityprofileof thespectrum.At theverysmalllevelsofpolarizationthatESPrecords,thisimpactcancreateanunwantedspectralshift and spurious polarization detections. The main goal of the tests done inJanuarywastoquantifythisimpact.
2. TestdescriptionThefirsttest(Jan22,2016)showedthatthe‘go’commandexecutesasexpected,inboth modes: when the SNR goal is not achieved before the time set in theInstrumentconfiguration,thentheexposurestopsnormallyatthisvalue;otherwiseit stops at the goal SNR value. In Polar mode, the four exposures have slightlydifferentexposuretimes,andweverifiedthatthisdoesnotcauseanyproblemforthequicklooknorthepipeline.ThesecondtestwastoinsertaSNR-modeflaginPH2,andcopytheSNRgoalgiveninPH2intothe‘go’command.ThiswasexecutedsuccessfullyonJan25,2016.Thethirdtest(jan25and26)wastoevaluatethelimitswhereSNRmodeshouldbeapplied.WecomparethesespectrawiththesamestarobservedwithsameexposuretimesandvaryingSNRandwithsimilarexptimesandSNRs.Tomimicthechangeofexposure time in a polarimetric sequence, we defocused the telescope during a
polarizationsequenceinSNRmode,inordertohaveequalSNRacrossthesequenceanddifferentexposuretimes.
3. ResultsJan25testID-OG#odometer
Texpmax[s]
SNRgoal Exp1 Exp2 Exp3 Exp4 Comments
E93-131889425p
60 950 Ef50740SNR695Texp60
Ef48440SNR683Texp60
Ef48324SNR676Texp60
Ef51111SNR695Texp60
Reference0SNRgoalnotachieved
E97-31889429p
45 - Ef49047SNR599Texp45
Ef50848SNR602Texp45
Ef49855SNR597Texp45
Ef50092SNR598Texp5
Reference1noSNRgoal
E93-151889441p
60 500 Ef52221SNR501Texp32.4
Ef43292SNR501Texp37.0
Ef22723SNR467Texp60
Ef4094SNR258Texp60
SNR-modewithdefocus
Foreachspectrum,themeanstellar(LSD)profilewascalculated.Inred,theSNR-modeprofileminustheReference0profile.Inwhite,forcomparison,asubtractionoftheprofilesofbothreferencespectra.Left:intensity,right:polarizationStokesV.Theplotshavebeenshiftedforclarity.Notethescale:welookatthe10-4level!ConclusionofJan25:Whenthefluxvariationsareextreme(facteur~15)duringapolarsequence,spurioussignalsareindeedproduced.Moredatawithmildervariationsareneeded.ASNR-modesequencew/ofluxvariationsisalsomissing.
Reference0-SNRmodewithextremefluxvariations
Reference0–Reference1
Jan26testID-OG#odometer
Texpmax[s]
SNRgoal Exp1 Exp2 Exp3 Exp4 Comments
E93-21889635p set1
180 300 Ef8358SNR300Texp67.5
Ef8301SNR303Texp69
Ef8370SNR300Texp68.1
Ef8516SNR301Texp66.8
SNRmodenormal3%varetime
E93-21889639p set2
180 300 Ef9118SNR301Texp62.5
Ef9012SNR302Texp63.8
Ef8581SNR301Texp65.2
Ef8697SNR304Texp65.6
SNRmode5%varetime
E93-21889643p set3
180 300 Ef8208SNR303Texp70.5
Ef8710SNR300Texp65.2
Ef9340SNR302Texp61.3
Ef8709SNR302Texp64.3
SNRmode14%varetime
E93-21889647p set4
180 300 Ef6919SNR301Texp82.3
Ef8390SNR300Texp67.8
Ef5812SNR301Texp96.4
Ef7824SNR301Texp73.1
SNRmode35%varetime
E93-21889651p set5
180 300 Ef7326SNR301Texp77.2
Ef8418SNR300Texp67.7
Ef5904SNR301Texp96.6
Ef7408SNR301Texp77.2
SNRmode35%varetime
E93-21889655p set6
180 300 Ef8606SNR302Texp66.1
Ef8943SNR300Texp63.4
Ef8636SNR302Texp66.6
Ef7857SNR301Texp71.9
SNRmode17%varetime
E93-21889659p set7
180 300 Ef7950SNR301Texp69.9
Ef7720SNR301Texp74
Ef8539SNR301Texp65.8
Ef7718SNR301Texp73.6
SNRmode11%varetime
E93-161889663p set8
180 - Ef8464SNR499Texp180
Ef7934SNR482Texp180
Ef8572SNR502Texp180
Ef8565SNR502Texp180
Etimemodereference
Exposuremeterplotsoftheflux/secondforset4(above)andreferenceset8(below).
ComparisonofintensityandpolarLSDprofilesforset1(SNRmodeinstableconditions),set4(SNRmodeinvariableconditions)andset8(etimemode).Nospuriousdetectioninpolarisseen.Smalldepthdifferencesinintensityspectra,probablyduetothedifferenceofSNRbetweenset8(~500)andothers(~300).Noneofthesetswithmodifiedfluxwithinthesequenceproducesaspurioussignalinpolarization.Thereisalwaysasmalldifferenceinintensityprofiledepth,butthishasnoconsequence.Conclusion:evenwith35%variationsinthefluxandexposuretimesduringapolarizationsequence,thereisnoappearanceofspuriouspolarsignals.ItisveryencouragingandmeanswecansafelyusetheSNRmodeinmostclearconditions,whenfluxvariationsonlyarisefromtheunavoidablecenteringand/orslightturbulencefluctuations.IwouldnotrecommendusingSNRmodeinpolarwhenfluxvariationsarelargerthan~30%overthesequence,tobesafe.
4. ApplicabilityandlimitsofESPSNR-QSO
- SNR-QSOmodecanalwaysbeappliedinStar+SkyandStar-Onlymodes,aslongasaSNRgoalisgivenbythePI.Thiscouldbethedefaultmodeinspectroscopy.Onecaveatisthecaseofvariablestars,orstarsofill-knowntemperatures,forwhichitwouldbebettertoslightlyincreaseratherthandecreasethee-timeinrealtime.ThesecasesarenotdealtwithinthecurrentsoftwareandweshouldurgePIstoputsomemargin(ie,higherSNRgoals)iftheyhavedoubts.
- SNR-QSOcanbeused(ascoded,with4differentexposuretimes)inthePolarmodewhenconditionsareclearandforcertaintypesofstars.
- Unsuitablestarsmaybespectroscopicbinaries,fastrotators,extremevariables.SothePIshouldhavethepossibilitytooptoutofSNRmode.
set1-set8:SNRmodecomparedtoetimemodeset4-set8:SNRmodewith35%etime/fluxvariationscomparedtoetimemode
- UnsuitableconditionsforSNRmodeincludecloudynightswherethefluxmayvarybyafactor>1.3duringasequence.Thesemaybebaddataanywayforspectropolarimetrysincethephotocenterofexposuresvaries.
- TheuseofSNRQSOisthusacombinationofPIoptionandskyconditions.Itshouldbepossiblefortheobserver(orthesoftware)tobypassthePIwillingnesstouseSNRmodeifconditionsarenotgoodenough,resultinginverydifferentexposuretimesinapolarsequence.WeshouldavoidsituationswheresomeexposuresofapolarsequencearestoppedbyetimeandothersbySNR.
- EnforcingtheuseoftheSNRmodeshouldbedoneatPH2level,sinceweneedpreciseinformation.Requiredparametersare:magnitude,Teff(FT),SNRgoal(OB),maximumexposuretimeperexposure(inIC).DefaultvaluescanbechosenforSNR_wave,IQlimit(verylargeimpact),Airmasslimit(verylowimpact),extinctionlimit(verylargeimpact),sincewetryobservinginanyconditionswithESP.Weneedtochecktheconsistencyofthesenumbersbeforehand,forinstancewhensavingtheOBsortheOGsandwithawebapplication.
- Technicalimplication:weneedamechanismtoconstantlyvalidatetheexposuremetercalibrationforSNR.Itcanbedonedailyoncalibrations,whentheexposuretimeexceeds15s.Ifpossible,theopticalalignmentoftheexposuremetershouldnotbetouched.
5. Exposuremetercalibration(update)
Verificationoftheexposuremetercalibrationanditsdependencywiththetemperatureinthespectrograph.SNRmeasuredbythepipelineat730nmasafunctionoftheSNRcalculatedinrealtimebytheexposuremeter,inpolarmode.Inwhite,alldatafrom2015whenexposuretimeis>10s.Inred,themostrecentdata,fromDecember2015toJanuary2016.Ontheright,alineardependencywithESPtemperatureiscorrectedforandthescatterisreduced.Thereisnotemperaturedependencyinthespectroscopicmode.
Itseemsthetemperaturecorrectioncouldbeadjusted–stillsomescatterinhighSNRregime.MostrecentdatamayrepresentdifferentthermalconditionssincetheinstallationoftheGRACESwalls,whichcouldexplaintheneedfora(minor)adjustment.TheSNRcalculatedforthelongcomparisonexposuresdoneeachdayofanESPrunatleastoncecanbeusedtocheckthereliabilityoftheexposuremeteralignment,especiallyincasesomethingismovedinsidethespectrograph.SNR_expo=705+-20inpolarmodeon60sHCexposuresSNR_expo=570+-20inspectro-SSmodeon60sHCexposuresTheplotbelowshowsthebehaviorofthisSNRasafunctionoftemperature[valuesbetween16-17ocorrespondtotherunQ13-Nov23toDec03,temperaturemainlydecreasingduringtherun;valuesbetween13-15ocorrespondtotheJanuary2016run,temperatureincreasingduringtherun].Thereisnoobvioustrendwiththisparameter.
Conclusion:theSNRmeasuredbytheexposuremeterdailyduringarunarefairlystableandcanbeusedtocheckthevalidityoftheexposuremeterpositionandcalibration.Ifmoreprecisionisneeded,aprocedureinvolvingtheflatfieldlampandexposures>15scouldbeputinplace,withafrequentmonitoring.Thatmaynotbeneeded,butweneedtokeepaneyeonthecomparison60sexposureduringrunsandatcheckout.IfSNRdropsto<500(polar)or<350(spectro),egthefluxdropsbyafactor2,awarningshouldbesenttotheRemoteObserverandSNR-QSOshouldbeputonholduntilitisinvestigated.
6. ReportonSNRmodeperformancefromFeb2016tomidAugust2016Timegain:16A11.5h,16B5.7h/16A391h,16B95.97hrequested(3%,6%)TimegainonprogramsthatrequestedSNRmode:-inPOL:16A5.3h,16B1.8h/16A11.7h,16B7.87hvalidated(45%,23%)-inS+S:16A5.8h,16B3.2h/16A11.6h,16B10.74hvalidated(50%,30%)ComparisonoftherequestedandmeasuredSNR:top)atthewavelengthrequestedbythePI,andcomparedtotheLEpipelinemeasurement,bottom)atthedefaultwavelength(730nm)andcomparedtotheexposuremeterSNRestimate(alignedbyconstruction).Thegreenandredlinesshow20%variationinSNR.Onlyvalidatedexposuresareshown.
Diamonds=polarmodeCrosses=Star+Skyspectro
16B
16B
16A
16A