snr-qso with espadons: tests, impacts, and...

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



SNRmodenormal3%varetime

E93-21889639p set2

180 300 Ef9118SNR301Texp62.5




SNRmode5%varetime

E93-21889643p set3

180 300 Ef8208SNR303Texp70.5




SNRmode14%varetime

E93-21889647p set4

180 300 Ef6919SNR301Texp82.3




SNRmode35%varetime

E93-21889651p set5

180 300 Ef7326SNR301Texp77.2




SNRmode35%varetime

E93-21889655p set6

180 300 Ef8606SNR302Texp66.1




SNRmode17%varetime

E93-21889659p set7

180 300 Ef7950SNR301Texp69.9

Ef7720SNR301Texp74



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

snr-qso with espadons: tests, impacts, and...

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