BFOSCBolognaFaintObjectSpectrograph&Camera

USERMANUAL

R.Gualandi1,R.Merighi1

TECHNICALREPORT

Version2.025-03-2001

1BolognaAstronomicalObservatory-viaRanzani1-40126Bologna-Italy

INDEX

1. Introduction2. Generalities3. InstrumentCharacteristic

1. Optics2. SlitsWheel3. FiltersWheel4. GrismsWheel5. ShutterandFocusingtheCamera

4. Calibrationlamps5. CCD6. ObservingProcedures

1. FocusingandPointing7. Appendix

1. TransmissioncurvesforJohnson-Kron-Cousin:U,B,V,R,Ifilters2. TransmissioncurvesforGunnG,R3. Grismsefficiencycurves4. PlotsofHe-Arlampfordifferentconfigurations

1.INTRODUCTION

BFOSC-BolognaFaintObjectSpectrograph&Camera-isaninstrumentbuittoallow,withasimpleconfigurationchange,theacquisitionofbothimagesandspectra.Atpresent,identicalinstrumentsareinuseatseveralobservatories:CopenhagenAstronomicalObservatory,AsiagoAstronomicalObservatory,AstrophysicalInstituteofAndalusiaandEuropeanSouthernObservatory.

2.GENERALITIES

Theprincipalinstrumentcharacteristicsare:

Collimator-cameraopticalsystemactingasafocalreducer;Parallelbeamarea;Interchangeableslits;Easyinterchangeofgrismsandfilters;Highefficiency.

Theinstrumentisbuildaroundastiffframeholdingthecollimator,thecameraandthedetector.Onitarealsomountesthethreewheelsforslits,filtersandgrisms.ThelastBFOSCopticalelementisthecamera,whichlastlensisthecryostatwindow.

ThedetectorisanEEVLN/1300-EB/1CCDwith1300x1340pixels,ARVisarcoated,backilluminated.

Eachwheelhaseightpositions.Filtersandgrismswheelsaremountedintheparallelbeamarea,betweenthecollimatorandthecamera,whiletheslitswheelismountedonthetelescopefocalplane.BetweenthefiltersandgrismswheelsisinstalledawheelactingasshutterfortheCCD.Theopticalaxisoftheinstrumentistilted110°withrespecttothetelescopeopticalaxis,inordertoreduceinstrumentdimensionsandincreaseitsstiffness.

ABFOSCschemeisshownhere:

3.INSTRUMENTCHARACTERISTICS

Collimatorfocallenght 252.1mmCollimatorlinearfield 52.9x52.9

mmBeamdiameter 31.5mmCamerafocallenght 146.3mmCameralinearfield 30.7x30.7

mmReductionratio 0.58Spectralrange 330-1100nmMaximumspectralresolution

4200

Projectedpixeldimension 0.58arcsec/pix*Fielddimension 13'x12.6'*

*usingEEVCCD.

Incaseofproblemswith theEEVCCDabackupdetector isavailable.ThisCCD,aThomsonxxxx,has1024x1024pixelswithafieldof9.6'x9.6'andasamplingof0.562arcsec/pix.

3.1Optics

OpticshavebeenbuiltusingFK54andUBK7galsses.Thisinordertogiveagoodtransmission(cut-offat360nm)inthe UV. Optics have been coated with a single layer of MgF2, centered to 500 nm, to improve anti-reflectioncharacteristics. Reflection losses are ~ 1.5% at central wavelength and increase to ~ 2.5% at the extremes. Thefollowinggraphshowstheopticstransmission.

3.2SlitsWheel

The slit wheel, mounted on the telescope focal plane, can hold seven different slits, beeing one of the aperturesintentionallyleftemptytoallowdirectimageacquisition.Atpresentfourdifferentslitsareavailable:1.5",2",2.5"and5",aspecial2"slitforechellespectraandamasktobeusedforpolarimetry.Theslitlengthislargeastheusablefieldwhiletheechelleslitis9"long.

3.3FiltersWheel

Liketheslitswheel,alsothefilterswheelhassevenavailablepositions.Atpresentcanbemounted:

U,B,V,R,IJohnson-Kron-CousinfiltersG,R,Z,IThuan-Gunnfilters.

On the filterswheel canalsobemounted thecross-disperser (#10,#11,#12) usedwith theechellegrism#9 toallowmediumdispersionspectroscopy.Adifferentialfilter,usedasorderseparator,ismountedonthefilterwheelinconjuctionwiththegrism#13.TwoHartmannmaskscanalsobemountedtocontrolthecamerafocusing.

3.4GrismsWheel

Theavailablegrismsaremorethanthenumberofpositionsonthewheelitself(7asusual).Letusseeindetailtheircharacteristics:

Grism# Lblaze(nm)

Lgrism(nm)

Dispersion(nm/mm)

DL(nm)

LLEEV(nm)

LLThom.(nm)

L/pixEEV(nm/pix)

3 390 430 17 0.55 330÷642 330÷580 0.274 480 580 22 0.83 380÷470 394÷786* 0.40*5 650 700 22 0.75 480÷980 520÷905 0.406 390 400 11 0.39 330÷535 330÷495 0.177 530 525 11 0.41 420÷600 430÷625 0.108 650 700 8.8 0.30 610÷818 620÷785 0.10

9Ech 17orders 2.6 0.12 350÷1020 335÷94010C.D. 380 390 46 1.7 330÷64011C.D. 520 500 34 1.3 400÷70012C.D. 730 700 91 3.7 530÷102013 510 525 3.6 0.12 495÷560

*Grism#4hasa"freespectralrange"smallerthanthevalueshowninthetable.Infactthetableshowsthefirstorderspectralcoveragebuttherangefreefromsecondorderoverlapendsat700nm.

Grisms#10,#11,and#12arecross-disperserstobeusedmainlywithechellegrism#9andaremountedonthefilterswheel.Theyareoptimizedwithrespecttotheworkingband:grism#10hasapeackefficiencyintheblue,grism#11inthevisibleandgrsim#12inthered.Alsothenumberofordersonthedetectorvarieswiththecross-disperser:grism#10gives13orders,grism#11gives9ordersandgrism#12gives6orders.In the followingfig A are shown the different grisms spectral range and resolution. The point indicate the blazewavelenght.

3.5SHUTTERANDFOCUSINGTHECAMERA

BFOSCshutterhasbeenmadeusingawheel,placedintheparallelbeamarea,thatmovesturning90°eachtime.Onithas been made two apertures allowing the light to reach the detector. The speed of the wheel is electronicallycontrolledinsuchawaytokeepconstanttheshutterspeedrotationduringtheclosingoropeningphase.AccelerationanddecelerationphasesaretimedtoappenoutoftheCCDfieldofview.Forthisreasontherearen'tilluminationeffectduetoshuttermovement.

Exposuretimeshavea0.1secresolutionbuttheminimumexposuretimeis3seconds.

Thecamerais focusedusingtheHartmannmasksmountedonthefilterswheel.Sincethecamerafocuschangewithtemperature,acompensationtablehasbeenprepared.Thecamerafocusprocedureisknownandundercontrolofthetecnicalpersonnel.

4CALIBRATIONSLAMPS

SpectralwavelenghtcalibrationisobtainedthroughexposuresofanFehollow-cathodlampfilledwithHe-AR.

PlotsofthecomparisonspectrumindifferentconfigurationsareshownintheAppendix4.

5CCD

TheCCDmountedonBFOSC isanEEVLN/1300-EB/1back illuminatedandARcoated (Visar).ThedetectorRON is3.06e-/pixandthegainis2.22e-/ADU.NormallytheinstrumenthastheslitorientedinE-Wdirectionbutitispossibletorotatetheinstrumentinanydirection(physicalconstrainstobetakenintoaccount).DispersionisalwaysalongCCDcolumns.

CCD�Q.�E.�curve fig. B

EEVCCDCHARACTERISTICSDetector EEVLN/1300-EB/1Controller PhotometricsSeriesST133B/100KHzand1MHzArray 1300x1340pixelsSpecialFeatures coatingARVisar,backilluminatedQuantumEfficiency 80%@500nm,32%@900nm,>50%@300nm

Pixelsize 20x20micronPixelscale 0.58arsec/pixelFieldofview 13'x12.6'Read-outtime 2sec@1MHz,18sec@100KHz(standardwork:100KHz)Read-outnoise 3.06e-/px@100KHz(standardwork:100KHz)Conversionfactor 2.22e-/ADU@100KHzDynamicalrange 16bitFull-wellcapacity Sensitivitymode:117000elettronsResponsenon-linearity <1%for16bit@100KhzResponsenon-uniformity <±4%overentireccdareaOperatingtemperature -100°CImageprocessingsoftware Winview,onlineOutputdataformat FitsImagedimension 3.5MbyteTypicaldarkcharge 15.1e-/pixel-hourat-97°CLiquidnitrogenholdtime ~12hours

6OBSERVINGPROCEDURES

TheobservercontrolsBFOSCusingaPCthathoststheCCDcontrollerandtheBFOSCcontroller.

TheWinViewCCDcontrolprogramallowstocontroltheCCDimageacquisition,thereal-timeimagedisplayandtheBFOSCcontrol.InparalleltotheWinViewwindowispresentedonthePCscreentheBFOSCstatuswindow,thatgivestotheobservertheupdatedinstrumentconfiguration.FromtheWinViewwindowitispossible,usingtheBFOSCbutton,toaccesstheBFOSCcontrolwindow.

6.1FocusingandPointing

Aspreviouslysaid,BFOSCfocusingismadeusingHartmannmasks.ThisisthecamerafocuswithrespecttotheBFOSCfocalplane.TofocusthetelescopeontheBFOSCfocalplanethesuggestionistoobtain,atthebeginningofthenight,asetofexposuresofastaratdifferenttelescopefocusvalues(centeredonthelastusedfocusvalue)andexaminetheimages using the IRAFprocedure starfocus, on the reductionworkstation in the control room.DONOTCHANGETHEFOCUSUSINGTHECAMERA.Exposuretimesshouldbenotlessthan15-20sec.inordertohaveagoodseeingmeasure.

Inordertomonitorseasonalseeingvariations,observersarerequestedtomeasureseeingduringthenight (possiblyusingtheIRAFpsfmeasurecommand).

7APPENDIX

7.1PlotsforJohnson-Kron-Cousin:U,B,V,R,Ifilters

7.2PlotsforGunn:G,R,I,Zfilters

7.3Efficiencycurvesforgrisms:#3,#4,#5,#6,#7,#8,#9E

7.4PlotsfortheHe-Arcalibrationlampforthedifferentconfigurations:#3,#4,#5,#6,#7,#8,#9E

Suggestedexposuretimesforspectralcalibrationlamp:

#3 30sec#4 7sec#5 6sec#6 50sec#7 50sec#8 8sec#9 8sec

Fig. A

Fig. B

Neareachwheelisshowntheaperturenumber(from1to8).Duringamovement,fromonetoanotherposition,thewheelimagestarttorotateshowingthatBFOSCisinanon-operativestate.Itisnotpossibletostartanexposureifthewheelsaremoving.The number on the right of the camera image is the value of thecamerafocus.The last symbolon thewindowshowsa rapresentationofwhat theCCD should "see" in the current instrument configuration (sky, skybehindaslit,spectrum).Thisinordertohelptheusertounderstandtheinstrumentoperativestate.Thelastimageshowstheshutterstatus(openorclose).

Plot for Johnson-Kron-Cousin filters

Plot for Gunn filters

The tags Slit, Filter, Grism are used to move, using the arrows, the wheels in the desiredconfiguration; the Camera tag allows to change the camera focus and the Shutter tag allows tomanuallycontroltheshutteropeningorclosing.PressingtheSendswitcheachcommandissenttotheinstrument.In thecentralareaof thewindowareplacedthecommandtocontrol the imageacquisition. It ispossibletoselecttheimagetype(bias,dark,ecc.)and,activatingtheFastReadoutoption,toincreasetheCCDreadoutspeed,losinginthiscaseefficiency.Thisoptionisusefullonlytoobtainexposuresforcheckingthetelescopepointing.TheStartbuttondoeswhatissays,thatisstartsanexposurewhichlenghtisshownonthewindowoverit(theexposuretimeisexpressedintenthofsec.).TheStopbuttonwillterminateanexposurebuttheimageacquireduptothatmomentwillbelost.On the right thebuttonConvertidocumento storesondisk theacquired frame inFITS format,allowingthepossiblitytoentertheobjectnameintheFITSheader([ObjectKeyword]).IntheFITSheaderofeachimagewilbestoredthewheelsposition,theJ.D.,U.T.,S.T,RAandDecoftheobject.