Adv. Space Res. Vol 11,No. 11 pp. (1 1)217—(11)220, 1991 0273—1177/91$0.00+.50Printedin GreatBritain. All rightsrreeived. Copyright© 1991 COSPAR

THE BERKELEY EUY SPECTROMETERFOR

ORFEUS

M. Hurwitz* andS. Bowyer**

nterforE~~I’Astrophysic;Universityof California, Berkeley,CA 94720,U.S.A.Centerfor EUVAstrophysicsandAstronomyDepartment,Universityof Caljfornia,

Berkeley,CA 94720, U.S.A.

ABSTRACT

The Space Astrophysics Group is currently fabricating a novel EUV spectrometerfor theORFEUS-SPASmission. It usesa set of four varied line-spacesphericaldiffraction gratings toobtain0high resolution (AJ,~l.>7000 ) spectraof point sourcesat wavelengthsbetween390 and1200 A. The spectraare recordedwith two detectorunits, eachcontainingcurvedsurfacemicro-channelplatesand a delay-lineanodereadoutsystem. An independentoptical systemdetectstheimageof the sourcein the entranceapertureandtracksthe sourceas it drifts during an observation,enabling a reconstructionof the spectrapostflight. The overall systemperformanceis discussedandillustratedby syntheticspectra.

INTRODUCTION

ORFEUS-SPAS,the Orbiting RetrievableFar andExtremeUltraviolet Spectrometerson the Astro-SPAS spaceplatform, is ajoint projectof NASA and DARA in the FederalRepublicof Germany/1/. The BerkeleyEUV spectrographfor this mission will provide simultaneouscoverageof the390—1200A bandwith aspectralresolution(A/A?~)in excessof 7000 for pointsources. Theinstru-ment incorporatesa set of four novel, spherically figured, variable line-space(SVLS) diffractiongratings to spanthe large wavelengthinterval. Two spectraldetectorunits containing sphericallycurvedmicrochannelplates(MCPs) and delay-lineanodereadoutsystemsencodethe arriving pho-tons in digital format for telemetry. An additional optic directs the imageof the source in theentranceapertureonto a sealedFUV detector. This fine guidancesystemis usedfor coalignmentof the telescopeand star tracker optical axes andfor tracking of targetsas they drift within theentranceapertureduring an observation.

THE BERKELEY EUV SPECTROMETER

In Figure 1 we show aschematiclayout of the opticalcomponentsof the spectrograph.Theinstru-ment containsfour diffraction gratings, arrangedside by side such that each is illuminated by a“wedge” on the annularapertureof the telescopeprimary mirror. Eachgrating uses—20% of theavailablegeometricalareaof the 1 m diameterprimary andhas a focal ratio of about f15 (ratherthan the full f12.4 from the primary mirror). An individual grating dispersesonly 1/4 of the total390—1200A bandpassandoperatesover a relatively small rangeof diffraction angle~3,allowinghigh diffraction efIlciency to be maintained. Two spectraldetectorseach acceptthe spectrafromtwo adjacentdiffraction gratings.

Eachdiffraction gratingis spherical,with a 1 m radiusof curvature. Unlike the conventionalRow-landspectrograph,in which the groovespacingis uniform, the groovespacingon our optics variesas a fourth orderfunction of position on the optic. Furthermore,the distancesfrom the telescopeimage to thegratingcenter,andfrom the grating centerto the detectorsurface,arenot equalto thevalues on the correspondingRowlandcircle. The astigmatismandspectralaberrationsof this spec-trograph are substantiallylower than what would be achievable with a conventionalRowland

J&SR 11:11—0

(11)217

(11)218 M. Hurwitz and S. Bowyer

spectrographanduniform line-spacediffractiongratings, whethersphericalor toroidal. A more

SVLSdetaileddescriptionof SVLS spectrometerscanbe found in /2/. Two MCP spectral detectors

(~l___~ThRATINGare used. The MCPs are spherically curved,with a radius chosento provide a closematchto the tangentialfocal surfaceof the diffractiongratings. A flat detector would have limitedtheresolutionto —1500. Eachdetectorcontainsa chevronpair of MCPs, the electroncharge

FOR FINE GUIDANCEclouds from which strike a delay-line anode OFF AXIS ELLIPSOID

readoutsystem/3/.

The fine guidancesystemconsistsof a smalloff-axis ellipsoid,which interceptsa fraction of

,._TELESCOPEthe beamfrom the primary mirror andimagesFOCUSthe target onto a sealed-tubeMCP detector

equippedwith a wedge-and-stripreadoutsys-DETECTORtem /4/. The ellipsoid is coatedwith Al/MgF2. ~FINE GUIDANCEThe detectoris sealedwith aBaF2window and

.._SPECTRALDETECTOR

containsa CsI photocathode,so the systemismost sensitiveto photonsin the 1400—1800Aband. Each pixel on the detector representsabout3” on the sky. To determinethe locationof the sourceto a precisionof about 1”, wewill place the detector so that the image isdefocusedto cover — 3 pixels and calculatea Fig. 1. Schematiclayoutof the BerkeleyEUVcentroidof the photonsrecordedduring a given spectrograph.integration period. The integration period isdeterminedby the expecteddrift rate of the target in the aperture;during one secondthe targetshould move no more than about 1”. Even a relatively faint target such as the hot white dwarfHZ43 shouldproduceover 100 events s~on the detector,enablinga centroid to be calculatedtoan accuracybetter than 1”.

INSTRUMENTPERFORMANCE

The telescopeprimary mirror will be III I II

coatedwith iridium, a stablematerialwhich providesgood normal-incidencereflectivity across the instrument ‘° -

bandpass.The diffraction gratingswillbe coatedwith the same metal. We -

have measuredthe groove efficiency çof a 6000 lines nim

1 sample gratingruled on the sameengineas the flight ~gratings and find that the rulingsachieve —55% of the theoretical G

efficiency /5/. We expect that thequantumdetection efficiency will beequal to that measuredfor the stable 2 -

photocathodeKBr by Siegmundet al./6/. The resulting curve of effective

1 I~ III ~ I)area versus wavelength is shown in ~ 800 000 1000 1100 1200W,veI,ngth (A)Figure 2.

Fig. 2. Predictedeffective area of the instrument vs.Prior to the launch of ORFEUS in wavelength. Fabricated instrument will display small1992, NASA will have flown two discontinuitiesat crossoverwavelengthsbetweengratingsother major instrumentswith spectros- due to variations in diffraction efficiency and coatingcopic capabilities in the EUV. These reflectivity.

TheBerkeleyEUV Spectrometerfor ORFEUS (11)219

are HUT 171 andEUVE181. Although therelativesensitivity of theseinstrumentsdependson quan-tities that may be difficult to estimate(e.g., the detectorbackgroundrate), for relatively brightsourcesit is the effective area that limits the achievable signal-to-noise,so it is the effectivearea—andspectralresolution—thatwecomparehere.

HUT containsanormal-incidencemirror which is only slightly smallerthanthat of ORFEUS. Thespectrographdesignis a classicalRowlandmount. At wavelengthsbetween840 and 1580 A, theHUT grating is usedin first order andshould achievea net effective areathat is —1.5 timesthat ofORFEUS. Between420 and925 A, the HUT grating is usedin secondorderandcan achieveaneffective area roughly equal to that of ORFEUS. But becausemanytargetswill be substantiallyfainter in that bandthanat first-orderwavelengths,an aluminum filter will certainlybe neededforcontinuum measurementsto attenuatethe first-orderradiation; it may well be neededalsofor stu-dies of line radiation. This filter reducestheHUT effective areaat the shortwavelengthsto 1/6 to1/10 that of ORFEUS,dependingon wavelength. Thespectralresolutionof HUT is —400, which is— 1/20 that of ORFEUS. The EL/YE spectrometeroverlapsthe ORFEUSbandpassbelow —7.60 A.Its effective area is 1/4 to 1/20 that of ORFEUSat overlapwavelengths.The resolution of theEUVE spectrographis about275, roughly 1/30that of ORFEUS.

To demonstratethe powerof this instrument,we havesimulatedthe spectraof a sourcethat is alikely candidatefor observationduring the ORFEUSmission:ahot whitedwarf. BecauseORFEUSwill provideuniquedata aboutthe interstellarmedium as well as thepoint sourcesthemselves,wehave also estimatedthe sensitivity of the instrumentto detectionof absorption line features. InFigure 3 we show a simulatedspectrumof the hot white dwarf HZ43 as would be observedwiththis instrument.

150cr—1——111111 III

1400 — —

1000 — -

I::: -

~600- -

______________ I

400 500 600 700 800 000 1000 1100 1200Wavol,ngth (A)

Fig. 3. Samplespectrumof hot white dwarfHZ43. Countsper spectralbin for a3600 s integrationare plotted vs. wavelength. Column densityof H I assumedto be 2 x 1011 cm

2 column of He Iassumedto be 1/10 that of H I. Absorption edgesof interstellarH i and He I are.shown, as arestrong resonancelines of thesespecies. Becausethe spectralresolutionchangesdiscontinuouslyatgrating crossoverwavelengths,the simulatedcounts per spectral bin display those discontinuities(markedwith an arrow).

We display the background-subtractedphoton counts per spectral resolution element versuswavelengthfor an integrationtime of 3600 s. Discontinuitiesin the spectrumoccurat wavelengthbreaksbetweenthe diffraction gratings and also at the absorptionedgesof neutral hydrogenandneutral helium. Absorption lines due to the strongestresonancetransitionsof interstellarneutralhydrogenand neutral helium are shown. Thesestrong absorptionfeaturesare detectedat a highlevel of significance;it is of interestto evaluatewhat equivalentwidth an absorptionfeaturewouldhave that would be detectedin this spectrumat some threshold,e.g., 3 cr significance.We show

(11)220 M. Hurwitzand S.Bowyer

this minimum detectableequivalentwidth in Figure4. Giventhe measuredcolumn densityof H ItowardsHZ43, weexpectto detectline featuresof manyelementsin a variety of ionization states,greatlyimproving our understandingof the local interstellarmedium. Although only a few sourcesare expectedto be asEUV-bright as HZ43, with longerintegrationtimes it will be possibleto per-form similar studieson a variety of white dwarf targets. Other sourcesweplanto observeincludestarsfrom many regionsof theH—R diagram,.interactingbinaries,andothercelestialobjects.

6C

50 — —

*II~400 500 600 700 800 900 5000 5100 1200

- Wavelength (A)

Fig. 4. Minimum equivalentwidth (mA) for 3 a detectionof an absorptionline in the spectrum

shownin Figure3 vs. wavelength.

ACKNOWLEDGMENTS

This researchwassupportedin part by NASA grantNGR—003--450.

REFERENCES

1. M. Grewing,TheORFEUSmission,this volume (1990).

2. T. HaradaandT. Kita, App!. Opt., 19, 3987 (1980)~

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4. C. Martin, P. Jelinsky, M. Lampton,R.F. Malina, andH.O. Anger, Rev. Sd. Instrum., 52,1067 (1981).

5. M. Hurwitz, S.Bowyer, J. Edeistein,T. Harada,andT. Kita, Appi. Opt., 29, 1866 (1990).

6. O.H.W. Siegmund,E. Everinan,J.V. Vallerga, andM. Lainpton,App!. Opt., 27, 1568 (1988).

7. A.F. Davidsen,R.A. Kimble, S.T. Durrance,C.W. Bowers,andK.S. Long, in: Extreme Ultra-

violet Astronomy,ed.R.F. Malina andS. Bowyer,-Pergamon,New York 1990,p. 427.

8. S. Bowyer andR.F. Malina, TheExtremeUltraviolet Explorer mission,this volume (1990).