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Chin.J.Astron.Astrophys.Vol.0(200x)No.0,000–000 ChineseJournalof (http://www.chjaa.org) Astronomyand Astrophysics The World Space Observatory (WSO-UV) 8 Current status 0 0 1 ⋆ 2 2 2 3 2 MichelaUslenghi ,IsabellaPagano ,CristianPontoni ,SalvatoreScuderi andBorisShustov n a 1 INAF–IASF-Milano,ViaE.Bassini15,I-20133Milano,Italy J 2 INAF–CataniaAstrophysicalObservatory,ViaS.Sofia85,I-95123Catania,Italy 4 3 INASAN,Moscow,Russia 1 ] h p Abstract ThispaperreportsonthecurrentstatusoftheWorldSpaceObservatoryWSO-UV, - aspacemissionforUVastronomy,plannedforlaunchatthebeginningofnextdecade.Itis o basedona 1.7mtelescope,withfocalplaneinstrumentsincludinghighresolutionspectro- r t graphs,longslitlowresolutionspectrographsandimagingcameras. s a [ Keywords: spacevehicles:instruments—telescopes—instrumentation:spectrographs— 1 instrumentation:highangularresolution—ultraviolet:general v 0 8 1 INTRODUCTION 0 2 UV spectroscopicandimaging capabilitiesare fundamentalfor astrophysicssince thermalphenomenaat . 1 temperatures T>10,000 K, with flux emission mostly in the UV, occur in a wide range of astrophysical 0 events.Also,theelectronictransitionsofthemostabundantmoleculesintheUniverse(H ,CO,OH,CS, 2 8 CO+, CO ) are in the UV range. This results in UV providing the most sensitive tools to trace the dis- 2 2 0 tribution of (baryonic)matter in the Universe,other than to diagnosethe chemicalcomposition,physical : v propertiesandkinematicsofastronomicalobjectsofalltypes. Xi Inrecentyears, therehavebeenthreemajorinstrumentsworkingin the UV:HubbleSpacetelescope (HST), Far Ultraviolet Spectroscopic Explorer (FUSE) and Galaxy Evolution Explorer (GALEX). The r a firsttwoareobservatory–likemissions,whereasGALEXisdedicatedtoanall–skysurveyandiscurrently providingwidefieldandlowresolutionspectraofalargenumberofastronomicalobjectswhichwillrequire detailedUVfollow-ups.However,afterthefailureoftheHSTSTISspectrographin2004,nofacilitiesto getmediumto highresolutionspectra in the classical UV domain(1000–3000A˚) havebeen available to thecommunity. AccesstoUVisbecomingproblematic.EvenwiththeplannedupgradeofHSTwiththeCosmicOrigin Spectrograph(COS), duringthe service mission SM4, HST is plannedto work till 2013,thusposing the problemofdevelopingnewfacilitiesforUVastrophysicsinthepost-HSTera,beforetheadventoffuture large(8mclass) UV telescopes,currentlyunderdiscussionbutnotyetincludedin theplansofanyspace agency(andthentobescheduledunlikelybefore2020–2025). The World Space ObservatoryUltraviolet – WSO-UV – is a multi-nationalproject grown out of the needsoftheastronomicalcommunitytohavefutureaccesstotheultravioletrange.Plannedtooperatefor 5(+5) years starting from 2011, it will fill in the gap between HST and the future large UV telescopes, complementing the wavelength coverage of the IR James Webb Space Telescope (JWST), which will be operativeinthesameperiod. 2 M.Uslenghietal. 1)PrimaryMirrorUnit(PMU) 2)MechanicalFrame 3)PMU basic frame 4)Radiatorwithheatingelements 5)Tube 6)SecondaryMirrorUnit(SMU) 7)ExternalBaffle 8)Telescope Light Protective Cover (LPC) 9)Primary Mirror Baffle 10)Secondary Mirror Baffle 11)Knife edge rings 12)Optical Bench (OB) 13)Field Camera Unit (FCU) 14)Spectrographs 15)External Electronics Box 16)Scientific Instruments Protective Cover Fig.1 GeneralviewoftheT–170Mtelescope. Table1 MaincharacteristicsofWSO-UV. Spacecraft Spacecraftmasswithpropellant 2900kg Payloadmass 1600kg InstrumentationCompartmentpowerconsumption 750W Datatransmissionrate(S-band) 2Mb/s Servicetelemetrydatatransmissionrate 32kb/s Platformstartrackerpointingaccuracy 30arcsec Stabilizationandpointingaccuracy 0.03arcsec Spacecraftangularrateinstabilizationmode 2x10-5degree/sec Spacecraftslewrate 0.1degree/sec Maximumdurationofscientificobservationincontinuousmode 30hr Telescope OpticalSystem Ritchey-Chre´tien Telescopeentrancepupildiameter 1.7m Effectivefocallength 17m ParameterValueF/ratio 10 FoVdiameter 30arcmin(148,48mm) Scale 12,13arcsec/mm Wavelengthrange 100–310nm(withextensiontothevisible) Primarywavelength 200nm 2 MISSIONOVERVIEW TheWorldSpaceObservatory-UVisaninternationalcollaborationledbyRussiatobuildaspacetelescope optimized in the UV range and devoted to investigate numerous astrophysical phenomena from plane- tary science to cosmology (Barstow et al. 2003, Pagano et al. 2007). The satellite will be based on the ”Navigator”platform,aservicemoduleusedalsoforotherRussianprojects(e.g.ElektroandRadioAstron). ⋆E-mail:[email protected] TheWorldSpaceObservatory 3 Fig.2 InstrumentalCompartment,showingthelocationofthefocalplaneinstruments. Table 1 summarizesWSO-UV spacecraft characteristics. Telescope, launcher (Zenith 2SB) and platform (Navigator) will be developed in Russia, whereas focal plane instruments will be provided by Germany, ChinaandItaly,withcontributionsfromUK,andSpain.GroundSegmentisunderdesignmainlyinSpain andRussia,withpossiblecontributionsfromothercountries.Thespacecraftwillbeputinageosynchronous orbitataheightof35800kmandaninclinationof51.4degrees. 3 TELESCOPE The WSO-UV telescope T–170M is a new version of the T–170 telescope designed by Lavochkin Association (Russia) for the Spectrum-UV mission. It is a Ritchey-Chre´tien with a 1.7m hyperbolicpri- marymirror,focalratioF/10(platescale12.13arcsec/mm)andacorrectedfieldofviewof0.5degrees.The optical quality of the two mirrors is λ/30 rms at 633 nm. The primary wavelength range is 100-350nm withextensionintothevisiblerange,buttheopticsareoptimizedat200nm. The telescope general view is given in Fig. 1: the main structural elements are the Primary Mirror Unit (PMU), the SecondaryMirror Unit(SMU) and the InstrumentalCompartment(IC). There are three attachmentpointsofthetelescopetothespacecraftservicemoduleinthebottomframespart.TheOptical Bench (OB), carryingthe scientific instrumentsand the Fine GuidanceSensors, is mountedon the PMU frame.TheSMUisattachedtothetelescopewithaspider. 4 FOCALPLANEINSTRUMENTS WSO-UV will have spectroscopic and imaging capabilities. The telescope will host two spectrographs and one imager. There will be a high resolution echelle spectrograph (High Resolution Double Echelle Spectrograph–HIRDES),andaLongSlit(lowresolution)Spectrograph(LSS).Theimager,FieldCamera Unit (FCU), will allow diffractionlimited, deep UV and opticalimages. Fig. 2 shows the locationof the instrumentsintheInstrumentalCompartment. 4 M.Uslenghietal. Fig.3 SchematicoverviewofHIRDES(Kappelmannetal.2006). 4.1 HIRDES HIRDES (Kappelmannet al. 2006, see Fig. 3) designis based on the heritage of the ORFEUS (Orbiting andRetrievableFarandExtremeUltravioletSpectrometer)missions(Barnstedtetal.1999).Itcomprises two echelle instruments, UVES (178-320nm) and VUVES (103-180nm), with high spectral resolution (R∼50,000).ThedetectorsofthetwochannelsarephotoncountingdevicesbasedonMicrochannelPlates, readoutbymeansofaWedge&StripAnodebasedontheORFEUSdetectordesign. EachofthechannelshasitsownentranceslitlyinginthefocalsurfaceoftheT-170Mtelescope,ona circlewithdiameter100mmwhichalsohosttheLSSslit.Thepointingwillbemonitoredbytwo(visible) sensors(oneforUVESandoneforVUVES)oftheInternalFineGuidanceSystem(IFGS),whichispart of HIRDES and will allow compensating the jitter of the spectral images due to slight variations in the telescopepointing. AnindustrialphaseAstudyforallthespectrographs(includingalongslitlowresolutionone,initially designed to be accomodated in a single box), has been completed in 2001 by Jena-Optronik (funded by the the German Space Agency DLR). A Phase B1 study for VUVES and UVES has been completed in May 2006by Germany,with Russia collaborationand with industrialsupportby Kaiser–Threde(whilea modifiedversionofLSS,nowconsideredasaseparateinstrument,isnowunderstudyinChina). With the present design, the limiting UV monochromatic magnitudes (SNR=10 in 10h) are 18 and 16 for UVES and VUVES, respectively. An Exposure Time calculator is available on the web (http://astro.uni-tuebingen.de/groups/wsouv/exptime calc.shtml). The spectral resolution provided by HIRDESissimilartothatprovidedbyHST/STIS,buthigherthanthatwhichwillbeprovidedbyHST/COS. Asfarassensitivityisconcerned,WSO-UV/HIRDESiscomparabletoHST/COSanddefinitelybetterthan HST/STIS. 4.2 LSS TheLongSlitSpectrographwillprovidelowresolution(R∼1500-2500)spectraintherange102-320nm (possibly divided in two sub–channels: 102-161 nm and 160-320 nm), using a 1”×75” slit, with spatial TheWorldSpaceObservatory 5 Fig.4 LayoutoftheFieldCameraUnit. resolution0.5-1arcsec.LSSstudy,ongoinginChina,iscurrentlyinphaseA,withaPhaseBexpectedto becompletedbyJanuary2008. 4.3 FCU TheFieldCameraUnit(FCU)willincludethreechannels(Scuderietal.2007): FUVchannel It covers the far UV providing medium resolution images. To satisfy the high sensitivity requirementbelow 200nm,thischannelis optimizedin the range115-190nm, with 0.2arcsec/pixel scale. Since ituses thefocalratioof the telescope,thisdesignminimizesthe numbersofopticalele- ments(onlyapick-upmirrorisrequired,todeviatetheopticalbeamtowardsthedetector),maximizing the throughput. The FUV channel will be equipped with a 2k × 2k pixels, photon counting, MCP detector (with CsI photocathode) and will have a FoV=6.0 × 6.0 arcmin2. This channel shall be equippedwith a filter wheelhostingbroadandnarrowbandfilters, neutralfilters, anda R∼100light disperser,allowingalsolowresolutionslitlessspectroscopy NUVchannel It operates in the range 150-280nm, providing”close to diffraction limit” images at 200 nm, with a 0.03 arcsec/pixel scale. The NUV channel will be equipped by a 2k × 2k pixel MCP detector (with CsTe photocathode) and will have a FoV=1.0×1.0 arcmin2. This channel shall be equippedwithtwofilterwheelshostingbroadandnarrowbandfilters,neutralfilters,polarizersanda R∼100grism,providingslitlessspectroscopicandpolarimetriccapabilities UVOchannel Nearultraviolet-visualdiffractionlimitimager,operatingintheinterval200-1000nm,with a0.07arcsec/pixelscale,equippedwitha4k×4kpixelsCCD,providingafieldofviewof4.6×4.6 arcmin2.Thischannelshallbeequippedwithasetofbroadbandandnarrowbandfilters,arampfilter, polarizersandaR∼250grism. FUVandNUVchannels,usingphotoncountingdetectors,willallowalsohightimeresolutionobser- vations,downtofewmstimescale.Fig.5ashowsapreliminaryestimateofthesystemthroughputofthe three FCU channels comparedto the cameras which have flown or will flew on board of HST (WFPC2, ACS/WFC, ACS/HRC and WFC3/UVIS, Bond H.E. et al. 2006). As it can be seen, the performance of 6 M.Uslenghietal. Total System Throughput DiscoveryEfficiency WSO-UV/FCU ACS/WFC ACS/WFC 2c) WFC3/UVIS oughput WFCA3C/USV/HISRC x area (arcse WFPC2 Thr WFCSUO-UV/ ghput ACS/HRC u WFPC2 o hr T Wavelength(nm) Wavelength(nm) Fig.5 Systemthroughput(a)anddiscoveryefficiency(b)oftheFCUcamerasasfunctionofthe wavelength,comparedwithHSTimagers. theFCUcamerascompareswellwiththeHSTinstrumentones.Anotherusefulquantitywhencomparing different instruments is the discovery efficiency, plotted in Fig. 5b, defined as the product of the system throughputandtheareaoftheFieldofView(FOV)asprojectedonthesky.DuetoitslargeFOV,UVOhas a discoveryefficiencyequalorgreaterthanACS/WFC. Inthecase oftheFUV, theperformanceareeven betterwhencomparedtoHSTbecausenocameraworkinginthisrangehasalargeFOV. The Italian Space Agency founded a Phase A/B1 for the FCU. Phase A has been completed in July 2007.PhaseB1willlastfourmonths. Acknowledgements TheparticipationintheWSO-UVprojectinItalyisfundedbyItalianSpaceAgency undercontractASI/INAFNo.I/085/06/0. References BarnstedtJ.etal.,1999,A&ASS,v.134,p.561-567 BarstowM.A.,BinetteL.,BroschN.etal.,2003,Proc.SPIE,Vol.4854,pp.364-374 BondH.E.etal.,2006,”WideFieldCamera3InstrumentMini-Handbook,Version3.0”,BaltimoreSTScI KappelmannN.,BarnstedtJ.,GringelW.etal.,2006,Proc.SPIE,Volume6266,p.25 PaganoI.,ShustovB.,Kappelmann,N.,etal.,2007,ProceedingsSeriesoftheItalianPhysicalSociety,F.Giovannelli &G.Mannocchi(eds.),Vol93,p.691 ScuderiS.,PaganoI.,FioriniM.,GambicortiL.,GherardiA.,GianottiF.,MagrinD.,MiccolisM.,MunariM.,PaceE., PontoniC.,TrifoglioM.,UslenghiM.,andShustovB.,2007,Mem.S.A.It,inpress DISCUSSION JOERNWILMS:WhatisthetimeresolutionoftheUVdetectors? MICHELAUSLENGHI:Timeresolutionwillbe∼10ms JIMBEALL:Thegeosynchronousorbitpresentssomechallengeswithrespecttosatelliteelectronics.Can youcommentonthis?Howdidyoupicktheorbit? MICHELA USLENGHI: We are aware of the problem and industry is now working on the electronics design on that base. The first choice for the orbit was an L2 one, the baseline (recently) changed to the geosynchronousoneduetocostissues. ThiswaspreparedwiththeChJAALATEXmacrov1.2.