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A&A 520, A2 (2010)DOI: 10.1051/0004-6361/200912911c○ ESO 2010Pre-launch status of the Planck missionAstronomy&AstrophysicsSpecial featurePlanck pre-launch status: The optical systemJ. A. Tauber 1 ,H.U.Norgaard-Nielsen 2 ,P.A.R.Ade 22 ,J.AmiriParian 3 ,T.Banos 4 ,M.Bersanelli 5 ,C.Burigana 6 ,A. Chamballu 7 ,D.deChambure 24 ,P.R.Christensen 8 ,O.Corre 4 ,A.Cozzani 9 ,B.Crill 10 ,G.Crone 9 ,O. D’Arcangelo 11 ,R.Daddato 9 ,D.Doyle 9 ,D.Dubruel 4 ,G.Forma 4 ,R.Hills 12 ,K.Huffenberger 10 ,A.H.Jaffe 7 ,N. Jessen 2 ,P.Kletzkine 9 ,J.M.Lamarre 13 ,J.P.Leahy 14 ,Y.Longval 18 ,P.deMaagt 9 ,B.Maffei 14 ,N.Mandolesi 6 ,J. Martí-Canales 9 ,A.Martín-Polegre 9 ,P.Martin 4 ,L.Mendes 15 ,J.A.Murphy 16 ,P.Nielsen 17 ,F.Noviello 18 ,M. Paquay 9 ,T.Peacocke 16 ,N.Ponthieu 18 ,K.Pontoppidan 17 ,I.Ristorcelli 19 ,J.-B.Riti 4 ,L.Rolo 9 ,C.Rosset 20 ,M. Sandri 6 ,G.Savini 21 ,R.Sudiwala 22 ,M.Tristram 23 ,L.Valenziano 6 ,M.vanderVorst 9 ,K.van’tKlooster 9 ,F. Villa 6 ,andV.Yurchenko 16(Affiliations can be found after the references)Received 17 July 2009 / Accepted 2 March 2010ABSTRACTPlanck is a scientific satellite that represents the next milestone in space-based research related to the cosmic microwave background, and in manyother astrophysical fields. Planck was launched on 14 May of 2009 and is now operational. The uncertainty in the optical response of its detectorsis a key factor allowing Planck to achieve its scientific objectives. More than a decade of analysis and measurements have gone into achievingthe required performances. In this paper, we describe the main aspects of the Planck optics that are relevant to science, and the estimated in-flightperformance, based on the knowledge available at the time of launch. We also briefly describe the impact of the major systematic effects of opticalorigin, and the concept of in-flight optical calibration. Detailed discussions of related areas are provided in accompanying papers.Key words. cosmic microwave background – space vehicles: instruments – instrumentation: detectors – instrumentation: polarimeters –submillimeter: general – telescopes1. IntroductionThe ambitious goals of the Planck mission 1 (Tauber et al. 2010)can only be met if its measurements can be calibrated to veryhigh accuracy. The accuracy of calibration on small angularscales depends directly on the uncertainties in the angular radiationpatterns of each detector, to a level unprecedented inmm-wave astronomy. The Planck goal to achieve photometriccalibration of 1% in the key CMB bands (70−217 GHz) impliesthat the beam characteristics (solid angle, shape) must beknown to sub-% levels. The impact of beam uncertainties hasbeen extensively analysed for WMAP (e.g., Hill et al. 2009;Nolta et al. 2009) andanalysesoftheeffect on the recovery byPlanck of some cosmological parameters have also been performed(Huffenberger et al. 2010; Rochaetal.2010), in bothcases confirming the importance of optical uncertainties.For this reason, the optical system of Planck is a key elementfor the mission, and its design, manufacture, and verificationprogrammes have been mission drivers in terms of cost andcomplexity. The success of the mission does not however dependentirely on the optical knowledge gathered on the ground. Inflightmeasurements of celestial sources are the principal sourceof information about the shapes of the main beams, and the1 Planck (http://www.esa.int/Planck) is a project of theEuropean Space Agency – ESA – with instruments provided by two scientificConsortia funded by ESA member states (in particular the leadcountries: France and Italy) with contributions from NASA (USA), andtelescope reflectors provided in a collaboration between ESA and a scientificConsortium led and funded by Denmark.ground knowledge allows us to tie the in-flight measurements tothe beam shapes below the level at which they can be measuredin flight. Confronting the ground predictions with the in-flightmeasurements allows us to build a reliable estimate of the opticalresponse to very low amplitude levels, and therefore to predictor constrain the level of unwanted optical systematics suchas straylight signals.The objectives of the ground activities related to opticswere to:– build a mathematical model that allows us to predict and verifythe in-flight performance with a combination of test andanalysis;– verify that the as-built optical system meets its major performancerequirements, and evaluate the uncertainties in theperformance predictions;– verify that a number of systematic effects caused by the opticsare either below a significant level, or can be dealt within-flight.This paper provides a summary of the activities carried out beforethe launch of Planck, culminatinginthepredictionofinflightoptical response and its uncertainties. We begin (Sect. 2)with a very brief summary of the development history and itsdesign requirements. Section 3 describes the main mechanicalelements of the system and some aspects of its manufacture thathave an important impact on its performance. The resulting opticalcharacteristics of the as-built system are described in Sect. 4,where readers can find a succinct description of the predictedArticle published by EDP Sciences Page 1 of 22
A&A 520, A2 (2010)Fig. 1. (Left) ThefullyassembledPlanck satellite and (right) itstelescopepriortointegration.Threeconical“V-grooves”(visibleontheleftasthree horizontal lines) isolate thermally and radiatively the warm Service Module (lower octagonal black box) from the cold payload module. Thetopmost (or 3rd) of the V-grooves, together with the large black baffle, form the cavity containing the Planck telescope. The white dots seen onthe telescope in the right panel are photogrammetry targets and were removed before integration of the telescope into the satellite; a focal plane isalso visible but unpopulated with horns.optical performance of the Planck detectors. Subsequent sectionsconsider the uncertainty in this prediction:– Section 5 describes the measurements of the geometry of thereflectors and telescope, and how they were combined withanalysis to predict their geometry and alignment in-flight.– Section 6 discusses the radio frequency (RF) measurementsperformed to verify the accuracy of the mathematical model(based on the GRASP software, GRASP Manual 2008) thatconverted the geometrical information into a prediction ofthe optical response in-flight.– Section 7 describes how the GRASP model was used to determineuncertainties on the predicted optical response.Section 8 addresses problems associated with the far-sidelobes,which may generate straylight signals. Section 9 estimates signalsproduced by thermal emission from the payload and thesatellite itself. Finally, Sect. 10 summarises plans for in-flightcharacterisation using celestial sources.2. BackgroundThe development of the Planck mission began with two proposalspresented to ESA in May of 1993: COBRAS (Mandolesiet al. 1993) andSAMBA(Pugetetal.1993). Each of theseproposed a payload formed by an offset Gregorian telescopefocusing light onto an array of detectors (based on HEMTLow Noise Amplifiers for COBRAS and very low temperaturebolometers for SAMBA) fed by corrugated horns. Thetwo proposals were used to design a payload where a singleCOBRAS-like telescope fed two instruments (a COBRAS-likeLow Frequency Instrument – LFI; and a SAMBA-like HighFrequency Instrument – HFI) sharing a common focal plane.The telescope for this (COBRAS/SAMBA) satellite was essentiallyidentical to the COBRAS design by Pagana (1993), namelyaclassicalGregorianparaboloid-ellipsoidcombinationobeyingthe so-called Dragone-Mizuguchi condition (which preservespolarisation purity on the optical axis). Subsequent studies culminatingin the so-called Red Book of 1996 (Bersanelli et al.1996) didnotmodifytheinitialdesignsubstantially,exceptforan increase in the reflector size to the maximum allowable bysatellite constraints at the time, and for the detailed design ofsurrounding elements, e.g., supporting structure and baffle (seeFig. 1). In 1997, the design of the focal plane was substantiallymodified to improve the efficiency of use of its centralarea and the manufacturability of the HFI, yielding today’s layout(see Fig. 2) inwhichthecentreofthefocalplaneisoccupiedby the very-low-temperature, high-frequency HFI detectors(Lamarre et al. 2010), surrounded by the higher-temperature,lower-frequency LFI detectors (Bersanelli et al. 2010).The new focal plane layout required a re-optimisation of thetelescope, which was carried out in 1999 (Fargant et al. 2000).Because of the long wavelengths involved relative to the size ofthe optics, physical optics methods were required to correctlymodel the detector patterns in the far field. However, the computationtimes required with physical optics are too long to allowmany iterations. Ray-tracing is a more efficient method butless accurate; however it is able to represent well enough theshape of the main beam for optimisation purposes. The optimisationwas therefore carried out using the optical ray-tracingsoftware CodeV, allowing variation of all the main parameters ofPage 2 of 22
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