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A&A 520, A11 (2010)DOI: 10.1051/0004-6361/200913039c○ ESO 2010Pre-launch status of the Planck missionAstronomy&AstrophysicsSpecial featurePlanck pre-launch status: The optical architecture of the HFIP. A. R. Ade 1 ,G.Savini 1,2 ,R.Sudiwala 1 ,C.Tucker 1 ,A.Catalano 3 ,S.Church 4 ,R.Colgan 5 ,F.X.Desert 6 ,E. Gleeson 5 ,W.C.Jones 7,8 ,J.-M.Lamarre 3 ,A.Lange 7,9,† ,Y.Longval 10 ,B.Maffei 11 ,J.A.Murphy 5 ,F.Noviello 10 ,F. Pajot 10 ,J.-L.Puget 10 ,I.Ristorcelli 12 ,A.Woodcraft 13 ,andV.Yurchenko 5,141 Astronomy and Instrumentation Group, Cardiff University, Cardiff,Wales,UK2 Optical Science Laboratory, University College London (UCL), Gower Street, WC1E 6BT London, UKe-mail: gs@star.ucl.ac.uk3 LERMA, CNRS, Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France4 Department of Physics, Stanford University, Stanford, CA 94305-4060, USA5 Department of Experimental Physics, National University of Ireland (NUI), Maynooth, Co. Kildare, Ireland6 Laboratoire d’Astrophysique Observatoire de Grenoble (LOAG), CNRS, BP 53, 38041 Grenoble Cedex 9, France7 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA8 Department of Physics, Princeton University, Princeton NJ 08544, USA9 Department of Physics, California Institute of Technology, Mail code: 59-33, Pasadena, CA 91125, USA10 IAS, Institut d’Astrophysique Spatiale, CNRS Université Paris 11, Bâtiment 121, 91405 Orsay, France11 The University of Manchester, JBCA, School of Physics and Astronomy, Manchester M13 9PL, UK12 CESR, CNRS, 9 Av. du colonel Roche, BP44346, 31038 Toulouse Cedex 4, France13 SUPA, Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK14 Institute of Radiophysics and Electronics, NAS of Ukraine, 12 Proskura St., 61085, Kharkov, UkraineReceived 31 July 2009 / Accepted 21 December 2009ABSTRACTThe Planck High Frequency Instrument, HFI, has been designed to allow a clear unobscured view of the CMB sky through an offaxisGregorian telescope. The prime science target is to measure thepolarizedanisotropyoftheCMBwithasensitivityof1partin 10 6 with a maximum spatial resolution of 5 arcmin (C l ∼ 3000) in four spectral bands with two further high-frequency channelsmeasuring total power for foreground removal. These requirements placecriticalconstraints on both the telescope configuration andthe receiver coupling and require precise determination of the spectral and spatial characteristics at the pixel level, whilst maintainingcontrol of the polarisation. To meet with the sensitivity requirements, the focal plane needs to be cooled with the optics at a fewKelvin and detectors at 100 mK. To limit inherent instrumental thermal emission and diffraction effects, there is no vacuum window,so the detector feedhorns view the telescope secondary directly. This requires that the instrument is launched warm with the coolerchain only being activated during its cruise to L2. Here we present the novel optical configuration designed to meet with all the abovecriteria.Key words. cosmic microwave background – space vehicles: instruments – instrumentation: detectors –instrumentation: polarimeters – submillimeter: general – techniques: photometric1. BackgroundThe Planck 1 High Frequency Instrument (HFI) will use very sensitivebolometric detectors cooled to 100 mK to measure polarisationand temperature anisotropies in the cosmic microwavebackground (CMB) on all scales larger than ∼5arcmintoanunprecedentedaccuracy of T ∼ 2 × 10 −6 .Itisintendedthatthesensitivity of the instrument will be limited only by the fundamentallimits set by CMB photon noise and the ability to removeastrophysical foregrounds. The anisotropy polarisation signatureis required to unambiguously reconstruct the spectrum of1 Planck (http://www.esa.int/Planck) isanESAprojectwithinstrumentsprovided by two scientific Consortia funded by ESA memberstates (in particular the lead countries: France and Italy) with contributionsfrom NASA (USA), and telescope reflectors provided in a collaborationbetween ESA and a scientific Consortium led and funded byDenmark.primordial perturbations and will enable cosmologists to testmodels for the origin and structure of the Universe (quantumfluctuations or topological defects) and to constrain the key cosmologicalparameters defining our Universe to an accuracy of apercent or better in most scenarios.Planck will be injected into a Lissajous orbit around the 2ndLagrangian point, L2, of the Sun-Earth-Moon system, whichsubtends a maximum angle of 15 ◦ as seen from the Earth. At thislocation, Planck is able to always maintain its payload pointedtowards deep space, shielded from Solar, Earth, and Lunar illuminationby its solar array. To scan the whole sky, Planck spinson a Sun-pointed axis with its telescope oriented at 60 degreesto it looking away from the Sun. A necessary requirement is thatthe HFI has sufficient pixels at each frequency in the cross scandirection to ensure complete beam sampling of the sky as thesatellite spin axis is stepped in increments of 2 arcmin. With thisstrategy the whole sky is mapped every 6 months.Article published by EDP Sciences Page 1 of 7
A&A 520, A11 (2010)HFI will measure the CMB radiation over the frequencybands where contamination from foreground sources is at a minimumand the CMB signal is at a maximum. Emission fromforeground contributions (the Galaxy and extra-galactic sources)will be removed from the sky maps by measuring the spectralsignature of the sky emission over a wide frequency range. TheHFI is therefore a multiband instrument with 6 bands from 100to 857 GHz. The four lowest frequency bands are spread acrossthe peak intensity of the CMB at frequencies centered near, 100,143, 217 and 353 GHz, respectively. In all these channels wedetect the polarisation signature and for all but the 100 GHzchannel we also directly measure total power in some sky pixelsto enhance the instantaneous detection of foreground sources.Detectors at the same frequency but different polarisation orientationare arranged to follow each other on the scanning path toallow nearly instantaneous measurements of the Stokes Q andU vectors. The overall sensitivity of HFI to the CMB is thereforedetermined by the inherent sensitivity of each sky pixel (designedto be close to the photon noise from a 3 K blackbody),the number of crossings of each sky element and the number ofsky horns for each channel.For extraction of the two main foregrounds at these frequencies,galactic dust emission and the infrared galaxy background,the HFI has two additional channels at 545 GHz and 857 GHz.These channels have similar or slightly better angular resolutionto the CMB channels and thus enable foregrounds which havespectra which rise steeply with frequency to be identified and removedefficiently. To maintain a beamwidth comparable to thatused in the CMB channels we use multi-moded horns. Thesenon-diffraction limited beams thus have increased throughputand give better instantaneous detection of point like sources.Unfortunately such horns scramble the polarisation informationso only total power is detected at these frequencies.The HFI frequencies have been carefully chosen to optimizethe detection of clusters of galaxies via the Sunyaev-Zeldovich(S-Z) effect. This effect arises from the Compton interaction ofCMB photons with the hot gaseous atmospheres of clusters ofgalaxies. The S-Z effect is expected to be the dominant secondarydistortion of the CMB, but can be separated very accuratelyfrom the primordial CMB anisotropies via its uniquespectral signature. The bands are set so that the S-Z decrementcan be observed in the 150 GHz band, the enhancement in the353 GHz band and the peculiar velocity extracted using measurementsfrom the S-Z null at 217 GHz. The HFI should detectmany thousands of S-Z clusters of galaxies Bluebook (2005),probing redshifts z ∼ 1. The HFI will also detect many thousandsof infrared galaxies. The production of complete near allskycatalogues of galaxy clusters and infrared galaxies with theHFI are important scientific goals of the Planck mission.The Planck sky scanning strategy has been chosen to optimizethe redundancy in the data by moving the spin axis by upto 10 ◦ from the anti-solar direction. An optimized scanning strategyis essential for detecting, controlling and removing systematiceffects which might affect the data. The cosmological resultsfrom Planck will thus be as free as possible from systematic errors.In the sections which follow we describe the optical architecturewhich meets with these scientific drivers to ensure thatthe spectral and spatial performance is achieved with the highestpossible on sky sensitivity. As is evident in the above discussion,in meeting with the science objectives there were many obstacleswith conventional receiver configurations which needed to beaddressed. Here we outline the design parameters and the novelinstrument architecture which we have developed to maximizethe science return for HFI and give representative componentand system level data.2. The Planck telescopeTo observe simultaneously in six frequency bands with multipledetectors to measure polarisation, total power and ensure that wehave cross scan beam coverage as well as sufficient sky pixelsto achieve the target sensitivity with some redundancy requires ahighly packed focal plane layout. This translates to requirementson the off-axis performance of the telescope to minimize straylightand inherent cross polarisation contributions (Tauber et al.2010). The highest angular resolution requiresfullilluminationof the telescope from the detector feed horns, whereas very lowside lobes require under-illumination of the telescope. Thus thephilosophy behind the telescope configuration and the consequentoptical layout for the focal plane horns is influenced by anumber of specific requirements peculiar to the Planck Mission(Tauber et al. 2010).To minimize diffraction and emission from telescope secondarysupport struts which add significantly to beam asymmetriesand detector thermal loading (loss of sensitivity) it wasdecided to use an off-axis Gregorian telescope with no supportstruts in the beam and no other structures (window, field stops orfilters) in front of the sky horns which directly view the secondary.The Planck submillimetre telescope is thus a simpleoff-axis Gregorian design with two elliptical reflectors providinga 1.5 m projected diameter. Although the performance ofthis a-planatic configuration is not as good at the field centeras a Dragone-Mizuguchi Gregorian (Dragone 1978) configurationwhich eliminates astigmatism, it is significantly better overthe large focal surface required to accommodate the distributedHFI and LFI feeds. The design ensures that there are no supportstructures in the beam, which could otherwise cause diffractionof the sky beam or radiate unwanted power to the detectors. Theemissivity of the telescope is expected to be
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ABSTRACTThe European Space Agency
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