Planck Pre-Launch Status Papers - APC - Université Paris Diderot ...

A&A 520, A9 (2010)– (v) an optical design that mixes bolometer and radiotechniques.These solutions had to be incorporated into a new type of architectureto be launched at ambient temperature and able to cooldown in space to temperatures appropriate for the bolometers’performance.The architecture of the HFI had to solve new optical, cryogenic,mechanical and electrical problems that often seemed incompatible.This made the operation of each sub-system dependenton the performance of several others. For example, the onekilogram bolometer plate at 100 mK required rigid positioningto maintain optical alignment, high strength support strutsto survive launch accelerations and 60 shielded, twisted pairwiring (one for each bolometer andthermometerandmandatoryto suppress electromagnetic interference (EMI)) while conductingonly a few µW to be lifted by the dilution cooler. Each ofthe techniques listed above had to be adapted, within very strictconstraints, into a single instrument concept.The HFI was proposed to the Centre National d’ÉtudesSpatiales (CNES) as the focal plane instrument of a dedicatedmission called SAMBA, and then as part of a medium missionof the program Horizon 2000 of the European Space Agency(ESA). It was selected for a phase A together with the COBRAS(Mandolesi et al. 1994)proposal.TheCOBRAS-SAMBAstudyresulted in the Planck mission that merged into a single telescopethe bolometers cooled at 100 mK of SAMBA and the COBRAS20 K amplifiers based on the use of high electron mobility transistors.The proposed performance and the expected scientificoutput of the Planck mission are described in the “Planck BlueBook” (The Planck consortium 2005).A number of technical developments necessary for HFIwere tested in pathfinder ground-based and balloon-borne instruments,like SuZIE (Holzapfel et al. 1997), Diabolo (Benoît et al.2000), BOOMERANG (Piacentini et al. 2002; Crill et al. 2003;Masi et al. 2006), and MAXIMA (Lee et al. 1999). Essential aspectsof the HFI design concept were verified with the balloonborneArcheops instrument (Benoît et al. 2002), whose focalplane instrument closely followed the design of HFI.This is one of the “Planck pre-launch status” papers. Its purposeis to give an overview of the design that meets the missionrequirements and to present the expected HFI performance.More details on HFI specific issues will be found in the companionpapers Ade et al. (2010), Maffei et al. (2010), Rossetet al. (2010), Pajot et al. (2010). The Planck mission is presentedin Tauber et al. (2010a) andtheopticsasasysteminTauber et al. (2010b). The next section of this paper is dedicatedto the high level scientific requirements that led to the HFI instrumentconcept, the optical optimisation and the measurementstrategy. Section 3 focuses on the sensitivity of the HFI and describesthe detection chain from the receivers to the telemetry.Expected sensitivities for the HFI are given in this section. Thereadout electronics is an essential piece of the detection chain.It is described in Sect. 4 together with the on-board data handling.Section 5 is dedicated to the cryogenic design and to theperformance of the HFI coolers, i.e. the open circuit dilutioncooler and the 4 K mechanical cooler and their associated temperaturecontrol systems. Section 6 addresses several miscellaneousissues, including the development philosophy and the calibrationapproach. This last aspect is developped further in Pajotet al. (2010).2. HFI architecture and optical optimisation2.1. Design and mission rationaleThe HFI was designed to have a sensitivity limited by the photonnoise of the observed radiation, which is possible in the millimetrerange with a passively cooled telescope (Lamarre 1986;Lamarre et al. 1995). The main features of the instrument, andof the Planck telescope and the spacecraft, result from this requirement.These initial aspirations for the HFI proved to beachievable following various instrumental developments and theimplementation of strict design principles.The HFI’s measurement noise on the CMB is less than thelevel of contamination expected from the foregrounds, even inclean regions of the sky. A mission concept was proposed to mapthe whole sky in six bands centred at 100, 143, 217, 353, 545 and857 GHz (Bouchet et al. 1996). The four low frequency bandsare dedicated to direct measurement of the CMB and are polarisationsensitive. The two high frequency bands are optimisedto identify the foregrounds and to separate them from the CMB.Even with wide spectral coverage, the removal of foregroundsis expected to be the limiting factor for the sensitivity (up toabout four times the photon noise limit), and this was adoptedas the sensitivity requirement for the HFI. A goal sensitivity oftwice the photon noise limit was also set to drive the design ofall subsystems and to provide margins in aspects of the instrumentationthat had never been explored before at this level ofperformance. The essential design goals are given in Table 1.Noise equivalent delta temperature and noise equivalent powerare goal values, i.e. twice better than the requirement and areconsistent with the overall mission goals given in the “PlanckBlue Book” (The Planck consortium 2005).The low frequency limit of the HFI spectral coverage was setmainly by the bolometer technology for very long wavelengths.The merging of the SAMBA concept and the COBRAS proposal(Mandolesi et al. 2010; Bersanelli et al. 2010)extendedthespectralrange for component separation.Beginning with its discovery, measurements of the CMBhave required control of systematic effects to high precision. Theimproved sensitivity expected from Planck-HFI requires commensurateimprovements in the control of systematic errors. Thereduction of systematic effects was a driver of every aspect ofthe design of the HFI instrument, as described in the followingsections.2.2. Optical, mechanical and thermal architectureThe thermal and optical architectures of the focal plane assemblyare tightly coupled, because every optical element warmerthan 1 K is a significant source of thermal radiation in the spectralrange suitable for CMB measurements. Stray light is also asource of parasitic radiation. Conversely, every optical elementabsorbs radiation power both from the nominal beam and fromstray light. The absorbed heat has to be lifted by the cryogenicstages, and may contribute to their cooling budget. This requiresnearly perfect control of stray light and a cascade of optical filtersat different temperatures.The HFI architecture is based on independent optical chainscollecting the light from the telescope and feeding it to bolometersor bolometer pairs (for the polarisation-sensitive bolometers,PSBs). Each of the 36 optical chains transmits the desiredfrequencies and blocks other frequencies with rejection factorsup to 10 10 .Italsoreducesthethermalradiationloadsonthecoldest stage by factors of at least 10 4 .InterferencemeshfilterPage 2 of 20