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

A&A 520, A3 (2010)Horizon 2000 medium-sized mission. Today Planck forms partof the “Horizon 2000” ESA programme.The Planck CMB anisotropy probe 1 , the first Europeanand third generation mission after COBE and WMAP(Wilkinson Microwave Anisotropy Probe), represents the stateof-the-artin precision cosmology today (Tauber et al. 2010;Bersanelli et al. 2010; Lamarre et al. 2010). The Planck payload(telescope instrument and cooling chain) is a single, highly integratedspaceborne CMB experiment. Planck is equipped witha1.5-m effective aperture telescope with two actively-cooled instrumentsthat will scan the sky in nine frequency channels from30 GHz to 857 GHz: the Low Frequency Instrument (LFI) operatingat 20 K with pseudo-correlation radiometers, and the HighFrequency Instrument (HFI; Lamarre et al. 2010) withbolometersoperating at 100 mK. Each instrument has a specific role inthe programme. The present paper describes the principal goalsof LFI, its instrument characteristics and programme. The coordinateduse of the two different instrument technologies andanalyses of their output data will allow optimal control and suppressionof systematic effects, including discrimination of astrophysicalsources. All the LFI channels and four of the HFI channelswill be sensitive to the linear polarisation of the CMB.While HFI is more sensitive and should achieve higher angularresolution, the combination of the two instruments is required toaccurately subtract Galactic emission, thereby allowing a reconstructionof the primordial CMB anisotropies to high precision.LFI (see Bersanelli et al. 2010, formoredetails)consistsofan array of 11 corrugated horns feeding 22 polarisation-sensitive(see Leahy et al. 2010, formoredetails)pseudo-correlationradiometersbased on HEMT transistors and MMIC technology,which are actively cooled to 20 K by a new concept sorptioncooler specifically designed to deliver high efficiency, long durationcooling power (Wade et al. 2000; Bhandari et al. 2004;Morgante et al. 2009). A differential scheme for the radiometersis adopted in which the signal from the sky is compared withastablereferenceloadat∼4 K(Valenziano et al. 2009). Theradiometers cover three frequency bands centred on 30 GHz,44 GHz, and 70 GHz. The design of the radiometers was drivenby the need to minimize the introduction of systematic errorsand suppress noise fluctuations generated in the amplifiers.Originally, LFI was to include seventeen 100 GHz horns with34 high sensitivity radiometers. This system, which could havegranted redundancy and cross-calibration with HFI as well asacross-checkofsystematics,wasnotimplemented.The design of the horns is optimized to produce beams of thehighest resolution in the sky and the lowest side lobes. TypicalLFI main beams have full width half maximum (FWHM) resolutionsof about 33 ′ ,27 ′ ,and13 ′ ,respectivelyat30GHz,44 GHz, and 70 GHz, slightly superior to the requirements listedin Table 1 for the cosmologically oriented 70 GHz channel.The beams are approximately elliptical with and ellipticity ratio(i.e., major/minor axis) of ≃1.15−1.40. The beam profiles will bemeasured in-flight by observing planets and strong radio sources(Burigana et al. 2001).AsummaryoftheLFIperformancerequirementsadoptedtohelp develop the instrument design is reported in Table 1.1 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.Table 1. LFI performance requirements.Frequency channel 30 GHz 44 GHz 70 GHzInP detector technology MIC MIC MMICAngular resolution [arcmin] 33 24 14δT per 30 ′ pixel [µK] 8 8 8δT/T per pixel [µK/K] 2.67 3.67 6.29Number of radiometers (or feeds) 4 (2) 6 (3) 12 (6)Effective bandwidth [GHz] 6 8.8 14System noise temperature [K] 10.7 16.6 29.2White noise per channel [µK · √s] 116 113 105Systematic effects [µK]