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

F. Pajot et al.: Planck pre-launch status: HFI ground calibrationresponse for sources with steep but with opposite spectral slopes(using high- or low-pass filters).2.4. Time responseWe measured the time response on the ground with signals modulatedat frequencies covering the range 0.01 Hz to 100 Hz.Specific care was taken to measure the time transfer functionat low frequency, since the absolute response for the low frequencychannels (CMB channels) will be measured on the signalcoming from the CMB dipole modulated by the spacecraft spinat ∼1/60 Hz.The ultimate determination will be verified using the in-flightdata by comparing the maps of the same bright sources, such asplanets, obtained with different scan directions and angles at differentphases of the mission.2.5. Optical polarisationThe absolute orientation of the polarisation and the cross-polarleakage are deduced from both the sub-system measurementsand the FPU characterisation. Polarisation orientation and crosspolarleakage are measured on individual horn assemblies andthe absolute orientation of the FPU is measured by taking intoaccount the geometry of the beam (Rosset et al. 2010).The pre-launch determination of the beams and polarisationparameters was achieved using measurements of the FPU(Maffei et al. 2010) andtheradiofrequency(RF)modelofthetelescope (Tauber et al. 2010b).The polarised emission of the Crab nebula was measured atthe IRAM 30 m telescope at frequencies near the HFI bands.These data are used to derive the final calibration of the skydata (Aumont et al. 2010). The galaxy polarisation is poorlyknown today and is not neglible. At low frequencies measurementsdo exist (Archeops, Benoît et al. 2002). New observationsat short wavelengths (e.g., PILOT balloon experiment, Bernardet al. 2007) willbeusedtoimproveourunderstandingofthepolarised emission on the sky at these frequencies.2.6. Optical efficiency, linearity, and sensitivityto the temperature of the cryogenic stagesThis group of parameters characterises the instrument behaviourover an extended range of operational conditions. The opticalefficiency is the fraction of photons collected by the real opticalsystem with respect to an ideal system of same spectral response.Bolometric detectors have a linear response when the optical signalis weak relative to the optical background, but a determinationof the linearity curve is needed for photometric calibrationat the percent level. In addition, knowledge of the dependenceof the signal on the temperature fluctuations of all optical componentsis essential to reduce the thermal systematic effects. Theinstrument thermal design includes very stable thermal regulationsystems that are designed to keep these effects below thedetector noise (see Sect. 5.3.2). The characterisation of the couplingcoefficients gives, if required, the possibility of removingsecond order correlations of thermal origin between channels.These parameters are measured at the level of the detector subsystems,during the focal plane calibration, and are confirmedwith in-flight measurements.2.7. Absolute calibrationThe absolute calibration allows one to convert the digital signalinto the sky brightness. The HFI does not use any internalabsolute reference signal, therefore the total power is notreliably measured by the HFI bolometers. The full sky mapsdeduced from the HFI data are instead insensitive to a constantemision level, such as the CMB monopole. A preliminary absoluteresponse was estimated during the focal plane calibrationwith a precision of 10% (and a relative pixel to pixel calibrationof 3%). The ground calibration sources allowed us to performthis measurement with an optical background that was representativeof that expected in-flight. The final determination will beperformed in-flight (Piat et al. 2002). The goal is a 1% radiometricaccuracy for the low frequency channels (ν 400 GHz: 545, 857 GHz). The FIRAS experiment on theCOBE satellite has provided the most accurate photometric calibrationfor extended sources in the millimeter and submillimeterwavelength range producing a spectral image of the sky in therange [0.1, 10 mm] (3000 GHz to 30 GHz), with a spectral resolutionof approximately 5% and a spatial resolution of 7 ◦ .FIRASused an absolute black body to provide a flux calibration with anaccuracy superior to 1% below 400 GHz and 3% above (Matheret al. 1999). The in-flight calibration of the submillimeter channelsof HFI will rely on this calibration. The CMB dipole component,produced by the proper motion of Planck with respect tothe rest frame of the CMB, will be used for the low frequencychannels. Therefore, the absolute calibration procedures can bedetailed as follows:1. For the channels below 353 GHz, the CMB dipole dominatesthe galactic signal over most of the sky. The observeddipole is the sum of two components, one resulting fromthe peculiar velocity of the solar system in the CMB restframe, another resulting from the orbit of the Earth, hencethe L2 point and Planck,aroundtheSun.Aslongasthecirclesdescribed on the sky have a different axis from that ofthe dipole (the angle is always larger than 10 ◦ ,whichprovidesmore than 15% of the dipole signal), a short-term relativecalibration can be obtained from the dipole: the relativevariation in the dipole signal is at most 0.9% per dayor 4 × 10 −2 %perhour,i.e.,betweentwoconsecutivedepointings.This allows a straightforward ring to ring relativecalibration limited only by the level of the CMB fluctuations.The use of WMAP data could improve the ring calibration,although the absolute calibration will be obtained in a selfconsistent way from the orbital dipole, observed on the surveytimescale (6 months), more accurately than 0.4% (Piatet al. 2002).2. For the channels above 353 GHz, Galactic emission is thedominant component. The Galactic signal has a high spatialfrequency component that makes relative calibrations fromring to ring impossible because rings have a large angularseparation (2 arcmin, about half of the FWHM beam of thesechannels). Observations of the Galactic disk at submillimeterwavelengths exist only at lower angular resolution (typically30 arcmin with balloon observations) with absolute calibrationsthat are poorer than 10 to 20%, and at the very low spatialresolution of FIRAS with an absolute calibration of 3%above 400 GHz. We will average HFI data over one weekperiods and more to obtain a pixel size of 7 ◦ identical to thatof FIRAS. This requires knowledge of the temporal variationin the relative response over this timescale without the benefitof an external calibration source. An instrument modelPage 3 of 15