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

A&A 520, A9 (2010)Table 3. Estimated average redundancy of observations in one year of operation for 2 ′ × 2 ′ pixels. The average observation time is per bolometer.Frequency (GHz) and mode 100P 143P 143 217P 217 353P 353 545 857Average obs time (s/sample) 2.9 1.8 1.8 0.85 0.85 0.85 0.85 0.85 0.85Average redundancy (1 year) 1380 850 1250 400 600 400 600 600 600Max. redundancy (1 year) 12 000 7700 12 000 3700 5600 3700 5600 5600 5600Min. redundancy (1year) 700 420 630 200 300 200 300 300 300Fig. 5. One can see the HFI and the LFI horns on this picture taken duringground tests of the Proto-Flight Model. An additional horn, mountedfor alignment tests, is visible at the end of the 100 GHz row.Fig. 4. Distribution of the beams on the sky and identification of opticalchannels (view from sky to telescope; units are degrees). Crossesindicate the polarisation orientation of the PSBs.3. Obtaining the desired performance3.1. Transforming photons in digital dataWhile the satellite rotates at one RPM, the image of the sky isconvolved with the beam of the optical system, including thetelescope, horn, internal optics, and the bolometers. Photons arealso selected in frequency by the optical chains. This producesatimelineofopticalpower,whichincludesanearlyconstantsignal produced by the thermal emission of the telescope andthe HFI cryogenic stages. The optical power (including the photonshot noise) is absorbed by the bolometers. The temperaturechanges of the bolometer are detected by current biasing of thebolometers and measuring the voltage at its output. Additionalnoise comes from the bolometer itself and from the readout electronics.The signal is then amplified, digitised, compressed anddelivered to the telemetry system.This results in a complex processing system involving anumber of free parameters and variables (Fig. 6). Among them,the temperature of the 100 mK stage is a critical variable, since itimpacts directly on the temperature of the bolometers, changingtheir impedance, response to optical signals, noise and time response.The temperatures of the 4 K and 1.6 K stages directlydrive the thermal emission of these stages. These effects werecalibrated and the relevant temperatures are actively controlledand residual fluctuations accurately measured so that their effectscan be removed if necessary.The parameters of the readout electronic unit (REU) areset by remote commands sent through the uplink to thespacecraft. Their effect on the bolometer response will bedetailed in Sect. 4.Additionalparasiticsignalsareexpectedfromelectromagnetic interferences, mechanical vibrations, and cosmicrays hitting various parts of the detectors.3.2. Time response – NEP trade-offLow temperature bolometers are the most sensitive receivers forwide band detection in the sub-THz frequency range. However,they have a finite time response set by their heat capacity andthermal conductivity. τ = C/G eff ,whereC is the heat capacityof the bolometer and G eff is the effective thermal conductance(Chanin & Torre 1984) oftheheatlinktothebolometerhousing (the physical thermal conductance and its effective reductionbecause of electrothermal feedback).Abolometertimeconstant less than 1/3 thetimetakenbyaGaussianbeamtosweep across a given point on the sky will not significantly limitthe signal bandwidth (Hanany et al. 1998). The beam size producedby the telescope and the optics, coupled with the scanrate (6 ◦ per second), sets the maximum frequency range of thevariations of the signal incident on the bolometers to a few tensof Hz (about 50 Hz for a 5 ′ beam). The time response of theHFI bolometers must be well-matched to this range, which isobtained for each bolometer species by tuning the thermal conductanceG.The voltage response of the bolometer to incoming radiationdepends on its temperature change for a given signal and is inverselyproportional to the thermal conductance. The quantumPage 6 of 20