Planck Pre-Launch Status Papers - APC - Université Paris Diderot ...
Planck Pre-Launch Status Papers - APC - Université Paris Diderot ... Planck Pre-Launch Status Papers - APC - Université Paris Diderot ...
A&A 520, A9 (2010)Fig. 9. Cosmic rays hitting the bolometers produce glitches with decay times depending on the part heated by the particle (after Tristram 2005).Fig. 10. Example of noise spectrum taken in the CSL facility. The blackcurve is a raw spectrum of three hours of data. The red curve is obtainedby decorrelating the TOI in the time domain with a template made ofother bolometer signals and smoothed. The green curve is a fit to thered power spectrum with a simple two-component noise model. Thefour high and narrow lines near 10, 30, 50 and 70 Hz are due to the 4 Kcoolers (see text).Fig. 11. Individual sensitivity of all bolometers measured during calibrationscompared with requirements and goal.because of unexpected difficulties in the manufacture of the longnarrow horns imposed by the geometry of the focal plane.We ignore the value of several parameters that will contributeto the noise during the mission, like the real environment conditions(particles and microphonics) or the effective thermal emissionof the telescope. This emission will be determined partlyfrom inevitable contamination by dust during the launch andprobably from the degradation of the surface of the telescopeby micro-meteorites during the mission. By extrapolating the resultstaken on the ground, one can derive estimates of the sensitivity(Table 5) thatareverysimilartothevaluespublishedup to now (The Planck consortium 2005). Average sensitivity isfor 14 months of operation for square pixels with the nominalbeam size. The goal ∆T/T sensitivity (The Planck consortium2005)isreported(inItalics)forcomparison.Sensitivitytopointsources assumes an R − J spectrum and does not account for theconfusion limit.These results show that the bolometer programme was successfulfrom the early design stages through to fabrication.The optical, thermal and electrical environment together withFig. 12. Distribution of the sensitivity (linear scale) of HFI’s 52 bolometersestimated from ground measurements and normalized by the sensitivityrequirement. About half the bolometers should have a sensitivitybetter than the goal.associated low-noise readout electronics were also designed andimplemented successfully. They demonstratethatacompletesystem was developped although based on a new architecturePage 10 of 20
J.-M. Lamarre et al.: Planck pre-launch status: the HFI instrumentTable 5. HFI performance expected from ground calibrations and for 14 months of operation.Channel 100P 143P 143 217P 217 353P 353 545 857Central frequency (GHz) 100 143 143 217 217 353 353 545 857Bandwidth (%) 33% 32% 30% 29% 33% 29% 28% 31% 30%Full width half maximum beam size ( ′ ) 9.6 6.9 7.1 4.6 4.6 4.6 4.7 4.7 4.3Number of detectors 8 8 4 8 4 8 4 4 4(∆T/T ) CMB per pixel (µK/K, I Stokes parameter) 3 2.2 4.8 20 150 6000Proposal goal (µK/K, IStokesparameter) 2.5 2.2 4.8 14.7 147 6700(∆T/T ) CMB per pixel (µK/K, Q and U Stokes parameters) 4.8 4.1 9 38 – –Proposal goal (µK/K, QandUStokesparameters) 4.0 4.2 9.8 29.8 – –Sensitivity to point sources (mJy) 14 10 12 30 40 35and new components and meets the most optimistic expectationsin the early stages of the Planck project.4. Readout electronics and on-board data handling4.1. Principles of operation of the readout electronicsThe performance of the bolometers cannot be separated fromthe performance of the readout electronics used to measure theimpedance of the thermometer part of the bolometer. The biascurrent deposits some power in the bolometer and changes itstemperature. AC biased readout electronics developped in thelate 80s (Rieke et al. 1989) havetheadvantageofproducinganoise spectrum that is flat down to very low frequencies, whileDC biased readouts show a large 1/ f component at frequenciesless than about 10 Hz. The AC bias readout electronics ofthe HFI instrument (Gaertner et al. 1997) includesanumberoforiginal features proposed by several laboratories (CRTBT inGrenoble, CESR in Toulouse and IAS in Orsay), which werevalidated on the Diabolo experiment and on the balloon-borneArcheops experiment. It was developped for space by the CESRin Toulouse.The particular features of the HFI AC bias readout are mainly(i) that the cold load resistors were replaced by capacitors becausethey have no Johnson noise; (ii) that the detectors arebiased by applying a triangular voltage to the load capacitorswhich produces a square current [I bias ]inthecapacitors,andasquare voltage [T bias ]thatcompensatesforthestraycapacitanceof the wiring (producing a nearly square bias current into thebolometer, as shown in Fig. 13); (iii) that a square offset compensationsignal is subtracted to the bolometric signal to minimisethe amplitude of the signal that has to be amplified anddigitized; (iv) that the electronicschemeissymmetricalandusesadifferential amplification scheme to optimize the immunity toelectromagnetic interferences; (v) and finally that every parameterof the REU (listed below) can be set by commands, whichis made possible by using digital-to-analog and analog-to-digitalconverters extensively.The modulation frequency f mod of the square bias currentcan be tuned from 70 Hz to 112 Hz by the telecommandparameters N sample ,whichdefinesthenumberofsamplesperhalf period of modulated signal, and by f div which determinesthe sampling frequency of the ADC. The optimal frequenciesare around 90 Hz.Each channel has its own settings:I biasT biasV balamplitude of the triangular bias voltage;amplitude of the transient bias voltage;amplitude of the square compensation voltage;G amp value of the programmable gain of the REU [1/3, 1,3, 7.6];N blank number of blanked samples at the beginning of halfperiodnot taken into account during integration of the signal;S phase shift when computing the integrated signal.All these parameters influence the effective response of the detectionchains, and were optimized during the calibration campaigns.They will be tuned again during the calibration andperformance verification (CPV) phase following the launch ofPlanck. Thescientificsignalisprovidedbytheintegralofthesignal on each half-period, between limits fixed by the S phaseand N blank parameters.The interaction of modulated readout electronics with semiconductorbolometers is rather different from that of a classicalDC bias readout (Jones 1953). The differences were seen duringthe calibration of the HFI, although the readout electronicswas designed (Gaertner et al. 1997) tomimictheoperationof a DC biased bolometric system as far as possible. With theAC readout the maximum of responsivity is lower and is obtainedfor higher bias current in the bolometer with respect tothe DC model. This is caused by the stray capacitance in thewiring which has negligible effects for a DC bias and a majoreffect for an AC bias. In our case, a stray capacitance of 150 pFinduces increases of NEP ranging from 4% (857 GHz bolometers)to 10% (100 GHz bolometers) and also affects the HFI timeresponse. Models were developped and will be published in aforthcoming paper.4.2. OverviewThe readout electronics consist of 72 channels designed to performlow noise measurements of the impedance of 52 bolometers,two blind bolometers, 16 accurate low temperature thermometers,all in the 10 MΩ range, one resistor of 10 MΩ andone capacitor of 100 pF. The semiconductor bolometers andthermometers of Planck-HFI operate at cryogenic temperaturearound 100 mK on the focal plane, with impedance of about10 MΩ when biased at the optimal current. The readout electronicson the contrary have to operate at “room” temperature(300 K). The distance between the two extremities of the readoutchain is about 10 m and could represent a point of extremesusceptibility to electromagnetic interference. The readout electronicchain was split into three boxes. These are the JFET box,located on the 50 K stage of the satellite at 2.2 m from the focalplane, the pre-amplifier unit (PAU), located 1.8 m further at300 K, and the REU (Fig. 14), located on the opposite side ofthe satellite, 5 m away. Each of the three boxes (JFET, PAU andREU) consists of 12 belts of six channels. The first nine beltsPage 11 of 20
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A&A 520, A9 (2010)Fig. 9. Cosmic rays hitting the bolometers produce glitches with decay times depending on the part heated by the particle (after Tristram 2005).Fig. 10. Example of noise spectrum taken in the CSL facility. The blackcurve is a raw spectrum of three hours of data. The red curve is obtainedby decorrelating the TOI in the time domain with a template made ofother bolometer signals and smoothed. The green curve is a fit to thered power spectrum with a simple two-component noise model. Thefour high and narrow lines near 10, 30, 50 and 70 Hz are due to the 4 Kcoolers (see text).Fig. 11. Individual sensitivity of all bolometers measured during calibrationscompared with requirements and goal.because of unexpected difficulties in the manufacture of the longnarrow horns imposed by the geometry of the focal plane.We ignore the value of several parameters that will contributeto the noise during the mission, like the real environment conditions(particles and microphonics) or the effective thermal emissionof the telescope. This emission will be determined partlyfrom inevitable contamination by dust during the launch andprobably from the degradation of the surface of the telescopeby micro-meteorites during the mission. By extrapolating the resultstaken on the ground, one can derive estimates of the sensitivity(Table 5) thatareverysimilartothevaluespublishedup to now (The <strong>Planck</strong> consortium 2005). Average sensitivity isfor 14 months of operation for square pixels with the nominalbeam size. The goal ∆T/T sensitivity (The <strong>Planck</strong> consortium2005)isreported(inItalics)forcomparison.Sensitivitytopointsources assumes an R − J spectrum and does not account for theconfusion limit.These results show that the bolometer programme was successfulfrom the early design stages through to fabrication.The optical, thermal and electrical environment together withFig. 12. Distribution of the sensitivity (linear scale) of HFI’s 52 bolometersestimated from ground measurements and normalized by the sensitivityrequirement. About half the bolometers should have a sensitivitybetter than the goal.associated low-noise readout electronics were also designed andimplemented successfully. They demonstratethatacompletesystem was developped although based on a new architecturePage 10 of 20
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