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

A. Mennella et al.: LFI calibration and expected performanceTo extrapolate the calibrated sensitivity from the 4.5 K inputtemperature in the test to flight conditions, we calculated theratio700Radiometer M-0∆T rms (T flightsky ) flight(Tsky + T noise )=∆T rms (T CSLsky) (T CSLsky + T noise) , (8)using the noise temperature found from the non-linear model fitfrom the receiver-level test campaign (Villa et al. 2010). This ratioranges from a minimum of ∼0.96 to a maximum of ∼0.98.Exact values for each detector are not reported here for simplicity.In Fig. 11,wesummarisegraphicallythein-flightsensitivityestimates from the three tests. In the following plots the sensitivityvalues are provided with errorbars, with the followingmeanings:– errorbars in sensitivities estimated from satellite-level datarepresent the statistical error in the calibration constants calculatedfrom the various temperature jumps and propagatedthrough the sensitivity formulas. They represent genuine statisticaluncertainties;– errorbars in sensitivities estimated from receiver and instrumentlevel tests data represent the uncertainty coming fromthe calculation performed according to the two differentmethods described in Sect. 5.2.3 and Appendix D. Inthiscase, errorbars do not have specific statistical significance,but nevertheless provide an indication of the uncertainties inthe estimate.Figure 11 shows that the in-flight sensitivity lies between therequirement and twice the goal levels for the 30 and 70 GHz receivers,and at about twice the goal for the 44 GHz receivers.The agreement between values extrapolated from the threetest campaigns is very good, apart from two noticeable outliers,LFI21S-1 and LFI24M-0, whichshowedahighernoiselevel during satellite level tests. Investigation showed that thisanomaly was caused by incorrect bias voltages on the front-enddevices during the test.After a thorough bias tuning activity conducted during inflightcalibration (see Cuttaia et al. 2009), a new bias configurationwas found that normalised the white noise sensitivity ofthese two receivers, as expected. Afulldescriptionoftheinflightcalibration results and scientific performance will be givenin a forthcoming dedicated paper.6.2. Noise stabilityDuring satellite-level tests, receiver noise stability was determinedfrom stable data acquisitions lasting several hours withthe instruments in their tuned and nominal conditions. Figure 12summarises 1/ f knee frequencies measured at instrument andsatellite levels compared with the 50 mHz requirement, andshows that the noise stability of all channels is within requirements,with the single marginal exception of LFI23S-11. Theslope ranged from a minimum of 0.8 to a maximum of 1.7.During satellite-level tests, there was substantial improvementin the noise stability relative to instrument-level tests, insome cases with a reduction in knee frequency of more than afactor of 2. This can be partly explained by the almost perfectsignal input balance achieved in the CSL cryo-facility, whichwas much less than 1 K compared to the ∼3 Kobtainedintheinstrument cryo-facility. Some improvement was also expectedWhite noise (uK*sqrt(s))White noise (uK*sqrt(s))600500400300200100Satellite level testsInstrument level testsReceiver level testsRequirement2*Goal0LFI18 LFI19 LFI20 LFI21 LFI22 LFI23 LFI24 LFI25 LFI26 LFI27 LFI28700600500400300200100Radiometer S-1Satellite level testsInstrument level testsReceiver level testsRequirement2*Goal0LFI18 LFI19 LFI20 LFI21 LFI22 LFI23 LFI24 LFI25 LFI26 LFI27 LFI28Fig. 11. Summary of in-flight sensitivities per radiometer estimatedfrom receiver, instrument, and satellite-level test campaigns.because of the much higher thermal stability of the CSL facility.In particular fluctuations of the sky and reference loads inCSL were about two order of magnitudes less than those in theinstrument facility (see Table 3). Because the highly balanced inputachieved in CSL will not be reproduced in-flight, we expectthat the flight knee frequencies will be slightly higher (althoughsimilar) than those measured in CSL.6.3. IsolationIsolation (see Eq. (B.3)) was measured during the satellite testsby changing the reference load temperature by 3.5 K. Figure 13compares the isolation measured during receiver- and satelliteleveltests. Several channels exceed the −13 dB requirement; afew are marginally below. One channel, LFI21S-1,showedpoorisolation of only −7dB. This result is consistent with the highvalue of the calibrated white noise measured for this channelPage 11 of 16