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

The resulting typical effect on a beam pattern is illustratedin Fig. 17. Weemphasizethattheuncertaintyinthisestimateis large. One of the features noted in the RFQM measurementsis that at high frequencies (i.e. at 100 and 320 GHz), the measuredpatterns show gain levels in the mid-side lobe regions thatare higher than expected. Figure 12 shows a discrepancy (labelled“3”) between the predicted and measured patterns, whichcould be caused by dust 15 ;acorrespondingfeatureisseeninthesame angular region at 320 GHz at a level 10 dB higher, whichis consistent with the frequency dependence of dust scattering.Adetailed2Dviewofthe100GHzmeasuredandpredictedpatterns(Fig. 18) reinforcestheindication.However,ifthesemeasuredlevels are truly due to dust, then they are at a higher amplitudethan even most conservatively modelled. An alternativeexplanation is that they are caused by diffuse reflection and scatteringfrom the walls of the CATR.J. A. Tauber et al.: Planck pre-launch status: The optical system9. Self-emissionThe payload and satellite radiate thermally within the detectorbandwidths; if the radiating surface fluctuates in temperature oremissivity, a corresponding signal fluctuation at the detector willbe generated. This is referred to as self-emission. The amplitudeof the detected signal depends on:– the amplitude of the temperature fluctuation;– the emissivity of the surface;– the RF coupling of the surface to the detector.The most troublesome self-emission signals are those that aresynchronised with the satellite spin rate or one of its harmonics,as they cannot be distinguished from a signal originating in thesky. The main sources of thermal fluctuations within the payloadmodule are the 4 K and sorption coolers, which are thermallylinked to the focal plane and the V-grooves (the upper one ofthese is the most relevant one). Their basic temporal frequenciesare not linked to the spin rate: the 4 K cooler basic frequencyis ∼40 Hz, and the sorption cooler has two basic periods: onerelated to the individual beds (varying between ∼1000 s at thebeginning of life to ∼500 s at the end of life) and one to the wholecooler cycle (six times longer). However, their spectra are broadand the sorption cooler especially contains weak components atand near 1/60 Hz and harmonics; these are taken as the worstcase values. The reflectors are thermally linked to the V-grooves,and therefore also have some very low level (of order ∼0.1 µK)residual fluctuation related to the coolers.Both the baffle andthetopoftheprimaryreflectormaybedirectly illuminated by the Moon, when the angle of the spin axisto the Moon is larger than 14.5 ◦ (respectively 25 ◦ ), in which caseaspin-synchronousfluctuationisexcited.Thesesituationswillbe avoided by mission planning but represent a useful worst caseto analyse.The optical coupling levels from the payload elements tothe detectors were estimated using GRASP and are shown inTable 7. Thetableshowsthattheworst-casespin-synchronoussignals at the detector due to self-emission are of order nK.Signals that are present at other frequencies can be correlatedto thermometer readings and are thus easy to remove.Farther from the payload, the Service Module contains awarm radiator that dissipates into space the desorption heat used15 The estimated obscuration by dust on the RFQM reflectors is largerthan that expected for Planck in-flight, i.e. ∼10 000 ppm, which shouldincrease the BRDF by a factor of ∼2.Fig. 18. When viewed on the same grid and at the same resolution, it becomesapparent that the pattern measured on the RFQM at 100 GHz(above) does not have the deep nulls seen in the most accurately predictedpatterns (below). It is possible that this filling-in of low-levelpower is due to scattering by dust on the reflectors (see Sect. 8.1).Asimilareffect is seen at 320 GHz (see Fig. 12) withahigherlevelof ∼10 dBi, which is qualitatively consistent with the effect of dust.by the sorption cooler. The amplitude of its temperature fluctuationis large (∼1 K)butitisveryweaklyopticallycoupledtothe detectors (with coupling below 10 −17 ). The inner edge of thesolar array may fluctuate with the spin but is also weakly coupled(10 −13 ). The strongest signal may be caused by the dailyoperation of the telemetry transmitter, which is estimated to inducean increase in temperature of several 100 mK at the top ofPage 17 of 22