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

A&A 520, A1 (2010)Fig. 12. The left panel shows the predicted initial cool-down profiles of the temperature stages in the coolers. The plateau at 170 K is created byheating, to prevent outgassing from contaminating the reflector and focal plane sufaces. The model does not represent the cool-down profiles of theactively cooled stages accurately: the right panel shows the profile measured during on-ground tests, which is expected to be close to the in-flightprofile. Figures courtesy of HFI (J.-L. Puget).sky at least twice. It will operate autonomously, driven from anon-board timeline which is uploaded daily during the 3 h periodof contact with the ground. The contact period will also be usedto downlink to ground the data which have been acquired overthe past 24 h.3.2. Payload commissioning and performance verificationFunctional commissioning started immediately after launch, firstaddressing critical satellite subsystems, and secondly the payload.At the time this paper is being submitted for publication,the commissioning activities are completed, and all on-boardsystems are behaving nominally.Initially, the telescope reflectors and the focal plane wereheated to prevent contamination by outgassing from otherpayload elements. As soon as heating was removed (abouttwo weeks after launch), the payload cooled radiatively ratherquickly, see Fig. 12. Duringthisphase,thecryo-chainwasgradually turned on and commissioned. The temperature profileachieved during cool-down was also used to tune and evaluatethe LFI’s radiometric performance. The coldest temperature of0.1 K was reached about 50 days after launch. At this time a onemonth phase of activities started, dedicated to the optimisationof the settings of the cryo-chain and the two instruments. Thisphase culminated with a two-week period of observations mimickingroutine surveying, after which small adjustments to thesettings could have been made (but were not necessary), beforethe start of the survey phase.3.3. Surveying strategyAfter the initial commissioning and performance verificationphases were completed, Planck started to survey the sky and wasscheduled to do so during 15 months 9 .Nointerruptionsoralterationsin the scanning strategy need to be made for polarisationcalibration or beam mapping, since the corresponding sourceswill anyway be observed. During this period the satellite movesin its orbit around L2 and L2 around the Sun. Its spin axis isactively displaced on the average 1 ◦ per day in ecliptic longitudeto maintain its anti-Sun direction (see Fig. 13). The instrumentField-of-View rotates around the spin axis and will coverthe full sky at least twice over within the nominal survey period.9 The satellite carries enough cryogens to allow an extra 12 months ofoperation.Fig. 13. From its orbit around L2 (Fig. 11), Planck will scan the sky asits Field-of-View rotates at 1 rpm. The spin axis is moved on average by1 ◦ /day (in 2 arcmin steps) to maintain the spin axis at a constant aspectangle to the Sun of 7.5 ◦ .Table 5. Scanning strategy parameters.θ 7. ◦ 5ω 2π/(6 months)φ 340 ◦n 1Step 2 arcminGeneral considerations on the exact choice of the path to be followedby the spin axis are described in Dupac & Tauber (2005)and Delabrouille et al. (2000). The cycloidal spin axis path selectedallows Planck to maintain a constant aspect angle to theSun and to cover the whole sky with each detector in the FOV. Itis defined by the following functions 10 :λ = θ sin[(−1) n ω(t − t 0 ) + φ] (2)β = −θ cos[(−1) n ω(t − t 0 ) + φ] (3)where λ is the angular distance from the fiducial point in Eclipticlongitude, β the angular distance from the fiducial point (theanti-Sun direction) in Ecliptic latitude, θ the spin axis precession10 These equations are not exactly followed by the mission planningsoftware, which corrects for the variation of the Earth’s orbital speedon the path of the cycloid, but the differences are small enough to benegligible for the purpose of characterising the survey coverage.Page 14 of 22