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

A&A 520, A4 (2010)Fig. 9. Schematics of the LFI showing interconnections and details of the DAE and REBA main functions and units.3.3.3. Noise temperatureThe LFI radiometer sensitivity is essentially independent of thetemperature of the reference loads. From Eqs. (5) and(12) itfollows that, to first order, the radiometer sensitivity is√2δT =τ · ∆ν (T sys + T sky ) √ 1 + η L , (17)whereη L =(T N,B /G F ) 2(T sys + T sky )(T sys + T load ) , (18)and T N,B is the noise temperature of the back-end, and G F thegain of the front end. For parameter values typical of LFI, wehave η L < 10 −3 ,sothatthedependenceofthenoisetemperatureon T load is extremely weak. The advantage of cooling the referenceload to 4 K, therefore, rests solely on better suppression ofsystematics, not on sensitivity.4. LFI configuration and subsystemsThe overall LFI system is shown schematically in Fig. 9. Inthissection we provide an overview of the instrument units and mainsubsystems. More details on the design, development and testingof the most critical components are given in companion papersthat are cited below.4.1. The front-end unit4.1.1. Focal plane designThe disposition of the LFI feeds in the focal plane is driven byoptimisation of angular resolution and by recovery of polarisationinformation. In addition, requirements need to be met onproper sampling of the sky and rejection of crosstalk effects.The central portion of the Planck focal plane is occupied bythe HFI front end, as higher frequency channels are more susceptibleto optical aberration. The LFI feeds are located as close aspossible to the focal plane centre compatible with mechanical interfaceswith HFI and 4 K reference loads (Fig. 10). Miniaturiseddesigns for the FEMs and OMT are implemented to allow optimaluse of the focal area. The 70 GHzfeeds,mostcriticalforcosmological science, are placed in the best location for angularresolution and low beam distortion required for this frequency(Sandri et al. 2010).AkeycriterionforthefeedarrangementisthattheE andH planes as projected in the sky will allow optimal discriminationof the Stokes Q and U parameters. The polarisation informationis obtained by differencing the signal measured by the mainarm(R0) and side-arm (R1) radiometers in each RCA, which areset to 90 ◦ angle by the OMT. An optimal extraction of Q and Uis achieved if subsets of channels are oriented in such a waythat the linear polarisation directions are evenly sampled (Leahyet al. 2010). We achieve this by arranging pairs of feeds withtheir E planes oriented at 45 ◦ to each other (Fig. 11). With thePage 10 of 21