The Planck-LFI : a Study of Instrumental and Astrophysical Effects

This thesis aims to address some aspects of the Low Frequency Instrument (LFI) on board the PLANCK satellite. As for any CMB experiment a great attention has to be devoted to all the possible systematic effects. Previous experiences in CMB experiments have in fact demonstrated that the more and the larger are systematic effects which contaminate the data and which have to be scrubbed in the data analysis, the less robust the final results will be. It is therefore of great importance, for not degrading the nominal angular resolution and sensitivity per resolution element, to carefully address and quantify all potential systematic effects. Through accurate and realistic simulations of PLANCK-LFI observations we study how LFI performances are affected by some of these systematic effects and how to control and further reduce these effects. This thesis is organized as follows. We give a brief overview of the origin of microwave sky fluctuations, including CMB anisotropy, foreground contaminations originated within our Galaxy (synchrotron, free-free and dust emission) and extra-galactic foregrounds (Sunyaev-Zel'dovich effect and point sources fluctuations). Since accurate simulations are needed, they must include complete and realistic simulated microwave sky at the various observing frequencies. Unfortunately our present knowledge of foreground emissions (both galactic and extra-galactic) is far from complete and approximations have to be made. All these problems are discussed in Chapter 2. A presentation of the PLANCK mission and its scientific capabilities is reported in Chapter 3: §3.2 reports on the selected orbit and scanning strategy; §3.3 describes the adopted telescope configuration and actual focal plane arrangement; §3.4 briefly outlines the LFI instrument and §3.5 reports LFI scientific capabilities. An introduction to the systematic effects addressed in this thesis is in §3.6 and the Flight Simulator code is presented in §3. 7. . The rest of the work deals with the results from different kinds of PLAN9K-LFI :?imulations. The off-axis position of the LFI instrument, as in the present baseline, results in optical aberrations in the angular response function of the instrument. The effect of these distortions (usually called main-beam distortions since they affect the very central part of the response function) on the nominal angular resolution is addressed in Chapter 4, firstly considering a pure CMB sky, and then a more realistic sky including galactic emission. §4.l uses approximated response functions while "real" optical simulated ones are considered in §4.4. The effective angular resolution is derived and the loss in capabilities of cosmological parameters extraction properly quantified. The angular response function of the LFI instrument at large angles out of the central part is extremely complex and depends not only on the telescope design but also on the whole optical system (shields, supporting structures, focal plane assembly). Signal and signal variations entering at large angles from the true direction of observation may produce errors on CMB measurem~nts. Chapter 5 addresses this issue using a simulated full pattern of the response function and considering signal coming from our Galaxy (§5.1 and §5.2). The level of this contamination and its spatial distribution on the sky are discussed in §5.4. As described in Chapter 2, PLANCK is a spinning space-craft with 1 minute period. Instrumental drifts occurring on time scales less than the spinning period are possible sources of systematic artifacts in final data. In general they produce "stripes" in the final maps. Chapter 6 considers typical instrumental drifts which are mainly due to gain fluctuations in the LFI amplifiers. A de-striping code for removing these artifacts is described in §6.6; its performances and possible residual striping are evaluated in §6. 7. Finally, Chapter 7 overviews simulations results and their implication on the optimization of the PLANCK design.