Observation of the Cosmic Microwave Background Brightness and Polarization with BOOMERanG

In this thesis I describe \boom (Balloon Observation Of Microwave Extragalactic Radiation ANd Geophysics), a telescope mounted on a long duration stratospheric balloon, devoted to the measurement of extragalactic radiation in the microwave band of the electromagnetic spectrum. This work is not a complete description of the experiment, rather it is a presentation of a part of the research work done during my doctorate. A full description of the experiment and of the results obtained with it in the last years are given in the several referred articles and collaborators Ph.D. theses. \boom is an experiment of wide scientific interest, which gave in year 2000 a main contribution to cosmology, crossing for the first time what was considered a knowledge {\it frontier} in the field: to obtain a high signal to noise ratio maps of the cosmic background radiation. From those maps was possible to measure the value of the total energy density of the Universe, $\Omega_0 \simeq 1$, with important consequences on the description of our Universe. In this work I particularly treat the 2003 setup of \boomn, which flew in January of that year from Antarctica, measuring cosmic background temperature and polarization anisotropy. In the cosmic microwave background a level of linear polarization of the order of 1-5\% of the temperature anisotropy intensity is expected. The angular power spectrum of that polarized signal should be characterized by features strictly linked to the cosmological model. Observation of the angular power spectrum of polarization can confirm the model and improve the precision in the measurement of the cosmological parameters, the numbers that quantitatively describe the evolution of the Universe. The work is experiment-oriented because such was my research work, even if space is given to description of data analysis tasks. In Chapter~\ref{chap:cmb} I introduce the cosmic background radiation science, particularly focusing on information obtainable from polarization measurements and on the relevant physical observables. In Chapter~\ref{chap:instrument} I describe the instrument, in a general view and in detail, component by component. In Chapter~\ref{chap:flight} I present the pre-flight calibration measurements performed at ground to characterize the instrumental sensitivity to a polarized signal. To make that possible, a custom apparatus was built, in order to illuminate the telescope with a linearly polarized plane wave. In the same chapter I briefly describe the flight, hiding stress and emotions of those 15 days. In Chapter~\ref{chap:pointing} I describe the techniques used to obtain the pointing solution: the measure, for each data sample, of the direction in sky observed by each detector. I particularly worked on this aspect of the raw-data analysis, in which data are treated to be used in map-making and power spectrum estimation procedures. Main steps in the raw-data analysis are the pointing solution, data deglitching and deconvolution, calibration, and beam measurement for each receiver. The pointing solution technique adopted is fully described. It is based on the Kalman filter, an optimal method of data integration. In this Chapter, the problem is presented, together with the involved geometry, the optimal method and the developed algorithms. Results are shown. Finally I describe the power spectrum extraction problem and techniques and I present preliminary results and compare them to the data realized by other experiments.