Development of high performance optics for the stratospheric italian polarimeter

The Cosmic Microwave Background (CMB) is one of the main tools of modern cosmology. Three generations of space missions and ground-based and balloon-borne experiments, have been devoted to the observation of its anisotropies, telling us about the history, age, geometry and the ingredients of our Universe. An invaluable amount of cosmological information is hidden in the polarized component of the CMB. The detection of the so-called B-modes – the tiny imprint of primordial gravitational waves in the CMB photons – would strongly support the paradigm of Inflation, shedding light on the very first moments after the Big Bang and consolidating our understanding of the early Universe. The Large Scale Polarization Explorer (LSPE) has taken up this challenge. Through its two instruments, it will observe the CMB polarization on large angular scales and produce wide maps of foreground polarization generated by synchrotron and interstellar dust emissions in our Galaxy. This thesis focuses on the optics of the Stratospheric Italian Polarimeter (STRIP), the eyes of one of the two LSPE instruments. We started from the development and electromagnetic characterization of the STRIP forty-nine feedhorns. Then we modeled our array in the focal plane of the instrument telescope and simulated the optical response. Aberrations of the angular response in the main beam region and the straylight contamination due to sidelobes pickup are two of the main sources of systematic errors. Therefore, we assessed their impact on the observation of the polarized signal from the sky. A three-year journey to deeply know our very first interface with the sky.