The polarization of the Cosmic Microwave Background (CMB) is at the center of attention of the cosmological community, as the observable possibly offers a view of primordial gravitational waves. A signature from an early-universe gravitational wave background would be imprinted on the parity-odd pattern of the CMB polarization, namely the B-modes. Achieving a non-zero primordial B-mode detection would serve as indirect evidence of the current most favorable scenario describing the initial perturbations of the universe, referred to as cosmic inflation. Several current- and next-generation telescopes are targetting high-accuracy measurements of the polarized microwave sky, with a particular emphasis on probing the inflationary paradigm. This effort faces two key challenges. The first is the impact of the galactic contaminants on the cosmological signal, which may be tackled by telescopes with increased frequency and sky coverage. The second concern refers to systematic errors arising from the instrument itself, highlighting the necessity of establishing a robust framework for their modeling. This thesis focuses on the modeling of a leading category of systematics that is associated with the telescope’s optics in the context of CMB analysis. A part of the thesis discusses the impact that combining non-ideal beams with a specific type of realistic polarization modulators, namely Half-Wave-Plates (HWPs), has on the reconstructed CMB B-mode spectra of a simulated satellite experiment. The rest of the analysis refers to the Simons Observatory (SO) Small Aperture Telescopes (SATs) that are currently being deployed from the Atacama Desert in Chile. Specifically, I first assess our expected beam calibration capabilities during science observations of the SO SATs in terms of the telescope’s scientific objective. I then expand on the calibration strategy optimization and pipeline infrastructure I developed for the commissioning phase of the first deployed SAT. This study aimed to assess the telescope’s anticipated pointing and beam reconstruction accuracy during its initial observations. Finally, I investigate how natural variations in beam patterns across the SO observing frequency bands influence the large-scale B-mode spectra of the SATs.