Test and calibration of the OLIMPO and LSPE experiments

In the last years, Cosmic Microwave Background (CMB) research moved from a discovery mode, with small experiments aimed at the investigation of the main characteristics of this radiation, to a precision mode, where complex large-scale experiments are designed to detect extremely small effects and characterize the finest details of this radiation. This transition implies extremely demanding requirements on the precision and accuracy of such instruments. The first aspect (precision) drove the development of large arrays of photon-noise-limited detectors with high mapping speed; the second one (accuracy) requires new methods of instrument calibration, and extremely accurate monitors of elusive systematic effects during the measurements. The focus of this thesis is on selected calibration and monitoring issues, which were implemented in two precision experiments, covering the two current mainstreams of CMB research: the OLIMPO experiment, aimed at spectroscopic measurements of the Sunyaev-Zeldovich (SZ) effect in the direction of galaxy clusters, and the LSPE-SWIPE experiment, aimed at the measurement of the polarization state of the CMB at large angular scales. In the following, in chapter 1, we introduce the scientific background for CMB measurements and the polarization effect, detailing then the SZ effect in chapter 2. In chapters 3 and 4 we describe, respectively, the OLIMPO and LSPE experiments and the systematic effects related to the temperature of the devices mounted on the different cryogenic stages; in particular, in chapter 4, we will focus on the SWIPE experiment. In chapter 5 we will focus on a class of systematic effects - related to temperature variations - which affect all CMB measurements, and describe in particular their effect in the OLIMPO and LSPE-SWIPE as balloon-borne experiments. In chapter 6 we describe the development of an accurate temperature monitoring system, which has been implemented first for OLIMPO and then re-implemented with improvements for LSPE-SWIPE. This monitoring system represents an essential tool to detect and mitigate temperature-related systematic effects.