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Description
Although the cosmic microwave background (CMB) is remarkably well described by a perfect blackbody at a temperature of 2.725 K, small deviations, referred to as CMB spectral distortions, are predicted by a wide range of physical processes in the early Universe. Detecting these signals, however, is pushing instruments to their sensitivity limits while requiring exquisite control of their systematic effects.
BISOU (Balloon Interferometer for Spectral Observations of the primordial Universe) is an astronomical balloon-borne pathfinder instrument developed as part of a preparatory study for a future space mission aimed at measuring spectral distortions of the CMB. The optical system is based on a differential polarising Fourier Transform Spectrometer (FTS) that receives inputs from both a sky-facing telescope and an internal calibration source. The FTS focal plane is equipped with bolometric detectors coupled to multimode feed horns, distributed across two distinct detection units dedicated to the low- and high-frequency bands, covering the 90-1500 GHz range. The goal of this work is to present the design of the BISOU optical system with an emphasis on identifying and characterising the main systematic effects likely to affect the instrument's performance.