Orateur
Description
While CMB temperature anisotropies have been characterised with precision, the CMB frequency spectrum has not been remeasured since the FIRAS instrument aboard COBE in the early 1990s. FIRAS showed that the spectrum is extremely close to a blackbody, with deviations from a perfect Planck spectrum below ΔI/I ∼ 10⁻⁵. Such small departures, known as spectral distortions, encode the thermal and energetic history of the early Universe, including epochs prior to the release of the CMB itself.
Spectral distortions are exceptionally faint signals: detecting them requires an exquisite understanding of the instrument, and anticipating from the earliest design stages how instrumental systematics will limit the final measurement. FOSSIL (FTS fOr CMB Spectral diStortIon expLoration) and BISOU (Balloon Interferometer for Spectral Observations of the primordial Universe) are two projects targeting their detection.
To assess the impact of systematic effects on the science measurement, we present an instrument simulator for FOSSIL and BISOU. This model forecasts the error induced on the science measurement by several instrumental systematic effects. This talk focuses on those linked to the optics. A laboratory breadboard of the instrument has been developed at the Institut d'Astrophysique Spatiale (IAS). It will allows us to characterise these systematics experimentally and to compare the simulator's predictions with measurements, a key step towards validating the model ahead of future missions. This presentation outlines the optical modeling process of the BISOU instrument. I will begin with an overview of the optical concept of its Fourier Transform Spectrometer (FTS), followed by the first overall configuration based on geometric optics. The design was refined and optimized through iterations using Gaussian beam propagation, enabling more realistic modeling of the instrument’s optical performance. The outcome is an optical configuration that incorporates key constraints such as size and thermo-mechanical limitations. I will present the current concept and the simulation results concerning the co and cross polarisation beam performance, including the ellipticity, together with the remaining steps required to finalize the design and prepare for the associated breadboard implementation.