The BISOU (Balloon Interferometer for Spectral Observations of the Universe) project studies the viability and prospects of a balloon-borne spectrometer, pathfinder of a future space mission dedicated to the measurements of the CMB spectral distortions, while consolidating the instrumental concept and improving the readiness of some of its key sub-systems. A balloon concept based on a Fourier Transform Spectrometer, covering a spectral range from about 90 GHz to 2 THz, adapted from previous mission proposals such as PIXIE and FOSSIL, is being studied and modelled. Taking into account the requirements and conditions of balloon flights (i.e. residual atmosphere, observation strategy for instance), we present here the instrument concept together with the results of the CNES phase 0 study, evaluating the sensitivity to some of its potential observables. For instance, we forecast a detection of the CMB Compton y-distortion monopole with a signal-to-noise ratio of at least 5.
BISOU (Balloon Interferometer for Spectral Observations of the Universe) is a CNES phase zero study investigating the feasibility of observing spectral distortion in the CMB signal using a balloon-borne spectrometer, which also will act as a pathfinder for a future space mission dedicated to the measurements of the CMB spectral distortions. The CMB frequency spectrum is an important probe of the cosmological model. In this paper, we describe the optical layout and outline the initial optical analysis of the signal path and the reference beam path. The major challenge outlined is the inclusion of the required optical train in a confined volume of the cryostat. We include a multimoded description of the optical response of the system from the multimoded pyramidal horn through the optical path containing mirrors and wire grids and predict beam patterns on the sky.
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