Characterization of BISOU systematic effects and optical-path asymmetry

Characterize the systematic effects of the BISOU cryogenic prototype, including the asymmetry between its two optical paths, which remain unresolved for this type of instrument.

Background

The BISOU cryogenic Breadboard model is being constructed to validate the balloon-borne differential Fourier Transform Spectrometer concept, investigate systematic effects, and test detector technologies. Because the instrument uses two optical paths—one observing the sky and the other viewing an internal 2.7 K blackbody reference—differences between these paths may produce systematic errors in measurements of the extremely small spectral distortions of the cosmic microwave background.

The paper specifically identifies the asymmetry of the two optical paths as an effect requiring study, while stating that systematic effects for this type of instrument are still unknown. Resolving these effects is necessary for validating the instrument concept and assessing whether it can achieve the sensitivity required for spectral-distortion measurements.

References

A cryogenic Breadboard model (BBm) is being built at the Institut d'Astrophysique Spatiale (IAS). This cryogenic prototype represents an important step toward validating the concept, studying systematic effects that are still unknown for this type of instrument, and testing new technologies such as detectors. In particular, the asymmetry of the two optical paths will need to be studied.

— Optical concept model of the future cosmology project BISOU  (2608.13257 - Gall et al., 13 Aug 2026) in Section 2, “The BISOU instrument concept” (p. 302)