Orateur
Description
The cosmic microwave background (CMB) spectrum measured by COBE-FIRAS is the most perfect blackbody observed so far, with upper limits on spectral distortions of |y| < 1.5×10⁻⁵ and |μ| < 9×10⁻⁵. Smaller distortions, left by energy injection at different epochs of cosmic history, are nevertheless expected below these limits. FOSSIL, a CMB spectral distortion mission proposed to ESA M8, aims to detect them with a differential Fourier Transform Spectrometer that compares the sky to an internal reference blackbody, the BBIR (BlackBody Internal Reference). This measurement principle turns the science goals directly into thermal requirements. The BBIR must be even closer to a perfect Planck spectrum than the CMB itself.
Following a brief description of the measurement concept, I will present the thermo-mechanical architecture of FOSSIL, from passive radiative cooling with V-grooves down to the 4.5 K instrument stage and the sub-kelvin cooling of the detectors. I will then present the BBIR design and the proposed solution to meet the thermal stability and photometric requirements driven by the science objectives. BBIR systematic effects can mimic a distortion and must remain well below the part-per-million level. Finally, I will present how thermal and spectral modelling is used to optimise the BBIR geometry, materials and thermal environment against these requirements.