The optical design of the QUBIC beam combiner

The Q and U Bolometric Interferometer for Cosmology (QUBIC) is a ground-based interferometer that aims to meet one of the major challenges of modern cosmology in the detection of B-mode polarization anisotropies in the Cosmic Microwave Background.B-mode anisotropies originate from tensor fluctuations of the metric produced during the inflationary phase of the early Universe. Their detection would therefore constitute a major step towards understanding the primordial Universe. The expected level of these anisotropies is however so small that detection requires instruments with high sensitivity and extremely good control of systematic effects. The QUBIC instrument is based on the novel concept of bolometric interferometry, and exploits the sensitivity advantages of bolometric detectors along with the greater control of systematics offered by interferometry.The instrument will directly observe the sky through an array of entry horns whose signals will be combined optically onto an array of bolometers cooled to around 300mK. The whole set-up is located inside a cryostat. The sensitivity of the instrument is maximised if equivalent baselines produce identical fringe patterns on the focal plane. This requires the minimization of wavefront aberrations for a wide field-of-view and a fast system.In this poster we present the quasi-optical design and analysis of the dual reflector designed to do this. We report on the loss of sensitivity for different levels of optical aberration in the combiner. The sensitivity of the QUBIC instrument is comparable with that of an imager with the same number of horns but with much greater control over systematics.