QUBIC VIII: Optical design and performance
  The Q and U Bolometric Interferometer for Cosmology (QUBIC) is a ground-based
experiment that aims to detect B-mode polarisation anisotropies in the CMB at
angular scales around the l=100 recombination peak. Systematic errors make
ground-based observations of B modes at millimetre wavelengths very challenging
and QUBIC mitigates these problems in a somewhat complementary way to other
existing or planned experiments using the novel technique of bolometric
interferometry. This technique takes advantage of the sensitivity of an imager
and the systematic error control of an interferometer. A cold reflective
optical combiner superimposes there-emitted beams from 400 aperture feedhorns
on two focal planes. A shielding system composedof a fixed groundshield, and a
forebaffle that moves with the instrument, limits the impact of local
contaminants. The modelling, design, manufacturing and preliminary measurements
of the optical components are described in this paper.
