Simulations and performance of the QUBIC optical beam combiner

MEDINA, M. C.; THE QUBIC COLLABORATION

Texas

Conferencia; SPIE Astronomical Telescopes & Instrumentation coference; 2018

QUBIC, the Q & U Bolometric Interferometer for Cosmology, is a novelground-based instrument that aims to measure the extremely faint B-modepolarisation anisotropy of the cosmic microwave background atintermediate angular scales (multipoles of 𝑙 = 30 - 200).Primordial B-modes are a key prediction of Inflation as they can only beproduced by gravitational waves in the very early universe. To achievethis goal, QUBIC will use bolometric interferometry, a technique thatcombines the sensitivity of an imager with the immunity to systematiceffects of an interferometer. It will directly observe the sky throughan array of back-to-back entry horns whose beams will be superimposedusing a cooled quasioptical beam combiner. Images of the resultinginterference fringes will be formed on two focal planes, each tiled withtransition-edge sensors, cooled down to 320 mK. A dichroic filter placedbetween the optical combiner and the focal planes will select twofrequency bands (centred at 150 GHz and 220 GHz), one frequency perfocal plane. Polarization modulation will be achieved using a coldstepped half-wave plate (HWP) and polariser in front of the sky-facinghorns. The full QUBIC instrument is describedelsewhere1,2,3,4; in this paper we will concentrate inparticular on simulations of the optical combiner (an off-axis Gregorianimager) and the feedhorn array. We model the optical performance of boththe QUBIC full module and a scaled-down technological demonstrator whichwill be used to validate the full instrument design. Optical modellingis carried out using full vector physical optics with a combination ofcommercial and in-house software. In the high-frequency channel we mustbe careful to consider the higher-order modes that can be transmitted bythe horn array. The instrument window function is used as a measure ofperformance and we investigate the effect of, for example, alignment andmanufacturing tolerances, truncation by optical components and off-axisaberrations. We also report on laboratory tests carried on the QUBICtechnological demonstrator in advance of deployment to the observingsite in Argentina.