Big Bang cosmologies predict that the cosmic microwave background (CMB) contains faint temperature and polarisation
anisotropies imprinted in the early universe. ESA's PLANCK satellite has already measured the temperature
anisotropies1 in exquisite detail; the next ambitious step is to map the primordial polarisation signatures which are
several orders of magnitude lower. Polarisation E-modes have been measured2 but the even-fainter primordial B-modes
have so far eluded detection. Their magnitude is unknown but it is clear that a sensitive telescope with exceptional
control over systematic errors will be required.
QUBIC3 is a ground-based European experiment that aims to exploit the novel concept of bolometric interferometry in
order to measure B-mode polarisation anisotropies in the CMB. Beams from an aperture array of corrugated horns will
be combined to form a synthesised image of the sky Stokes parameters on two focal planes: one at 150 GHz the other at
220 GHz. In this paper we describe recent optical modelling of the QUBIC beam combiner, concentrating on modelling
the instrument point-spread-function and its operation in the 220-GHz band. We show the effects of optical aberrations
and truncation as successive components are added to the beam path. In the case of QUBIC, the aberrations introduced
by off-axis mirrors are the dominant contributor. As the frequency of operation is increased, the aperture horns allow up to five hybrid modes to propagate and we illustrate how the beam pattern changes across the 25% bandwidth. Finally we
describe modifications to the QUBIC optical design to be used in a technical demonstrator, currently being manufactured
for testing in 2016.
The facility to realise the shape and extent of optical beams within a telescope or beamcombiner can aid greatly in the
design and layout of optical elements within the system. It can also greatly facilitate communication between the optical
design team and other teams working on the mechanical design of an instrument. Beyond the realm where raytracing is
applicable however, it becomes much more difficult to realise accurate 3D beams which incorporate diffraction effects.
It then is another issue to incorporate this into a CAD model of the system.
A novel method is proposed which has been used to aid with the design of an optical beam combiner for the QUBIC (Q
and U Bolometric Interferometer for Cosmology) 1 experiment operating at 150 GHz and 220 GHz. The method
combines calculation work in GRASP 2, a commercial physical optics modelling tool from TICRA, geometrical work in
Mathematica, and post processing in MATLAB. Finally, the Python console of the open source package FreeCAD3 is
exploited to realise the 3D beams in a complete CAD system-model of the QUBIC optical beam combiner.
This paper details and explains the work carried out to reach the goal and presents some graphics of the outcome. 3D
representations of beams from some back-to-back input horns of the QUBIC instrument are shown within the CAD
model. Beams of the -3dB and -13dB contour envelope are shown as well as envelopes enclosing 80% and 95% of the
power of the beam. The ability to see these beams in situ with all the other elements of the combiner such as mirrors,
cold stop, beam splitter and cryostat widows etc. greatly simplified the design for these elements and facilitated
communication of element dimension and location between different subgroups within the QUBIC group.
The expansion of the universe has red-shifted remnant radiation, called the Cosmic Microwave Background (CMB)
radiation, to the terahertz band, one of the last areas of the electromagnetic spectrum to be explored. The CMB has
imprinted upon it extremely faint temperature and polarisation features that were present in the early universe. The next
ambitious goal in CMB astronomy is to map the polarisation characteristics but their detection will require a telescope
with unprecedented levels of sensitivity and systematic error control. The QUBIC (Q&U Bolometric Interferometer for
Cosmology) instrument has been specifically designed for this task, combining the sensitivity of a large array of wideband
bolometers with the accuracy of interferometry. QUBIC will observe the sky through an array of horns whose
signals will be added using a quasi-optical beam combiner (an off-axis Gregorian dual reflector designed to have low
aberrations). Fringes will be formed on two focal planes separated by a polarising grid.
MODAL (our in house simulation package) has been used to great effect in achieving a detailed level of understanding
of the QUBIC combiner. Using a combination of scalar (GBM) and vector (PO) analysis, MODAL is capable of high
speed and accuracy in the simulation of quasi-optical systems. There are several technical challenges to overcome but the
development of MODAL and simulation techniques have gone a long way to solving these in the design and analysis
In this paper I outline the quasi-optical modelling of the QUBIC beam combiner and work envisaged for the future.
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.