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.
Special approaches unique to the waveband are required for the modelling of terahertz optical systems. Ray tracing is
inadequate because in typical instruments the propagating beams are not very many wavelengths in diameter and a
"quasi-optical" approach is required in which Fresnel diffraction effects can be efficiently and accurately simulated.
Typically, it is also necessary to be able to simulate the coupling of quasi-optical beams to feed antenna structures to
predict optical performance. In many systems the beams can be considered to be coherent and their propagation through
a beam guide consisting of re-focussing elements can be efficiently modelled using modal analysis, especially useful for
quick design purposes, beam control and optimisation. This modal approach has been extended to allow for aberrations
and truncation particularly relevant in compact mirror based systems. At the same time physical optics, although
computationally intensive, is also a useful tool when detailed analysis is required, particularly for providing crosspolarisation
information and high accuracy far-field beam patterns from large reflecting antennas, for example. However,
modal analysis in general is a very powerful tool, which enables one also to understand issues associated with throughput
when partially coherent systems are being considered. This is important for the efficient optical modelling of large arrays
systems now being developed for next generation astronomical instrumentation. In the presentation, we will discuss
these issues and present examples from real instrumentation. We also summarise our continuing work on the
development of computationally efficient modelling tools for fast quasi-optical design and analysis.
The Millimeter-Wave Bolometric Interferometer (MBI) is a ground-based instrument designed to measure the
polarization anisotropies of the Cosmic Microwave Background (CMB) and contains a number of quasi-optical
components, including a complex back-to-back system of corrugated feed-horn antennas. In this paper we use MBI as
an example to demonstrate the existing modeling techniques and as a focus to develop extended modeling capabilities.
The software we use to model this system targets the millimeter and sub-millimeter region of the electromagnetic
spectrum and has been extended to efficiently model the performance of back-to-back corrugated horns embedded in
larger optical systems. This allows the calculation of the coupling of radiation from the sky to the detector array through
a back-to-back horn feed system.
Optical design in the terahertz (THz) waveband can be challenging, especially for high-precision applications. In this
paper we summarise our experience with the quasi-optical design and subsequent performance of astronomical
telescopes designed to measure the faint temperature and polarisation properties of the Cosmic Microwave Background
Radiation, in particular QUaD<sup>1</sup>, the PLANCK Surveyor<sup>2</sup> and MBI<sup>3</sup>. These telescopes contain a range of quasi-optical
components including corrugated feed horns, on- and off-axis conic mirrors and lenses. Knowledge of their optical
performance and beam patterns is critical for understanding systematic effects in the reliable extraction of feeble
Although Physical Optics can be used to characterise electromagnetic systems to high accuracy, it is computationally
intensive at these frequencies and often not suitable for the initial design or preliminary analysis of large multi-element
optical systems. In general there is a lack of dedicated software tools for modelling the range of components and
propagation conditions encountered in typical systems and we have employed a variety of commercial and in-house
software packages for this task. We describe the techniques used, their predictions and the performance of the
telescopes that have been measured to-date.
MODAL is an optical design and analysis package targeting the millimetre and sub-millimetre region of the
electromagnetic spectrum. It is being developed at NUI Maynooth with the aim of integrating advanced modelling
techniques and access to High Performance Computing into a user-friendly and yet very powerful tool for an
(quasi-)optical designer. MODAL has been recently extended to allow integrated simulation of custom corrugated
horns and dielectric lenses. This made it possible to model an existing instrument (QUaD), with the goal of
optimising its performance.
Here we present new results from analysis of the predicted performance of the QUaD telescope, with particular
emphasis on polarisation information. They were obtained by using MODAL to model the whole telescope, with
the distortion of the primary accounted for, for a range of component tilts and separations.