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Development of the multi-mode horn-lens configuration for the LSPE-SWIPE B-mode experiment

Legg, Stephen, Coppi, Gabriele, Lamagna, Luca, de Bernardis, Paolo, Giuliani, Grazia, Gualtieri, Riccardo, Marchetti, Tommaso, Masi, Silvia, Pisano, Giampaolo and Maffei, Bruno 2016. Development of the multi-mode horn-lens configuration for the LSPE-SWIPE B-mode experiment. Presented at: Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VIII, Edinburgh, Scotland, UK, 26 June - 1 July 2016. Published in: Holland, Wayne S. and Zmuidzinas, Jonas eds. Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VIII (Proceedings). SPIE Proceedings , vol. 9914. Bellingham, WA: Society of Photo-optical Instrumentation Engineers, p. 991414. 10.1117/12.2232400

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The Large Scale Polarisation Explorer (LSPE) is a balloon-borne experiment aiming to measure the B-mode component of the CMB polarisation at large angular scales. Onboard LSPE, the Short Wavelength Instrument for the Polarisation Explorer (SWIPE) is a bolometric polarimeter observing in three bands centred at 140, 220 and 240 GHz. The telescope is a single large-diameter plano-convex lens with a cold aperture stop. A small number of multi-mode feed horns feeding bolometric detectors are used within the focal plane, achieving a sensitivity equivalent to that of 100’s of single-mode horns. Simulations have been performed to predict the multi-mode optical response of the horn-lens configuration for centre and off-axis pixels pertaining to each frequency band. The horn has been simulated to a high accuracy using the Method of Moments. Using the horn simulation result as a source, the optical response of the lens has been examined using the more approximate simulation technique; Ray-Launching Geometrical Optics (RL-GO). Solution accuracy and simulation time depend heavily on the choice of RL-GO simulation parameters including: mesh size; the number of launched rays; and how densely the horn source beam is sampled. Individual convergence studies have been performed for each of these parameters and a final model has been obtained as a compromise between simulation time and accuracy. The instrumental polarisation of the lens is predicted to be at the -50 dB level. Finally, the optimal location of where to place the telescope focus in relation to the horn to maximise on-axis gain has been investigated. Several techniques agreed that the ‘phase centre’ is around 20 mm behind the horn aperture at 140 GHz, increasing to 30 mm at 220 and 240 GHz. Taking into account beam truncation effects caused by the finite size of the telescope was found to reduce the overall variation in on-axis gain.

Item Type: Conference or Workshop Item (Paper)
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Subjects: Q Science > Q Science (General)
Q Science > QB Astronomy
Publisher: Society of Photo-optical Instrumentation Engineers
ISBN: 9781510602076
ISSN: 0277-786X
Last Modified: 28 Jun 2019 14:33

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