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

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 hornsfeeding bolometric detectorsare used within the focal plane,achieving a sensitivity equivalent tothat of100’s of single-mode horns.Simulations have been performed to predict the multi-mode optical response of the horn-lens configurationfor centre and off-axis pixels pertaining to each frequency band. The horn has beensimulated to a high accuracy usingthe Method of Moments.Using the hornsimulation result as a source, theoptical response of the lens has beenexamined 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 oflaunched rays; and how densely the horn source beam is sampled.Individual convergence studies have beenperformed for each of theseparameters and a final model has beenobtained as a compromise between simulation time and accuracy. The instrumental polarisation of the lens ispredicted to beat the -50 dB level. Finally, the optimal locationof 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 to30 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.