Systematic effects introduced by lenses at mm-wavelengths in CMB applications

Telescopes for the next generation of CMB (Cosmic Microwave Background) experiments could be based on either reflective (such as Planck, Clover) or refractive optics (BICEP, LSPE, SPIDER). Both techniques have advantages and disadvantages. On-axis lens based telescopes can be compact while off-axis reflective configurations can be large. The RF performances of mirror based telescopes are very well understood, whereas lens based systems have a lower technology readiness level: specifically, the systematic effects (aberrations such as chromaticity, birefringence, losses, standing waves and cross-polarisation) that they can introduce need to be accurately quantified at millimetre-wave. This paper reports on both RF modelling and preliminary experimental studies of a lens coupled to a feed-horn antenna for which the co- and cross-polarisation beam patterns are characterised