Systematic polarisation effects introduced by lenses and by axial birefringent half-wave plates used in CMB experiments in the millimetre region

A lot of effort is currently devoted to the design of ground based experiments dedicated to the detection of the Cosmic Microwave Background Radiation Polarisation. The very low level of the expected signals requires a detailed study of the spurious effects introduced by each element of the optics. We use the well-known general formulae for the reflection and transmission coefficients in isotropic medium-uniaxial birefringent crystal interfaces for any angle of incidence and arbitrary birefringence optical axis orientation. Ordinary and extraordinary ray tracings are carried out and arbitrary incoming polarisation is used to calculate instrumental polarisation and depolarisation effects.