Optical properties of dielectrics in the mm range: the case of spurious polarization induced by a vacuum window.

The noise of radioastronomy receivers is usually kept low by cooling the front-end into cryostats, where the internal and the external environments are interfaced by optical windows. Such dielectric windows can sistematically correlate the incoming unpolarized radiation and decorrelate its polarized component. Here, we present a study on the effects of dielectrics in high sensitivity microwave polarimetry, including a model of the induced spurious polarization, a selection of materials in term of their optical properties as well as measurements of their optical parameters.