Systematic effects induced by a flat isotropic dielectric slab

The instrumental polarization induced by a flat isotropic dielectric slab in microwave frequencies is faced. We find that, in spite of its isotropic nature, such a dielectric can produce spurious polarization either by transmitting incoming anisotropic diffuse radiation or emitting when it is thermally inho-mogeneous. We present evaluations of instrumental polarization generated by materials usually adopted in Radioastronomy, by using the Mueller matrix formalism. As an application, results for different slabs in front of a 32 GHz receiver are discussed. Such results are based on measurements of their complex dielectric constant. We evaluate that a 0.33 cm thick Teflon slab introduces negligible spurious polarization (< 2.6 × 10−5 in transmission and < 6 × 10−7 in emission), even minimizing the leakage (< 10−8 from Q to U Stokes parameters, and viceversa) and the depolarization ( ∼ 1.3 × 10−3). c