Polarization properties at the Nasmyth focus of the alt-azimuth TNG telescope

In alt-azimuth telescopes Nasmyth foci are suitable focal planes to reduce mechanical instrument complexity and costs. However, they present some disadvantages mainly due to the field rotation. This is a particularly crucial point in polarimetric observations. Because of the folding mirror, the radiation polarization state is so modified that, to avoid systematic errors, instrumental polarization has to be removed as a function of the telescope position. A model of the polarization introduced by the Telescopio Nazionale Galileo (TNG) at its focal plane is presented. The model takes into account physical and geometrical properties of the optical system, complex refraction index of the mirrors and their relative position, deriving instrumental polarization as a function of the pointing coordinates of the telescope. This model has been developed by means of Muller matrices calculation. Telescope instrumental polarization has been measured following some standard polarization stars at different telescope positions. The mathematical model, here discussed, was confirmed comparing the theoretical results and the experimental measurements at the TNG instruments.