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Polarization model for the multi-application solar telescope at the Udaipur Solar Observatory

Solar polarimetry aims at measuring the full set of Stokes vectors (I, Q, U, V) to extract the magnetic field information of the solar atmosphere. During the polarimetric observations, the oblique reflections from the telescope mirrors modify (crosstalk) or even produce polarization (instrumental polarization). For accurate polarimetric measurements of the source, it is important to correctly model and remove the instrumental polarization and crosstalk introduced by the telescope. The Multi-Application Solar Telescope (MAST) at the Udaipur Solar Observatory is a Gregorian-Coude telescope with a 50 cm off-axis parabolic primary mirror. It consists of nine mirrors that rotate as the telescope tracks the object and delivers a constant beam at the observing floor. Here, we present a formalism for an analytical estimation of the telescope's Mueller matrix using a polarization ray tracing algorithm. The model was experimentally verified at 6173 Å using the observations taken from the facility imaging spectro-polarimeter. The observations were split into two sets, during January and May 2018, to verify the model's consistency. The instrumental polarization was found to vary between 1.3% to 5.54%, and 3.5% to 4.3% throughout the observation from 9 AM to 4 PM during summer and winter respectively. The maximum value of the crosstalk (Q to V and U to V) was found to be 29.08% and 39.46% respectively. We obtained a reasonable match between the model and the observations with some offsets. We also discuss the possible reasons for the observed deviations and their effects.