Measurement of instrumental polarisation of the Kodaikanal tunnel tower telescope

Abstract

The measurement of the instrumental polarization for any obliquely reflecting telescope is necessary to measure the polarization state of the input beam precisely. To find out the instrumental polarization of the reflecting mirror(s), the complex refractive index of the reflecting coating has to be measured with fairly good accuracy. Here we present the complex refractive index measurements for the aluminium-coated mirrors of the Kodaikanal tower tunnel telescope using an ellipsometer. The design of the ellipsometer using a Babinet Compensator and the results of the experiment will be discussed. These results will be used to remove the instrumental polarization contribution from the observed Stokes profile of magnetic regions on the sun