Minimizing instrumental polarization in the multiangle spectropolarimetric imager (MSPI) using diattenuation balancing between the three mirror coatings

Special enhanced silver mirror coatings were designed and fabricated to minimize the polarization introduced by a three-mirror off-axis high-accuracy telescope. A system diattenuation of approximately 1% in the VIS-NIR was achieved by both reducing the diattenuation from each mirror individually and by balancing the diattenuations introduced by the three mirrors over the spectral range. This process of low-polarization engineering involves minimizing system polarization introduced by surface geometry, thin film coatings and birefringent elements, and measuring the system. In this report we will outline a methodology to minimize instrumental polarization aberrations, with an emphasis on achieving low diattenuation in the MSPI camera, given its off-axis geometry and coating design constraints imposed by the space-based application. This polarization balancing technique for mirror coatings can be applied to astrophysics applications.