Understanding polarization accuracy: the effect of mirror coating non-uniformity on instrument polarization

Astronomical mirror coatings are often metals protected by multiple layers of dielectrics. Varying the thickness and layering of dielectrics causes a significant dependence on the polarization properties (retardance, diattenuation, and depolarization) of reflected light across all wavelengths. Polarization further varies with angle of incidence and mirror shape. In models predicting polarization performance, assumptions on the properties and uniformity of coated optical surfaces are usually made. Here, we present how a non-uniformly applied coating affects polarization performance and causes depolarization across an aperture. We then assess the differences from assuming a uniform surface. Using the NSF's Daniel K. Inouye Solar Telescope as an example of a complex, many-optic, articulated system, we also compare depolarization effects of mirror coating non-uniformity to other known sources of systematic polarization error on DKIST.