Mirror contamination in space I: approach

This is the authors' abstract. We don't add key points for this paper.

We present a comprehensive model that can be employed to describe and correct for degradation of (scan) mirrors and diffusers in satellite instruments that suffer from changing optical UV-VIS properties during their operational lifetime. As trend studies become more important, so does the importance to understand and correct for this degradation. This is the case not only with respect to the transmission of the optical components, but also with respect to wavelength, polarisation or scan-angle-effects. Our hypothesis is that mirrors in-flight suffer from the deposition of a thin absorbing layer of contaminant, which slowly builds up over time. We describe this with the Mueller matrix formalism and Fresnel equations for thin multi-layer contamination films. Special care is taken to avoid the confusion often present in earlier publications concerning Mueller matrix calculus with out of plane reflections. The method can be applied to any UV-VIS satellite instrument, and in any orbit, both low and geostationary. We illustrate and verify our approach on the optical behaviour of the multiple scan mirrors of SCIAMACHY (on board of ENVISAT).