I used to work on spectropolarimetric techniques, and these are some of my works,
— In this 1992 paper, we developed the equaitions that to the instrumental polarization in the focal plane of a telescope including differaction effects. The figure below corresponds to Stokes Q (maybe U) for unpolarized imput ligt (the Airy disk in Stokes Q corresponding to umpolarized illumination). The two main results are (1) even an axi-symmetric telescope has intrumental polarization, and (2) seeing induces instrumental polarization because it breaks down the axi-symmetry of the optical system. This second issue is pursued and quantified in a follow up paper. We even dare to talk about the polarizing properties of grazing incidence X-ray telescopes, in particular, the mirrors of telecope of Chandra.
— I like this 1995 highly unquoted paper very much. I worked out the general constraints imposed by photon noise on the determanation of the Stokes parameters. It is mostly analytical and based on inequelities, and the fact that the intensity is positive and the degree of polarization smaller than one.
— I was also involved in the first set of solar spectro-polarimeters built at the IAC, LPSP and TIP (1996, 1999): ref1, ref2.