Spectropolarimetry and polarization-dependent fringes

An account is given of the formation of polarization-dependent fringes, the “enemies" of spectropolarimetry, present practically in all polarization devices. Typically, multiple reflections give rise to secondary beams coherent with the main beam, but with wavelength-dependent phase differences. Polarized fringes may appear in any particular Stokes parameter or their combinations, and may (or may not) be seen in the intensity. The mechanism of formation of fringes in polarization is demonstrated. Fringes of analyzers, beam-splitter and retarders are evaluated. Flat-fielding techniques are discussed and ways for the removal of polarized fringes are suggested. Techniques that are efficient in removing fringes in intensity may be inadequate or insufficient for polarization. Anti-reflection coating, for instance, may reduce fringes, but not below 10-3 of the intensity. This level would often be acceptable for intensity measurements but not necessarily for polarization, where one may often need to evaluate signals down to the level of 10-5, as in the observation of resonance line polarization and the second solar spectrum. Flat-fielding, by observing disk center, may be satisfactory to remove fringes for solar observations but this is not appropriate for stellar observations. The understanding of how these fringes are formed is essential both for the design of polarimeters and for their proper use