Polarimetry

Part of the optics used with the FAPOL polarimetry unit is a calcite mounted in the slit wheel of ALFOSC. When checking the calcite after some report of features in the images when doing polarimetry with FAPOL, it was noted that the calcite was rather dirty. It was tried to clean the calcite by blowing some air on it, but it was not possible to remove the dirt. In principle, for the reduction only normal flat fields taken with the specific filter used for the observation is sufficient. The question is if, and by how much, the dirt on the calcite might have influence on the results, and if there is any additional reduction step needed that would improve the results.