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. Liquid crystal variable retarders (LCVRs) will be used in the polarization modulation packages (PMPs) of the instruments SO/PHI (Polarimetric and Helioseismic Imager) and METIS/COR (Multielement Telescope for Imaging and Spectroscopy, Coronagraph) of the Solar Orbiter Mission of the European Space Agency (ESA). Optical retarders are dependent on the angle of incidence (AOI). Since the optical retardances during the polarization modulations are optimized for a particular AOI, other angles increase the polarimetric measurement error. Coronagraphs, such as METIS, are characterized by having wide field-of-view (FoV), which involves large incidence angles through the entire instrument. METIS PMP will work with collimated beams and an AOI up to ±7.0  deg. For this reason, a double LCVR configuration with molecular tilts in opposite directions was selected for METIS PMP, which provides lower angular dependence. The polarimetric performance of the METIS PMP flight model was measured at different AOIs and compared to a single LCVR PMP. The results shown in this paper demonstrate that the functional concept used in METIS guarantees the polarimetric performances at the wide FoV expected in METIS coronagraph. Moreover, a detailed theoretical model is showed and compared to the experimental data, finding successful agreement, which can be very helpful for the design of instruments characterized by wide FoV.

Polarimetric performance of a polarization modulator based on liquid crystal variable retarders for wide acceptance angles