Polarization modulators based on liquid crystal variable retarders for the Solar Orbiter mission

A polarization modulator based on Liquid Crystal Variable Retarders (LCVRs) will be used in the Polarimetric and Helioseismic Imager (PHI) for the Solar Orbiter mission to measure the complete Stokes vector of the incoming light. PHI is one of the six remote sensing instruments onboard of this space mission led by the European Space Agency (ESA) with strong NASA participation. It is an imaging spectro-polarimeter that will acquire high resolution solar magnetograms. Also the LCVRs will be used in the polarization modulator of the METIS instrument (Multi Element Telescope for Imaging and Spectroscopy). METIS is a solar coronagraph that will analyze the linear polarization for observations of the visible-light K-corona. The polarization modulators are described in this work including the optical, mechanical, thermal and electrical aspects. Both modulators will consist of two identical LCVRs with a relative azimuth orientation of 45o for PHI and parallel for the METIS modulator. In the first case, the configuration allows the analysis of the full Stokes vector with maximum polarimetric efficiencies. In the second setup, wide acceptance angles (≤ ±7°) are obtained. This works presents the preliminary results obtained for the full representative prototypes from the verification and environmental test campaign in progress currently. The main performances were measured and analyzed including polarimetric efficiencies, wavefront error transmission, beam deviation and transmittance. This valuable information will allow to consolidate the detailed design of these devices increasing its TRL to 6 and to proceed to the manufacturing of the Qualification Model (QM) and Flight Models (FM).