Space UV polarimeters

Several space missions are proposed or planned for the coming two decades dedicated or including mid- to high-resolution spectropolarimetry on a wide UV band. This includes the European instrument Pollux for the NASA HWO flagship mission, the NASA SMEX candidate Polstar, and the French nanosatellite demonstrator CASSTOR. We are developing UV polarimeters for these missions thanks to a R&D program funded by CNES. For the mid- and near-UV, i.e. above 120 nm, birefringent material (MgF2) can be used to produce a polarimeter. This is the baseline for Polstar, CASSTOR, and the MUV and NUV channels of Pollux. Prototypes have been built and tested with excellent results, and further tests are ongoing to fully characterize them. For the FUV channel of Pollux however, it is not possible to use this technology and we have instead studied a design based on mirrors only. We will present the various missions and instruments, their technical challenges, as well as the R&D work performed on UV polarimeters and the proposed design solutions.