Design of a far-UV polarimeter for POLLUX aboard NASA's HWO flagship mission

There are currently no high-resolution wide-wavelength range spectropolarimeters in the UV domain. This represents a critical gap in our observational capabilities. Overcoming this challenge holds promise for opening up new frontiers in the study of celestial objects, from star-planet interactions to cosmology. The design of the POLLUX spectropolarimeter for the HWO mission comprises four channels in the visible, near-UV, mid-UV, and far-UV ranges. Although MgF2 will be used for the polarimeters in the NUV and MUV, the challenge arises in the far-UV range (100-120 nm), where the absence of known transparent birefringent glasses poses a substantial hurdle. The solution lies in the development of a new type of spectropolarimeter, the reflective FUV polarimeter. After a first study conducted several years ago, the goal of this work is to improve the TRL of this new kind of spectropolarimeter. This article describes the proposed design of the FUV polarimeter for HWO, the optimisation process using the Mueller calculus, and the ongoing prototyping and tests conducted.