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image UV and visible polarimeter image polarimeter efficiency

UV and visible polarimeter
The F/13 beam from Arago's telescope enters the polarimeter, placed at the Cassegrain focus. Due to the wide wavelength range, it is not possible to find polarisation components with achromatic retardation. Arago's solution is to use a polarimeter with the highest efficiency of Stokes parameters extraction. The polarimeter consists of a modulator, i.e. a rotating stack of retardation plates, followed by a polarisation separator, which is a Wollaston prism. The modulator consists of 3 pairs of plates with different thicknesses and relative orientations, in optical contact. This whole assembly is rotated stepwise with increments of 30 degrees, to optimally modulate all the Stokes parameters at all wavelengths in the required UV+Visible wavelenth range. With 6 exposures, each with a different angular position of the modulator, we can extract the Stokes parameters (IQUV) with an optimum efficiency. The Wollaston prism is needed to sufficiently spatially separate the two orthogonal polarisation states, even at the lowest UV wavelengths where the birefringence is very low. The UV domain requires the use of specific materials, coatings, and optical surface qualities. The modulator and the polarisation separator of Arago are made of MgF2.
R&T development funded by CNES is ongoing to study other solutions for the polarimeter. A prototype of a robust achromatic polarimeter has been developed in 2015 as a backup solution for Arago. It consists of two birefringent wedges associated with a linear polariser, creating a spatial modulation of the polarisation in the orthogonal direction of the spectrum. This would allow a complete (IQUV) Stokes measurement with a single-shot exposure and without any moving parts. However, this technology has a low readiness level at this stage, which is why we rather propose the rotating modulator presented above as the baseline for Arago.