Overview of the optical path of the complete optical system, i.e., the Unit Telescope (UT) and SPHERE/IRDIS, showing only the components relevant for polarimetric measurements (image adapted from Fig. 2 of Paper I). The names of the (groups of) components are indicated in boldface. The black circular arrows indicate the astronomical target’s parallactic angle p, the telescope’s rotation with the altitude angle a, the offset angle of the calibration polarizer δcal, and the rotation of the HWP and image derotator with the angles θHWP + δHWP and θder + δder, respectively. Also shown are the parameters describing the instrumental polarization effects of the (groups of) components: the component diattenuations ϵ, retardances 𝛥 and the polarizer diattenuation d. The Stokes vectors Sin, SHWP, Sdet, L, Sdet, R and used in the instrument model are indicated as well. Finally, the top right of the image shows the data-reduction process that produces the measured (after calibration) Stokes vector incident on the telescope.
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