Spatial variation of the PSF polarisation e_α measured using a KSB+ Gaussian filter scale $r_{\mathrm{g}}=0\stackrel{″}{.}16$ (left panel) and its 2D third-order spatially interpolated model (right panel). Here we consider the central 20 000 × 20 000 pixels of this VIS stack (extending slightly beyond the primary WL region of interest; see Sect. 2.2), where the depicted coordinates (X_c, Y_c) are related to the pixel positions in the stack (X_s, Y_s) as (X_c, Y_c) = (X_s − 10799, Y_s − 10499). The whisker in the top left indicates a reference polarisation with |e| = 0.05.