Upper panel: instrumental polarization predicted by the model for an unpolarized source corresponding to matrix (17) (solid line) and including the systematic effects of the the analyzer (see Sect. 2.5 and Fig. 7 therein, dashed line). The curves show the Stokes parameters Q (blue), U (red), and V (green) as a function of hour angle. Here we considered the most common instrumental setup with Wollaston, HWP, and S13 optics. Middle: linear (green) and total (red) polarization degree corresponding to the upper Stokes values. Lower panel: polarization angle.