Computation of the astrometric bias with multiples (×1, 2.5,5) of a ground level (G) of differential birefringence based on fiber measurements in the case of α Cru and with a degree of linear polarization varying between 0 and 50% and polarization angles in the 0–2π range for different hour angles. The instantaneous (u, v) coverage with the four Unit Telescopes is displayed as an inset in the bottom right of each plot. The full (u, v) coverage is presented in Fig. 1.