Aperture polarimetry: P_Q and N_Q parameters as function of the aperture for the various observing series. P_Q parameters, represented by blue empty circles, are offset to the values corresponding to the aperture adopted for the measurement and reported in Table 1. This point is hightlighted with solid circles and dotted lines. N_Q parameters, represented by red empty circles, are offset by − 0.5% for display purpose. Again, the adopted values are highlighted with solid symbols and dotted lines. Each panel of this figure is similar to the left panel of Fig.1 and is explained in more detail in Sect. 2.2.