SPHERE/ZIMPOL polarized light imagery of HD 100546. a) Q_φ images in coronagraphic SlowPol mode. b) Q_φ images in FastPol mode. c) same as a) with labels, where the white dashed line indicates the disk major axis and the inner grey spot the coronagraph size. d) U_φ images in SlowPol mode, with color stretch twice as hard as in a). e) Unsharp masking of the Q_φ image (see Sect. 3.1.3). The predicted locations of b) (Quanz et al. 2015) and of c) in May 2015 with relative azimuthal uncertainty (S. Brittain, priv. comm.) are shown in purple and green. All images except e) are scaled by the squared distance from the star and are shown with linear stretch. North is up, East is left.