NOAA AR 11591 at the central meridian. Top panel: Continuum intensity map with a greyscale I_c = 0.1 to 1.2 I_qs. The rainbow and brown contours outline I_c = 0.9 I_qs, where the colour of the rainbow marks the reference angle, ψ, with green pointing towards the disc centre. Light and dark purple contours outline |B| = 625 G. Different formation heights are accounted for (Eq. (1), Δh = 255 km). The cyan cross denotes the umbral centroid. Lower panel: Absolute magnetic field retrieved along the I_c = 0.9 I_qs contour. Video is available online.