Top panel: sketch of the PA measurement method. The grey area sketches an elliptical luminosity distribution. The dotted circle and lines picture four quadrant-shaped masks defining two zones, labelled A and B. The mask rotates around its centre. For each angle θ between the axis of the mask and the semi-major axis of the galaxy, the total intensity I_A and I_B are computed within each couple of quadrants, as well as their difference Δ. Bottom panel: plot of Δ versus θ for one of the data frames of HE0435-1223 as an example. The value of θ for which Δ reaches a maximum indicates the PA of the galaxy. The second maximum is redundant, 180° further.