Top panel: sketch of the ellipticity measurement method. The grey area sketches the luminosity distribution of the galaxy. The thick rings and dotted lines represent the ring-shaped masks, split into quadrants, defining two zones, labelled A and B. The ellipticity of the mask is incremented from ε₁ to ε₂. For each ε_i, the average intensities in zones A and B, and , are computed, as well as their difference Δ. Bottom panel: plot of Δ versus ε_i for one of the data frames of HE0435-1223 as an example. The value of ε_i for which Δ = 0 indicates the ellipticity of the galaxy.