Evaluation of the SIE profile (cusp): reconstruction of the critical curve close to a cusp for a symmetric image configuration a) and for an asymmetric image configuration b). For a) and b), the y₁-value of the sources is the same, y₂ varied. The symmetry axis through the lens centre (black line) is drawn to better visualise the location of the cusp point (crossing of black line and red curve) and to compare it to the vertex positions (blue, yellow and green dots) for the different reconstructions, marking the reconstructed cusp point. The reconstruction of the fold point between A and B is also shown (red dot). Inset: detail of the cusp position reconstruction.