Tangential critical lines (red) and caustic lines (blue) for two different sources located at z = 1.0 and z = 1.7. As in Fig. 4, the circles show the measured positions of the images (input data for the model). The squares (orange and cyan) are the model-predicted image positions after optimization, and the asterisks (orange and cyan) the positions in the source plane. Although the cyan asterisk appears to over lay the caustic line, this is only an artifact, a product of the thick lines, and the size of the asterisks that we used to highlight the figure. The source is not over the caustic line.