Critical curves of alternative lens models. All critical curves are computed at the redshift of system 7 (z = 5.1727). The red curve corresponds to the driver model derived with the six spectroscopic systems and a grid of 20 × 20 points. The green curve uses the same grid configuration but is derived from the 25 systems with constrained redshifts. The blue curve uses the same 25 systems but it is based on a higher resolution grid of 25 × 25 points.