Difference in the sources brightness distribution in the source plane obtained when modelling the systems in 𝒟₁ with a lens following an elliptical power law profile with additional external shear. If S_X is the source surface brightness distribution obtained for the system X, these images are the result of S_Unperturbed − S_{1 keV} (left) and S_Unperturbed − S_{3 keV} (right). The local surface brightness of the reconstructed source can be shifted and modified in the perturbed models if some of the additional perturbations are absorbed by variations of the main lens properties. The general trend given by these images is that the main effect of the additional perturbation is an overall shift of the source, which is indicated by corresponding red and blue regions of the 2D brightness distribution. To highlight this effect, the black markers give the point of highest surface brightness (‘+’ for the unperturbed system and ‘x’ for the perturbed one).