Impact of an SPT described by the radial stretching , with f₂= 0.5, on time delays image pairs generated by a NIS, with core θ_c = 0.1 θ_E. Top panel: ratio between SPT-modified and original time delays (black inverted triangles) for each source position. The time delay ratios scale almost perfectly like (red curve), as predicted by Eq. (29). Bottom panel: numerical confirmation of the validity of the approximation performed in Eq. (29) which consists in neglecting the term ε_AB. The solid black curve illustrates |η_AB| as an analytical function of β and perfectly fits the quantity numerically evaluated for each source position.