Simulated impact of the mass of S1 on S2 images. Left: model consisting of two lens planes. The plane at z_L has the same mass model as the one shown in Fig. 3, while the plane at z_{S 1} is characterized by a SIE model with a velocity dispersion of σ_{v,S 1} = 100 km/s and the same geometry as the surface brightness profile of S1a. Right: normalized residual between the dual and single plane models, along with the noise. The S2 images exhibit a noticeable difference between the two models, indicating that the mass of S1a has a significant impact.