Posterior distributions for lens model (SIE + external shear) and source grid offset parameters (see Sect. 4.5 for details), for the simulated HST data shown in Fig. 4 (first row). We show individual posteriors for different values of r_pix (thin colored contours), and the marginalised posteriors (thick grey contours) have been obtained by combination with equal weights. Two-dimensional contours show 68% and 95% of the distribution volume, and true values are indicated by dashed lines (https://github.com/aymgal/SLITronomy-papers/blob/master/paper_I/mass_sampling/corner_plot_mass_sampling.ipynb).