Schematic view of the lensing configuration and coordinate frames. The observed image-frame coordinates r^img − r₀^img are mapped to source-frame coordinates r^src − r₀^src via the lens equation ${\mathbf{r}}^{\mathrm{src}}={\mathbf{r}}^{\mathrm{img}}-\alpha ({\mathbf{r}}^{\mathrm{img}})$. The source-frame coordinates correspond to positions r in the physical galaxy frame, where parametric flux and velocity profiles are defined. These coordinate transformations do not alter the model grid, which remains aligned and regularly sampled along the IFS spatial axes and the LOS. This schematic is inspired by Bartelmann & Schneider (2001) and Di Teodoro & Fraternali (2015).