Left: example of cell and grid-point locations on the bottom-left area of chip[1]. Dotted lines mark the 170.7 × 170.7 pixels square elements inside which we computed the median value of the distortion residual to use as grid point (empty circles) in the look-up table. For a given star (marked with an ∗), we used the four surrounding closest grid point to perform the bi-linear interpolation (sketched with the arrows) and evaluate the residual geometric distortion in that location of the detector. Right: Bi-linear interpolation outline. Each grid point P₁,...,P₄ is weighted by the corresponding area A₁,...,A₄ to associate the correction in ∗.