Forward modelling of the MRS webbPSF used for the estimation of iso-α trace. The detector trace is determined by collapsing the corresponding slice in the webbPSF model along β and projecting the α profile of the PSF to the detector using the estimated distortion solution. The centre of the webbPSF model is optimised to match the data on the detector and estimate the centre of the given point source exposure. For accurately centred slices this works well, while for slices further from the PSF centre, the discrepancy between model and data is larger; however, the source centre estimation is not significantly affected.