Demonstration of the steps taken to build residual table for PSF photometry. A cutout is taken around the source, and an analytical function is fitted and subtracted. The image is then resampled to a finer pixel grid (default: ×10). The residual image is then rolled (discretely shifted along x and y) such that the location of best fit is at the image centre. This is repeated for several bright isolated sources to obtain an average residual.