Example simultaneous fit of a PSF model to ten stars, in a (rare) difficult case of unfocused observation. The frame chosen for this figure could still be used for photometry, not leading to an outlier in the light curves. The kernel, at the top left, transforms a two-pixel FWHM Gaussian into the PSF at the bottom left. Note that for our lens photometry we used the kernels and not the PSFs (see Sect. 3.6 and Fig. 3). The bottom row shows the residuals of the joint fit. Similar residuals were obtained in all conditions of seeing, focusing, and tracking.