Completeness (red dots) and contamination (blue open circles) as a function of the i-band PSF magnitude for the COSMOS Wide-depth best (top left), median (top right), and worst (bottom left) seeing stacks. The bottom right figure is for the UltraDeep depth. The error bars are Poisson errors. This is star/galaxy separation compared to HST observations, where star–galaxy separation is taken as truth. (Color online)