(a) The mean error in the position of the origin determined as a function of the number of photons incident on the CCD. The theoretical data was obtained using a computational model for photon and instrument noise, and the mean was calculated from a sequence of 100 different profiles for the PSF (background: ∼430 photons per pixel, effective pixel size: 160nm, quantum efficiency: 93%, σr: 160 electrons, grid size: 7 × 7 pixels). (b) The average value of r2 for the least-squares fit as a function of the number of photons.