We consider the error in the fitted position in units of σ_IQ as a function of the S/N. The discrepancy between the expected relationship (solid line) and observed values (points with error bars) is due to the fact that the form factor model does not accurately capture the non gaussianities of the PSF. This is explained in detail in Appendix A. The outliers observed are thought to be due to model stars affected by proper motions not detected by simultaneous astrometry.