Difference between recovered and true radial velocity of a simulated apparent binary star, as a function of source separation and of the degree of PSF mismatch, for two ranges of flux ratios between the two stars. The curves show the median of the absolute velocity difference of each stellar pair. Also shown is the median offset for the case of a perfect PSF (red line) to show the behaviour expected in the absence of any systematic error. For clarity, deviations larger than the velocity dispersion have been set to a value of 10   km   s^-1, the assumed velocity dispersion in the cluster.