Empirical estimate of the errors on radial velocities. Top panel: normalised frequency distribution of empirical uncertainties () derived from the difference between velocities measured from the upper (RV_U) and lower (RV_L) spectrum of all the stars observed with the 580 set-up. The dashed line shows the position of the 68th percentile of the distribution. Middle and bottom panels: same empirical uncertainties binned as a function of the S/N of the spectra (middle panel) and of the full width half maximum of the CCF (bottom panel). Error bars on each bin are equal to , where σ_bin and N_bin are the standard deviation and the total number of values for each bin, respectively. The number of values per bin is not constant, but it ranges from ~200 (in the central bins, S/N ~ 40 − 50) to ~50 (bins of the lowest and highest S/N) in the middle plot, and from ~900 (CCF_FWHM ~ 20 km s^-1) to ~20 (CCF_FWHM ~ 110 km s^-1) in the bottom panel.