Errors due to the zero point of wavelength calibration. The top and the middle panels show the normalised frequency distributions of empirical uncertainties () derived from stars observed multiple times in different epochs for the lower and upper wavelength range, respectively. The dashed lines show the position of the 68th percentile rank. The bottom panel shows the distribution of the differences between radial velocities observed with both the UVES 580 set-up and the GIRAFFE HR15N set-up. The dashed line shows the position of the median of the differences.