Top: thorium flux ratio as a function of time, which is the total flux emitted by the thorium lamp at a given time relative to a reference time. Middle: RV drift of the thorium line at 4833.77 Å measured with the old (blue) and new (orange) DRS. On both data sets, we fitted a model corresponding to a linear drift plus a linear correlation with the thorium flux ratio. In the legend, we report the measured slope of the linear drift. Bottom: RV drift residuals after subtracting the fitted linear drift. In the legend, we report the R Pearson correlation coefficient between those residuals and the thorium flux ratio, as well as the ratio between the rms of those residuals and the median error in RV for this spectral line to detect a non-Gaussian behaviour.