Difference in the spectrograph drift measurement between the information provided by simultaneous FP calibrations and a linear interpolation between daily wavelength solutions (dark blue dots). Light blue dots correspond to times when the FP was down, and thus no simultaneous drift measurement was performed. The magenta dots correspond to a 30 day smoothed version of the RV difference and the black dash-line corresponds to our correction model for spurious FP drift measurements (equal to zero when the drift difference is negligible).