In order to speed up the FmodErr procedure the idea is to compute the MC z step looking at the calibrated signal and then null it in one step (the fine tuning will be done tuning the modulation frequencies). In order to understand if the calibration has to be continuously measured or a fixed value is enough the calibration factor has been measured over months (using the MC z displacement in the FmodErr procedure). The calibration is quite stable but it is safer to remeasure it quickly during the procedure (making a small step in MC Z of 100 um).