Top: example of PM determination by means of the least square fitting for one of the cluster stars (#98166), with magnitude K_s = 12.4 mag. Each point is the star position in different epochs after been locally-transformed to the reference system of the master catalog. Bottom: CSE in the PM, computed as the error on the slope in the linear fit shown above, as a function of K_s magnitude. The thin red line marks a 3σ clipping on the error distribution; the thick blue line is the median error. Numbers list mean errors in bins of 0.5 mag. The figure inset shows the histogram of errors for 11.5 <K_s< 15 mag before (gray) and after (blue) the 3σ clipping.