Range of PM precisions among the clusters. Shown are the 1D PM uncertainties as a function of the F814W magnitude, for the two clusters NGC 1868 (left panel) and NGC 2108 (right panel). The two clusters have very different data sets available for the PM calculations, in terms of time baseline and numbers of exposures and represent the lower and upper end of the quality scale of our PM measurements. For NGC 1868 we reach a median nominal PM precision for the best-measured stars of about 55 μas yr⁻¹, whereas for NGC 2108, the median nominal precision of the best-measured stars is about 11 μas yr⁻¹. The solid line in both panels follow the median σμ as a function of the magnitude. The different sequences in the right-hand panel correspond to PMs determined using time baselines of ∼3.0 yr (blue), ∼15.1 yr (purple) and ∼18.2 yr (orange).