Top: distance distribution of the 75 globular clusters included in the present study. Bottom: standard uncertainties on the PM in declination as a function of number of cluster members nMemb used in the solution. The diagonal line represents a fit to the relation where we find a = 0.3 [mas/yr]. A similar dependence on the number of members is found for the parallax uncertainty (with a = 0.15 [mas]) and for σμ_α* (where a= 0.25 [mas/yr]).