Outline of the procedure to correct the PMs for high-frequency systematic effects. Panel (a) shows the VPD of sources towards NGC 1850. Likely cluster members were selected based on their PMs (within the red circle, which has a radius of 0.1 mas yr⁻¹ in the case of NGC 1850) and their positions in the CMD. Panel (b) shows the m_F814W vs. m_F438W − m_F814W CMD of all sources with measured PMs towards NGC 1850 (grey dots) and the final selection of well-measured cluster stars (black dots). Panels (c) and (d) show, separately for each PM component, the maps of the local mean raw PM field (before the high-frequency correction). Each point in the map is colour-coded according to the median PM of its closest 200 well-measured cluster stars. The corresponding maps of the a posteriori corrected PMs are displayed in panels (e) and (f). The points in these maps are coloured according to the same range in colour as in panels (c) and (d).