Proper motion uncertainties of stars in the Quintuplet (left) and Arches (right) clusters for Fields 1 through 5 (top to bottom) in each cluster. The proper motion uncertainties include the astrometric uncertainties from each individual epoch in addition to the standard deviation in the geometric transformation from epoch 2 to epoch 1. The deeper K_s observations in the central Quintuplet field provide good astrometric accuracies down to K_s ~ 19 mag, while Fields 2–5 are truncated at K_s = 17.5 mag (dashed lines), beyond which astrometric accuracies degrade. The wide spread in Quintuplet Field 1 is caused by crowding. For the purposes of membership derivation, only sources with K_s< 18 mag (dashed line) are used in Field 1 for compatibility with the outer fields. In most Arches fields (right panels), the astrometric accuracies are small down to K_s ~ 18 mag, with the exception of Field 2, where proper motion derivations are limited to K_s< 17 mag.