Resumen
Proper motions are the best and fastest screening tool to look for open cluster members, but when dealing with deep observations, crowdiness and small-size proper motions become troublesome. In general, the vector-point diagram can be applied on small areas of the sky and the proper motion errors impose a limit on the smallest proper motions that are significant. Moreover, the deeper the data, the further the star, and the smaller its proper motion. Herschel's Convergent Point method is an alternative way to study proper motions that uses the whole celestial sphere to represent them. A correct interpretation of the geometry introduced by this method is necessary to exploit all the kinematical information contained in the proper motions.
