M. Geffert (1,3), P. Bonnefond (2), G. Maintz (3) and J. Guibert
(2)
Send offprint requests to: M. Geffert, Bonn
(1) Sternwarte der Universität Bonn, Auf dem Hügel
71, D-53121 Bonn, Germany
(2) Centre d' Analyse des Images, Bâtiment Perrault, Observatoire
de Paris, 77 avenue Denfert Rochereau, F-75014 Paris, France
(3) Observatoire de Bordeaux, BP. 89, F-33270 Floirac, France
Received September 4, 1995; accepted January 19, 1996
The astrometric accuracy of triple image Carte du Ciel plates has been analysed using plates of the open cluster NGC 1647. A new multithreshold technique was used for the treatment of the triple image plates. Accuracies ranging from 100 mas to 200 mas were found. We determined proper motions with a median accuracy of 1.6 mas/a for 2220 stars in the field of NGC 1647. The membership probabilities of stars in the cluster region agree very well with the data of Francic (1989). Due to a significant difference of the proper motions of SZ Tau and the cluster members, SZ Tau seems not to be a member of NGC 1647.
Key words: astrometry --- stars: individual: SZ Tau --- Galaxy: open clusters: individual: NGC 1647