The astrometric accuracy of “Carte du Ciel" plates and proper motions in the field of the open cluster NGC 1647

¹ Sternwarte der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
² Observatoire de Bordeaux, BP. 89, F-33270 Floirac, France
³ Centre d' Analyse des Images, Bâtiment Perrault, Observatoire de Paris, 77 avenue Denfert Rochereau, F-75014 Paris, France

Send offprint request to: M.  Geffert, Bonn

Received: 4 September 1995
Accepted: 19 January 1996

Abstract

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.