Astrometric Accuracy on Tautenburg Schmidt Plates

Abstract

Using a big Schmidt telescope for astrometric purposes we take advantage of a coma free field with sharp images of all objects up to the 20th magnitude. The main disadvantage of a Schmidt telescope is in its relatively short focal length and the necessary bending of the photographic plate during exposition. It is reduced to a minimum in the case of the Tautenburg Schmidt telescope (134/200/401). We have a focal length comparable to that of astrometric astrographs (the scale is 51″/mm) and a relatively slight bending of the 24 x 24 cm plate covering a 3.°1 x 3.°1 field. On the other hand this is a relatively small field for a wide–field telescope so that a sky survey becomes too expensive, and the scale also gives no opportunity of defining highly accurate relative proper motions (p. m.), e. g. internal p. m. of stars in clusters, as is the aim of long focus astrographs. Therefore, we should concentrate on the determination both of positions of faint objects, e. g. extragalactic radio sources and of absolute p. m. of stars with respect to a great number of galaxies in selected areas, making in this way a contribution to the approximation of an inertial system.