PARALLAXES OF STARS IN THE HYADES CLUSTER REGION - A COMPARISON OF HAND AND AUTOMATIC MEASURES

Parallaxes and proper motions are presented for 30 members of the Hyades Cluster and 223 field stars in 13 fields located near the center of the cluster. The series of photographic plates are extensive and they were measured twice; first by hand using a conventional two-screw measuring machine and also with a PDS microdensitometer. A comparison of the two sets of measures reveals that the internal and external errors in parallax average about ± 0″.005 for the central stars from automatic PDS measures and about twice this amount for the data measured by hand. The parallaxes of the member stars yield a distance modulus of 3.30, in excellent agreement with the average of many prior distance determinations for the cluster. The parallaxes of the field stars confirm a small amount of coma or higher-order terms in the optical system of the Van Vleck refractor. They also confirm that within a field of one half degree radius centered on the optical axis and over a range in visual magnitude from 8.5 to 13, the errors in parallax vary only slightly over the most optimum conditions.