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Description
The first epoch plates of 8 globular clusters, available in the plate collection of the Pulkovo Observatory, were taken with the normal astrograph at the end of the last and beginning of this century. These plates had been measured previously by N. Gamalei [1948, IzPul, 141] in order to determine the proper motions of the clusters. From 40 to 62 stars were used for this purpose and these were assumed to be cluster members on the basis of their proximity to the cluster centers. The epoch differences ranged from 30 to 45 years. As this time interval has now increased considerably (by about a factor of two), it is planned to redetermine the proper motions of these clusters, using more precise criteria for the selection of stars belonging to the cluster. In the previous study (N. Gamalei [1948, IzPul, 141]) only a limited number of field reference stars were measured, and as more complete data on the cluster, cluster members and field stars is desirable all the star images on the plates in an area 80'x80' will be measured in the present program. For future studies of the orbits of galactic clusters it is important that the absolute proper motions be on one system and therefore special attention should be devoted to this problem. The present paper is a continuation of an investigation of M13 and deals with the determination of the proper motions and U, B, V magnitudes of separate stars and also a redetermination of the proper motions of the cluster. The U, B, V magnitudes of the majority of 923 stars with measured proper motions were found from plates (table 2) taken with the 2 meter Schmidt telescope at Tautenburg and kindly lent to us by Professor N. Richter. The photoelectric standards as given by Savedoff [1956AJ.....61..254S] (table 7) and also U B V magnitudes of six bright field stars (table 8) observed by Mayer were used for calibration. A comparison of the determined B and V magnitudes of stars within an area of about 10'x10' centered on the cluster with those obtained by other authors indicates that the probable external errors do not exceed +/-0.03mag (table 15). However as the photoelectric magnitudes of fainter field stars were not available, a slight systematic error may be present in the derived magnitudes of these stars. Data on the plates used for the determination of the proper motions were given in Kadla [1963AZh....40..691K]. The relative proper motion of M 13, redetermined on the basis of a larger number of cluster members (80 in all, measured on 4 pairs of plates with a mean epoch difference equal to 61.4 years) is found to be {mu}x=+0.0008", {mu}y=-0.0014" with a probable error of +/-0.0002".
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