THE BVR CCD PHOTOMETRY OF GLOBULAR CLUSTER M13

The BVR CCD photometry was performed for the globular cluster M13 down to $V=19^m$ over the region from the center to the west 13 arcmin and the CMD of M13 is investigated. The major photometric error in the crowded field which is due to the variation in the sky background was corrected by the median filtering method and the direct sky method. Some characteristics of the CMD of M13 obtained in the present study are as follows: Firstly, the distribution of stars on the CMD is well consistent with the mean lines of Sandage(1970) along the red giant branch(RGB), horizontal branch(HB) and asymtotic giant branch(AGB). Secondly, some gaps exists along the RGB and blue horizontal branch(BHB). Thirdly, the UV-bright stars are more concentrated at the inner region of the cluster.