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The astrometric capability of the Hubble Space Telescope Planetary Camera (WF/PC I) is investigated, motivated by a study of the internal velocity distribution of globular clusters. The ultimate astrometric accuracy of the HST PC will be determined by 1) the precision to which the aberrated images can be "centered", and 2) the accuracy to which the distortions across the PC field can be modeled. A series of overlapping PC exposures for two clusters, NGC 6752 and M15, are utilized to examine these issues.
We have made use of maximum-likelihood image reconstruction to address the first issue, with good success. Reconstruction improves both the detectability and precision of the image centers. A preliminary exploration of the second issue, that of modeling the distortion across the PC field, is also presented, using positions derived from the multiple, overlapping exposures.
