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The Hubble Telescope Ritchey-Chretien design produces optical distortions in the field of view of the telescope, which because of residual misalignments, must be calibrated on-orbit. The method chosen involves exploiting the metric invariance of a rich star cluster with respect to repointing of the telescope; that is, the measured separations between pairs of stars, after calibration, should not depend on where the telescope was pointed. We report the analysis of an extensive series of measurements of the ecliptic open cluster M35, taken in January 1993, for the purpose of determining distortion polynomial coefficients and other parameters necessary to reduce HST astrometric observations with Fine Guidance Sensor \# 3. Implications for the accuracy of HST astrometry are discussed.
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