Precision Astrometry in Globular Clusters with HST's WFPC2

Details Precision Astrometry in Globular Clusters with HST's WFPC2 Precision Astrometry in Globular Clusters with HST's WFPC2

Sep 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999dda....31.1001k&link_type=abstract

American Astronomical Society, DDA meeting #31, #10.01

Astronomy and Astrophysics

Astronomy

Scientific paper

Using 15 images of the same field in a globular cluster, we achieve a positional accuracy better than 0.01 pixel for each star, so that internal proper motions are measurable over intervals as short as two years. Comparison of the dispersion of thousands of proper motions with that of thousands of radial velocities should, when the latter are available, yield the most accurate distances for globular clusters (good to a few per cent). The proper motions themselves should yield a wealth of new information about the dynamics of the clusters. Our methods are aimed at removing the systematic error connected with the positioning of a star with respect to pixel centers. The crucial step is to obtain a very accurate PSF, which we derive by an iteration between PSF shape and star positions in the individual images. We introduce the concept of the effective PSF, which is a continuous function derived by ingegrating the instrumental PSF over a pixel at all possible positionings. We avoid global plate solutions by transforming each star from image to image relative to a tight grid of its immediate neighbors. We have also developed procedures for dealing with the spatial variation of the PSF, and strategies for effective dithering of multiple images.

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