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J. Anderson (UC Berkeley)
The distortion solutions currently available for HST's WFPC2 are accurate to only about 0.05 pixel in the PC and 0.02 pixel in the WF cameras. This has in fact been the most serious limitation to accurate astrometry with WFPC2.
We have improved the accuracy of these corrections by a considerable factor. The astrometric improvement is dramatic in typical fields, where stars are few. Even in crowded fields such as globular clusters, the new-found ability to make more globally based transformations offers a decided improvement over the local transformations to which we had been restricted.
Our new solution comes from a re-reduction of all of the images in the Omega Cen calibration fields. These two fields have been observed repeatedly over the past seven years at a variety of orientations and offsets. The outer calibration field has been observed well over 1200 times, and the inner field more than 300 times.
By studying this large data set as a whole, we can derive a much more accurate distortion solution than is possible from a single observing program at a single orientation. In particular, we (1) characterize the distortion present in each chip to 0.015 pixel in the PC and better in the WFs, (2) examine the effects of breathing on the plate scale and on the distortion, and (3) accurately determine the the astrometric transformation from one chip to another, including how the chips have moved with respect to each other over time.
This work has been funded by an archival-research grant from STScI.