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A.E. Dolphin (NOAO), E.K. Grebel (U Wash), P. Seitzer (U Mich), D. Geisler (U Concepcíon), P. Guhathakurta (UCSC), P.W. Hodge (U Wash), I.D. Karachentsev (SAO), V.E. Karachentseva (U Kiev), A. Sarajedini (Wesleyan U), M.E. Sharina (SAO)
The availability of HST WFPC2 data poses an interesting problem in stellar photometry - while the images are of impressive clarity and resolution, their undersampled nature makes accurate photometry difficult. Because a star's PSF contains a high fraction of the total counts in the central pixel, and because the position of the star's center within that pixel strongly affects its PSF, standard techniques involving centroiding and PSF fitting will result in an additional random error in the photometry. As this error depends on the star's position within the chip rather than the number of counts received, it is independent of the star's magnitude, thus affecting even the brightest stars in a frame.
HSTphot is a photometry program designed with these problems in mind. It uses a library of Tiny Tim-based PSFs, which accounts for PSF variations caused by a star's position on the frame as well as its centering within the pixel. Using this PSF library, HSTphot makes a two-pass search for stars, followed by an iterative solution. Solutions are made using PSF fitting, and include a centering of the star through PSF fitting, rather than through centroiding.
In addition to the careful treatment of the undersampled images, HSTphot is fairly automated, and is thus well-suited for batch processing of large observing projects. It is being used in the data pipeline for the HST snapshot survey of nearby galaxies (Seitzer et al.), to provide initial photometry of every observed object, and to provide a uniformly processed database of photometry for the entire survey. HSTphot CMDs and analysis for several of the snapshot survey galaxies will be presented, as well as a comparison between HSTphot and other photometry programs, such as DAOPHOT and DOPHOT.
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The author(s) of this abstract have provided an email address for comments about the abstract: dolphin@noao.edu