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R.S. Hill, E.M. Malumuth (Raytheon ITSS), E.P. Smith, J.P. Gardner, S.R. Heap (LASP/GSFC/NASA)
We compare two popular packages for finding and measuring galaxies in deep fields: FOCAS and SExtractor. These packages perform similar tasks, including galaxy detection at a given isophote defined in terms of sky noise, several kinds of aperture photometry, source splitting, and star-galaxy discrimination. The analysis is of a set of images of 5 arcmin fields showing numerous galaxies with R magnitudes ~ 18 to 23, observed using the KPNO 2.1 m telescope in U, B, V, R, and I bands, in support of a Space Telescope Imaging Spectrograph (STIS) observing program. Results are also compared for STIS wide-band CCD images. Differences between the detections and photometry done using FOCAS and SExtractor are evaluated statistically; for a subset of objects, these results are compared to those of customized analysis using IDL. The photometry of relatively bright, isolated galaxies is robust for almost any reasonable algorithm, whereas the splitting of blends and the photometry of faint sources are more sensitive.