Detection, Photometry, and Completeness of FOCAS Catalogs for the HST Medium-Deep Survey

We discuss the methods which we have developed and applied to optimize the detection efficiency of faint galaxies imaged with the HST Wide-Field and Planetary Cameras both prior and subsequent to the HST refurbishment mission in December 1993. Our methods are based on the FOCAS (Tyson and Jarvis, ApJ, 230, LI53, 1979) automated detection software package, which we have applied to compile galaxy catalogs. Using this package, we examine photometry and detection efficiency as a function of image convolution filter width (p). We measure completeness and reliability for the catalogs from image simulations, and we also compare a WF/PC data frame with an exposure of the same field obtained with the 3.6 m Canada-France-Hawaii Telescope. Further, we investigate the quality of galaxy scale lengths and axial ratios derived by FOCAS on WF/PC data. We summarize several results applicable to WF/PC and WFPC2 data. The object detection efficiency increases monotonically with increasing p, independent of galaxy half-light radius (r hl ), although only marginal improvement in efficiency occurs for p> rhi. Second, for both WF/PC and WFPC2, the FOCAS 95% completeness limit shifts toward brighter apparent magnitude as a function of increasing rhi, best fit by a second-order polynomial but roughly equivalent to a 0.8 mag per 1" increment in half-light radius. As a result of the elimination of the HST spherical aberration and the use of filters with high throughput, the 95% completeness limits for WFPC2 F606W and F814W data are ≥ 1.1 mag fainter than those for WF/PC F555W and F785LP data, respectively, for a given integration time and sky background level. Fixed-aperture magnitudes grow fainter with increasing rhi, for a given intrinsic flux. Finally, 1"-diameter aperture photometry in WF/PC and WFPC2 data serves to parametrize the detection efficiency fairly independently of galaxy scale lengths for 0".2≤rhi≤", i.e., the spread in detection efficiency over the range 0".2≤rhi≤2", for galaxies of a given fixed aperture magnitude, is minimized when the diameter of the fixed aperture is 1".