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

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

Jun 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995pasp..107..590n&link_type=abstract

Publications of the Astronomical Society of the Pacific, v.107, p.590

Astronomy and Astrophysics

Astronomy

6

Techniques: Photometric

Scientific paper

We discuss the methods which we have developed and applied to optimize the detection efficiency of faint galaxies 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 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 (rho). 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 rho, independent of galaxy half-light radius (r_hl), although only marginal improvement in efficiency occurs for rho > r_hi. Secondly, for both WF/PC and WFPC2, the FOCAS 95% completeness limit shifts toward brighter apparent magnitude as a function of increasing r_hl, 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 higher 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 r_hl for a given intrinsic flux. Finally, 1"-diamter aperture photometry in WF/PC and WFPC2 data serves to parameterize the detection efficiency fairly independently of galaxy scale-lengths for 0.2" <= r_hl <= 2", i.e., the spread in detection efficiency over the range 0.2" <= r_hl <= 2", for galaxies of a given fixed aperture magnitude, is mimimized when the diameter of the fixed aperture is 1". (SECTION: Computing and Data Analysis)

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