|STIS||version 1||version 2|| |
|WFPC2||version 1||version 2||HDF-N SExtractor catalog||NICMOS||version 1||version 2|| |
Detection of faint sources near brighter objects poses a difficult problem for automated photomtetry programs. SExtractor fares better than most, but nevertheless no single set of deblending thresholds, cleaning parameters, etc., was deemed acceptable. In particular, the set of parameters that did the best job for detecting the typical faint, relatively isolated galaxies, would tend to over-merge galaxies near bright stars (counting them as part of the star rather than keeping them as separate objects). While this has a negligible effect on faint-galaxy statistics, it did produce a catalog that did not have separate entries for a number of objects that might be good candidates for spectroscopy. To recover these objects, SExtractor was run multiple times with different detection thresholds. Objects that were considered to be "over merged" in the first catalog were deleted from that catalog and the entries for their various subcomponents were taken from a catalog with a higher detection threshold. A few obvious artifacts due to diffraction spikes were deleted from each catalog as well. The object numbering and the IMA_FLAGS parameter indicate which SExtractor run each catalog entry came from. The number of objects added and deleted for the different catalogs were WFPC2: 24 deleted, 31 added; STIS: 26 deleted, 8 added; and NICMOS: 14 deleted, 29 added.
The WFPC-2 catalog of HDF-N published by Williams et al. (1996) used a modified version of FOCAS for source detection and photometry. To aid in comparison to HDF-S, we have constructed a SExtractor catalog of HDF-N.
Technical information on the construction of the catalogs for the individual instruments is available in the README* files that are distributed with the catalogs (click on any of the links in the table above).
This page was last updated on September 23, 2002