What limits to do source confusion place on the depth attainable in deep GALEX exposures?
Source confusion is an important concern in deep GALEX images and is the dominant factor limiting the depth attained in deep exposures. The standard GALEX pipeline relies upon the SExtractor program (Bertin & Arnouts 1996, A&AS, 117, 393; http://astromatic.iap.fr/software/sextractor/) to obtain source catalogs for each image. As SExtractor was not designed to work in crowded fields, we recommend that users wanting to obtain photometry for deep fields not rely upon the standard GALEX catalogs. These are the “xd-mcat” files for each tile, or equivalently the PhotoObjAll table in the MAST database (http://galex.stsci.edu/GR4/). Many of the sources in these catalogs at faint magnitudes will be blends of more than one source. The GALEX exposure time calculator does not take into account the source confusion when determining the photometric errors for a given source flux and exposure time and thus will give erroneously small errors for faint sources in deep exposures. The best way to obtain