What are some common problems with extracting photometry from GALEX images?
In general, extracting photometry from sources in GALEX images is similar to measuring fluxes of sources in the optical and infrared. However, GALEX data users should note that there are a few differences. Probably the most important difference from optical or infrared images is the extremely low background levels observed in the UV. For high Galactic latitude fields, typical background count rates are ~10^-4 cts/sec/arcsec^2 in the FUV and 10^-3 cts/sec/arcsec^2 in the NUV. These count rates correspond to surface brightnesses in AB mag/arcsec^2 of 28.8 and 27.6 in the FUV and NUV, respectively. This means that AIS exposures, in particular, with exposure times as low as 100 sec, will have average background counts in the FUV and NUV of ~0.01 and ~0.1 cts/arcsec^2, respectively. Statistics such as the median and mode are not useful with so few counts. Some source detection software written for optical or infrared data, such as SExtractor, relies upon these statistics to determine the av