subroutine ommag Read in task parameters Open OM OSW Source List file Read in the detection-limit count rate. Read in the source list table extension keywords from the primary header Extract the background limiting counts from the SRLLIST binary-table extension. Convert this to a limiting magnitude. Extract coordinates, photometry radii, count rate, count-error rate and background counts, for each source, from the SRCLIST binary-table extension Open OM ODF Priority Window Data Auxiliary file Get the frame time from the OM ODF Priority Window Data Auxiliary file Get the ccd mode Loop over source list For the ultraviolet filters, extraplolate the source count-rate to a 35-pixel aperture radius. Correct the count-rates for the CCD dead-time. Calculate the source magnitude using the corrected count-rate. end loop Write count rates and magnitudes to SRCLIST extension of OM OSW Source List file Add the limiting magnitude and write it to the source-list file. end subroutine ommag