Use a FITS image with a good (i.e. better that 1" RMS) WCS to locate USNO sources. Measure the instrumental magnitudes of these sources and derive a phtometric zeropoint. The image photometry is done with mido.sh and I frequently have to review several OTW and bash codes to verify how this works. I give a good explanation and example of this to make things clear.

  
% usno_photcal --help