With an image that has a well-calibrated WCS we can sompute a photometric zeropint.

% % wcs_photom_zp 
Usage: wcs_photom_zp a.fits 17.0 3.0 0.5 11.5 red 4.0 V 
arg1 - image name 
arg2 - V magnitude limit in USNO catalog
arg3 - search Radius in arcmin
arg4 - stellarity low (0.5) 
arg5 - stellarity hi (11.5) 
arg6 - color of markers (red) 
arg7 - radius of circle markers in pixels (5) 
arg8 - name of standard filter (V,Bmag,Rmag)