import os, re

def psfsize(cfgfile, freq):
    """
    Returns the PSF FWHM of cfgfile in arcsec.  Only really works for snapshots
    at zenith.
    """
#    print "psfsize's cfgfile:", cfgfile
    avgsep = os.popen('/users/rreid/bin/avgsep < ' + cfgfile)
    for lin in avgsep:
        if re.search('Nominal resolution acc. to rmssep:.*" at 300 GHz',
                     lin):
            fwhm = re.search('rmssep:\s+([^"]+)"', lin).groups()[0]
            break
    avgsep.close()
    return float(fwhm) * 300.0 / freq