plot_detimage: always ignore proton charge

                      description='show detector image')

    pars.add_argument('file', metavar='filename', help='file to process', nargs='+')

    pars.set_defaults(minchan=0, maxchan=TDC_MAXCH, npix=TDC_MAXPIX,

                      log=False, edge=False, threshold=None, ignore_pcharge=False)

                      log=False, edge=False, threshold=None, ignore_pcharge=True)

    pars.add_argument('--num-pix'  , '-N', dest='npix',  metavar='N', type=int,

                    help='set number of pixels, (default %(default)s)')

    pars.add_argument('--edge'     , '-E', dest='edge',  action='store_true',

                    help='edge detection')

    pars.add_argument('--threshold', '-T', dest='threshold',  metavar='THR', type=float,

                    help='set threshold (need more explanation here)')

    pars.add_argument('--ignore-pcharge', '-C', dest='ignore_pcharge',  action='store_true',

                    help='ignore empty proton charge')

    # detector image does not depend on proton charge, so ignore it by default (ignore_pcharge=True)

    #pars.add_argument('--ignore-pcharge', '-C', dest='ignore_pcharge',  action='store_true',

    #                help='ignore empty proton charge')