Data reduction has been accomplished using the IRAF package achieving a mag internal error, while external magnitude uncertainty from standard zero points amounted to mag. Surface brightness profiles have been obtained in the four bands down mag arcsec -2 . Table 1 reports a full summary of the galaxy photometry, including aperture magnitudes and detailed measurements of the single knots. Internal mag uncertainty is 0.001 for aperture photometry and 0.004 for knot magnitudes.
Error bars account for photometric errors and uncertainties in the sky level.
The g surface brightness profile of N50 is shown in Fig. Error bars including photon noise and sky level uncertainty have been taken into account in the plot. Note in the figure the blue colour bump about from the centre due to the knotty ring. A smooth colour gradient is also evident along galaxy radius with the outermost regions sensibly bluer than the centre. As expected, N50 colours are slightly bluer than high-mass ellipticals, and intermediate between E and Sa Hubble types.