A series of prime focus plates taken on the 3.9-m Anglo-Australian Telescope has been measured with an iris photometer, and calibrated using a faint photoelectric sequence. A colour-magnitude diagram has been obtained for about 300 faint stars in ω Cen. The main sequence turn-off point clearly lies below V = 18, confirming an earlier result that the difference in luminosity between the turn-off and the horizontal branch is larger than usual. However, this result probably does not imply an exceptionally large age for ω Cen. The improved precision of the present data permits an analysis of the intrinsic spread in colour among stars on the upper main sequence. The resultant distribution in colour is skew, after correction for field star contamination, and is consistent with a (presumably primordial) abundance spread similar to that found in the highly evolved giants and RR Lyrae stars. It appears that the majority of the ω Cen main sequence stars have low metallicity, with perhaps a small proportion (∼10 per cent in the present sample) of relatively metal rich stars. However, the skew distribution in (B - V) may be due to non-linearity in the relation between turn-off colour and metallicity, and may not reflect a skew metallicity distribution. Because of the small sample of stars and uncertainties in the field star distribution, it is impossible to rule out the simplest alternative hypothesis, that the total intrinsic abundance spread on the main sequence is negligible. Some suggestions are made for future observational programmes which could resolve this ambiguity. © 1981 Royal Astronomical Society.
