Galaxy number counts – I. Photographic observations to B = 23.5 mag

We present galaxy number–magnitude counts and colour distributions from objectively defined catalogues of faint $$({b}_{j} \lt 23.5\,\text{mag},\,{r}_{F}\lt 22\,\text{mag})$$ galaxies on three new fields in U, b_j, r_F and I pass-bands, determined from COSMOS machine measurements of Anglo–Australian Telescope photographs. We have recalibrated three further fields analysed in a similar way in earlier papers, and used all six to investigate the variation in count normalization from field-to-field. The new data define the mean faint galaxy number-magnitude relation to ± 9 per cent. The aim is to determine whether the previously observed large scatter in the faint counts is due to galaxy clustering or some other systematic effect. After a detailed assessment of contamination, absorption, and photometric and other errors, we conclude that the observed variation in both blue and red pass-bands can be accounted for by the combination of known galaxy clustering properties and several of these small measurement errors. We confirm the trend of the galaxy counts to have steeper slopes when measured at shorter wavelengths. Finally, we present the stellar number-magnitude counts on four fields, and make some brief comments on these counts.