CCD-GALAXY PHOTOMETRY AND THE CALIBRATION OF PHOTOGRAPHIC SURVEYS

We present CCD UBVRI aperture photometry and total B-band magnitudes f or a sample of 155 galaxies with B less than or equal to 18 covering 1 4 UK Schmidt fields. The data, taken on a variety of telescopes, have been acquired in order to calibrate photographic photometry on these f ields. Using these data, we discuss the calibration of photographic ga laxy surveys at these magnitudes, and show that the accuracy of such p hotometry is potentially better than +/-0.05 mag. However, data from b oth the COSMOS and APM automatic plate measuring machines show strong surface-brightness-dependent systematic errors, which primarily manife st themselves as a much increased scatter (similar to +/-0.25 mag). Th e cause of these effects is almost certainly related to the limited dy namic range of the automated machines. They have the potential to intr oduce scale errors of up to similar to 0.1 mag per magnitude into the photographic magnitudes, and we discuss the implications of this for b right galaxy number-magnitude counts. We show evidence for a scale err or in the APM galaxy survey magnitudes. Our revised APM magnitudes all ow a standard, non-evolving cosmological model to fit the APM galaxy c ounts in the range 17 < B < 20, although this model continues to overp redict the galaxy count at brighter magnitudes.