CCD-GALAXY PHOTOMETRY AND THE CALIBRATION OF PHOTOGRAPHIC SURVEYS

We present CCD UBVRI aperture photometry and total B-band magnitudes for a sample of 155 galaxies with B less than or equal to 18 covering 14 UK Schmidt fields. The data, taken on a variety of telescopes, have been acquired in order to calibrate photographic photometry on these fields. Using these data, we discuss the calibration of photographic galaxy surveys at these magnitudes, and show that the accuracy of such photometry is potentially better than +/-0.05 mag. However, data from both the COSMOS and APM automatic plate measuring machines show strong surface-brightness-dependent systematic errors, which primarily manifest themselves as a much increased scatter (similar to +/-0.25 mag). The cause of these effects is almost certainly related to the limited dynamic range of the automated machines. They have the potential to introduce scale errors of up to similar to 0.1 mag per magnitude into the photographic magnitudes, and we discuss the implications of this for bright galaxy number-magnitude counts. We show evidence for a scale error in the APM galaxy survey magnitudes. Our revised APM magnitudes allow a standard, non-evolving cosmological model to fit the APM galaxy counts in the range 17 < B < 20, although this model continues to overpredict the galaxy count at brighter magnitudes.