The APM galaxy survey. II Photometric corrections.

We describe the methods that we have used to establish accurate photometry in a survey of two million galaxies brighter than b_3_ = 20.5 covering over 4300 square degrees of the South Galactic cap. We apply a field correction for vignetting and differential desensitization which is accurate to <~ 0.04 mag across each of 185 Schmidt plates. Images in the overlapping regions of neighboring plate pairs are used to establish a uniform magnitude system over the entire survey. We discuss the residual magnitude differences in the overlaps and the propagation of plate-to-plate magnitude errors across the network of plates. We argue that the rms plate zero-point error in the final matched survey is 0.04 mag. CCD photometry of 40 faint galaxy sequences is used to calibrate the matched magnitudes and to set stringent limits on large-scale gradients in the matched survey photometry.