Exceptions are stars brighter than about 11th magnitude and fully resolved galaxies. The external accuracy is currently governed by the density and magnitude range of the PPM astrometric standards used and is limited by residual plate distortions and magnitude- dependent terms to an average of 0.5 arcsec. Internal accuracy for the majority of images brighter than 1 magnitude above the plate limit are typically 0.1 magnitude, with the same exceptions as above. For bright galaxies we recommend using the semi-major axis size as a measure of brightness, since the magnitude system used is based on an internal calibration of stellar profiles.
All magnitudes are in the natural photographic system.