Automated galaxy surface photometry - I. Technique, calibration and validation

Abstract

Two-dimensional surface photometry of a very large number of galaxies on a deep Schmidt plate has been obtained using the Automatic Plate Measuring System (APM). A method of photometric calibration, suitable for APM measurements, via pixel-by-pixel comparison with CCD frames of a number of the brighter galaxies is described and its advantages are discussed. The same method is used to demonstrate the consistency of measurement of the APM machine when used for surface photometry. For galaxy surface brightness below 22.5 B<it>μ</it>, errors introduced by the machine measuring process are found to be small enough to allow the possibility of 10 per cent (0.1 mag) accuracy in the photometry. This means that, at low surface brightness, machine-induced errors are as small as a number of other effects inherent in all galaxy surface photometry, and thus do not compromise the vast multiplexing advantage of being able to measure all the galaxies in a photographic plate simultaneously.