Digital stacking of tech pan films and the photometry of faint galaxies

Abstract

We describe the processing and co-addition of digitised scans of Tech Pan films, and discuss the scientific and photometric gains obtained from use of co-added films. This work is based on SuperCOSMOS scans at 10 fim resolution of small regions of the Virgo cluster, which form part of a large-scale survey for low surface brightness galaxies in Virgo. Use of co-added Tech Pan films allows the detection and surface photometry of such galaxies to very faint limits, and therefore provides an invaluable tool for a deeper sampling of the cluster galaxy luminosity function over large areas of sky. The calibration steps necessary before the films can be stacked are described in detail, as well as the stacking process itself. Median-stacking is found to be more effective than averaging or simple adding of individual images, mainly due to its ability in removing spurious features present on single films. We also discuss several of the properties of stacked Tech Pan films, such as background variations, the extent of galaxy profiles and the detectability of small, faint galaxies.