CROWDED-FIELD PHOTOMETRY FROM HST-IMAGING

We present a thorough investigation of stellar photometry based on HST imaging of crowded fields at 85 '' and 10 '' from the centre of the h igh-surface brightness elliptical galaxy M 32. The Principal Investiga tors of the present archive data have elsewhere presented an impressiv e colour-magnitude diagram of the field at 85 '' Based on the same dat a we enlarge on their photometric analysis and supplement with error e stimators that more clearly show the implications of severe image crow ding on the stellar photometry. We show that the faintest stars (I gre ater than or similar to 25.0, V greater than or similar to 26.0) are f ound too bright by several tens of a magnitude. For the field at 10 '' we conclude that it is not possible to obtain reliable stellar photom etry, standard deviations being larger than 0.4 mag. Artificial-star e xperiments show that only very few of the brightest stars of the lumin osity function can be expected to represent single objects, the majori ty being either spurious or not as bright as measured. The paper as su ch introduces and demonstrates basic guide lines which may be used whe n dealing with stellar photometry of severely crowded fields.