THE STELLAR POPULATIONS OF PIXELS AND FRAMES

Derived from first physical principles, a few simple rules are present ed that can help in both the planning and interpretation of CCD and IR -array camera observations of resolvable stellar populations. These ru les concern the overall size of the population sampled by a frame as m easured by its total luminosity, and allow us to estimate the number o f stars (in all evolutionary stages) that are included in the frame. T he total luminosity sampled by each pixel (or resolution element) allo ws us, instead, to estimate the depth to which meaningful stellar phot ometry can be safely attempted, and below which crowding makes it impo ssible. Simple relations also give the number of pixels (resolution el ements) in the frame that will contain an unresolved blend of two star s of any kind. It is shown that the number of such blends increases qu adratically with both the surface brightness of the target and with th e angular size of the pixel (or resolution element). A series of examp les is presented illustrating how the rules are practically used in co ncrete observational situations. Application of these tools to existin g photometric data for the inner parts of the bulge of M31, M32, and N GC 147 indicates that no solid evidence has yet emerged for the presen ce of a significant intermediate-age population in these objects.