The Stellar Populations of Pixels and Frames

Derived from first physical principles, a few simple rules are presented that can help in both the planning and interpretation of CCD and IR-array camera observations of resolvable stellar populations. These rules concern the overall size of the population sampled by a frame as measured by its total luminosity, and allow to estimate the number of stars (in all evolutionary stages) that are included in the frame. The total luminosity sampled by each pixel (or resolution element) allows instead to estimate to which depth meaningful stellar photometry can be safely attempted, and below which crowding makes it impossible. Simple relations give also the number of pixels (resolution elements) in the frame that will contain an unresolved blend of two stars of any kind. It is shown that the number of such blends increases quadratically with both the surface brightness of the target, as well as with the angular size of the pixel (or resolution element). A series of examples are presented illustrating how the rules are practically used in concrete observational situations. Application of these tools to existing photometric data for the inner parts of the bulge of M31, M32 and NGC 147 indicates that no solid evidence has yet emerged for the presence of a significant intermediate age population in these objects.