Heterochromatic Atmospheric Extinction

These problems exist to a lesser degree in intermediate band systems, but narrow band systems are relatively immune from these complexities. We do not treat either kind of system here. If our results were incorporated into a photometric reduction program, and standard stars and program stars stars carefully selected by metallicity and luminosity class, a standard magnitude system could be established that would be directly comparable to synthetic systems.

As a bonus, measurements of intrinsic flux variations at the millimagnitude level would become more secure. We describe our own operational photometric transformation program that incorporates only the linear part of the dependence on colour of atmospheric extinction. Our results and prescriptions are useful for aperture photoelectric photometry, but our implementation is aimed at CCD photometry of stellar populations.