Derivation of Illuminations from Stellar Colors

Most photometric measurements through colored or interference filters are given in terms of magnitudes and/or color indices. However, results of computations related to stellar models are given in terms of monochromatic fluxes, the distribution of which is to be compared to the observations. It is thus interesting to translate magnitudes and colors (as observed from outside the Earth’s atmosphere) in terms of monochromatic illuminations, related to the stellar fluxes attenuated by interstellar extinction. Although such a transformation may not be possible when the filters are rather broad and when the stellar flux distribution has sizeable discontinuities, it may lead to satisfactory results when moderate width (100–300 A) filters are suitably distributed along the spectrum and when the response curve of the telescope and photometer system is well known. This is the case for the Geneva Observatory photometric system.