Variable phot::fourPiTenPcSq

Variable Documentation

constexpr double phot::fourPiTenPcSq = 4.0 * std::numbers::pi_v<double> * (10.0 * utils::pc) * (10.0 * utils::pc)

4 pi (10 pc)^2, in cm^2 – the standard distance magnitude systems assume

ST, AB, and Vega magnitudes are all defined in terms of a flux, not a luminosity, but the Flambda/Fnu-domain quantities SLUG actually converts (e.g. FilterCollection::phot()’s own results) are specific luminosities, with no particular distance baked in. Converting one of these to a magnitude therefore requires first adopting some fiducial distance to turn the luminosity into a flux – by convention, the same 10 pc the absolute-magnitude scale itself is defined at. This has no analog for the Fnu system’s own physical units (there is no standard distance for a bare flux-per-frequency density), which is why PhotConvert’s direct Flambda <-> Fnu conversion below never uses this at all.