Frogel et al. See IRTF photometry manual for above references.
M are less reliable than earlier spectral types.
Avoid using them if possible. The Koorneef standards may be of lower quality than the others. This is likely a result of the different detector response of these two systems. The wavelength of 4.8 microns for the M filter is adopted to be consistent with the published magnitudes until such time that the true effective wavelength of the IRTF M filter is established for both the InSb and Bolometer systems. Use the Sinton and Tittemore M standards for the InSb and the CalTech M standards with the Bolometer, since these magnitudes were measured with the InSb and Bolometer, respectively.