def mag_to_fnu(self, mag):
        """Convert a magnitude in this band to a f_ν flux density.

        It is assumed that the magnitude has been computed in the appropriate
        photometric system. The definition of "appropriate" will vary from
        case to case.

        """
        if self.native_flux_kind == 'flam':
            return flam_ang_to_fnu_cgs(self.mag_to_flam(mag), self.pivot_wavelength())
        raise PKError('dont\'t know how to get f_ν from mag for bandpass %s/%s',
                      self.telescope, self.band)