docs: def mag_to_fnu(self, mag):
          """SDSS *primed* magnitudes to F_ν. The primed magnitudes are the "USNO"
          standard-star system defined in Smith+ (2002AJ....123.2121S) and
          Fukugita+ (1996AJ....111.1748F). This system is anchored to the AB
          magnitude system, and as far as I can tell it is not known to have
          measurable offsets from that system. (As of DR10, the *unprimed* SDSS
          system is known to have small offsets from AB, but I do not believe
          that that necessarily has implications for u'g'r'i'z'.)
  
          However, as far as I can tell the filter responses of the USNO
          telescope are not published -- only those of the main SDSS 2.5m
          telescope. The whole reason for the existence of both the primed and
          unprimed ugriz systems is that their responses do not quite match. For
          my current application, which involves a completely different
          telescope anyway, the difference shouldn't matter.
  
          """
          # `band` should be 'up', 'gp', 'rp', 'ip', or 'zp'.
          if len(band) != 2 or band[1] != 'p':
              raise ValueError('band: ' + band)
          return abmag_to_fnu_cgs(mag)