docs: def flux_units(self, units):
          """
          A setter for the flux units
  
          Parameters
          ----------
          units: str, astropy.units.core.PrefixUnit
              The desired units of the zeropoint flux density
          """
          # Check that the units are valid
          dtypes = (q.core.PrefixUnit, q.quantity.Quantity, q.core.CompositeUnit)
          if not isinstance(units, dtypes):
              raise ValueError(units, "units not understood.")
  
          # Check that the units changed
          if units != self.flux_units:
  
              # Convert to new units
              sfd = q.spectral_density(self.wave_eff)
              self.zp = self.zp.to(units, equivalencies=sfd)
  
              # Store new units
              self._flux_units = units