A setter for the flux units

        Parameters
        ----------
        units: str, astropy.units.core.PrefixUnit
            The desired units of the zeropoint flux density

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