def make_energies_hdu(self, extname="ENERGIES"):
        """ Builds and returns a FITs HDU with the energy bin boundries

        extname   : The HDU extension name            
        """
        if self._evals is None:
            return None
        cols = [fits.Column("ENERGY", "1E", unit='MeV',
                            array=self._evals)]
        hdu = fits.BinTableHDU.from_columns(
            cols, self.make_header(), name=extname)
        return hdu