def halfmax_points(self):
        """Get the bandpass' half-maximum wavelengths. These can be used to
        compute a representative bandwidth, or for display purposes.

        Unlike calc_halfmax_points(), this function will use a cached value if
        available.

        """
        t = self.registry._halfmaxes.get((self.telescope, self.band))
        if t is not None:
            return t

        t = self.calc_halfmax_points()
        self.registry.register_halfmaxes(self.telescope, self.band, t[0], t[1])
        return t