This is an optical term that refers to the mirror of a reflecting telescope that is ground to the figure of a parabola or ellipsoid. Such a mirror is said to be "aspheric" – literally, "non-spheric". Parabolization is necessary to ensure that a mirror will bring the light rays to a common focal point. Parabolization is not necessary with mirrors of longer focal length. Because a sphere has the same radius of curvature across its entire surface, it is simpler to grind mirrors and lenses to a spherical shape. This generally results in a smoother and more optically precise surface. For this reason, refractor lenses and Maksutov telescope optics, which employ spherical surfaces, normally present somewhat better images.