Complex sentence: The mirror has to be carefully ground and polished (figured) to an extremely accurate shape, usually a paraboloid, although telescopes with high focal ratios may use spherical mirrors since the difference in the two shapes is insignificant at those ratios.
Split into simpler sentences: The mirror has to be carefully ground, polished and figured to an extremely accurate shape, usually a paraboloid. Telescopes with high focal ratios may use spherical mirrors since the difference in the two shapes is insignificant at those ratios.