Aplanatic corrector designs for the extremely large telescope.

The next century is knocking on our door, bringing with it the possibility of telescopes even bigger than the 8-10-m-class instruments that have proliferated over the past decade. The fixed spherical reflector is the most economical and pragmatic way to construct an extremely large primary mirror (30-50 m in diameter). Although spherical mirrors have virtues such as manufacturability and identically figured segments, they also create great amounts of spherical aberration and coma.