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 proli
ferated over the past decade. The fixed spherical reflector is the most eco
nomical and pragmatic way to construct an extremely large primary mirror (3
0-50 m in diameter). Although spherical mirrors have virtues such as manufa
cturability and identically figured segments, they also create great amount
s of spherical aberration and coma. Here we show that there are several cat
optric (all-reflecting) corrector designs that enable a fast telescope base
d on a spherical primary mirror. (C) 2000 Optical Society of America OCIS c
odes: 110.6770, 350.1260, 110.0110.