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Large Telescope Designs With A Spherical Primary Mirror

Very large telescopes of the future will be much more practical to build if the large primary mirror can have a spherical surface. This greatly reduces the time and expense required to optically figure a large surface. Several two-mirror designs are discussed which have a spherical primary mirror and a sub-aperture diameter aspheric secondary mirror. The light path reflects off of both mirrors twice, and the system is corrected for spherical aberration, coma, and astigmatism. Designs with small obscuration are possible if astigmatism correction is dropped. Scanning arrangements for huge Arecibo-type systems are also discussed.