Optical issues for giant telescopes with extremely fast primary mirrors

Existing design rules break down as we plan for a new generation of giant optical telescopes of 20, 30, 50, even 100 meters in diameter. One might expect these telescopes to converge on the design universally accepted for similarly sized radio telescopes, with their highly aspheric, ∼ f/0.4, primary dish. But most of the optical design concepts now under consideration have favored spherical or relatively slow paraboloidal surfaces, leading to a much larger telescope, more subject to wind buffeting, and requiring gargantuan enclosures for protection. This paper explores issues and limitations for building and operating telescopes as the primary focal ratio is reduced to a value as small as f/0.4. Such compactness will be particularly important for mechanical stability, cost control and for large telescopes that must move continuously on a track, as in the 20/20 concept. We find that fabrication and alignment methods for telescopes using numerous small (1-m class) segments are driven to long focal ratios, while those using few large, actively controlled segments can be made as fast as f/0.5.