Very large optics of the future

Very large is a relative term when applied to optics. Astronomers work at the dual frontiers of state of the art and available resources. The fundamental limit is dollar resources: ~100 million for terrestrial telescopes and 1 billion for space telescopes. The challenge is to build the largest systems to meet increasingly exacting science requirements within these limits. New concepts, materials, methods, and control technology are increasing the upper limit to size to a point scarcely imagined a few decades ago. We trace the history of successive largest telescopes and see where the arrow of history points. Relatively traditional telescope designs reach the 5-m class, of which the 5-m Palomar telescope is the prime example. Now astronomers are reaching for 10 m, made possible by new technologies to make, test, support, and control the optical system. Beyond this size the multiple mirror telescope offers advantages; its limit is set by the number of 10-m class optical systems that can be beam combined. In space, the 2.5-m Hubble telescope has exceeded the billion dollar limit but will be worth it in terms of the science to be returned. A 20-m optical infrared telescope can probably be assembled in space within this limit, and new concepts and technologies may make future space telescopes of this size work at optical diffraction limits.