Making mirrors for giant telescopes

Advances in our understanding of the Universe depend on improvements in sensitivity and angular resolution that can come only with larger telescopes.Telescope diameters increased by almost an order of magnitude in the last century, but that growth has been sporadic, limited mainly by the ability to make bigger mirrors that hold their shape against the dynamic effects of gravity, wind and temperature.Three major advances in mirror technology occurred in the 1980s, including the lightweight honeycomb mirrors made at the Richard F. Caris Mirror Lab at the University of Arizona.In this informal paper, I will describe these technologies and show how they enabled the current generation of 8-to 12-m telescopes and how they are now being used to build telescopes of 25 to 39 m.