Bojan
You're right about glass, but what about the glass that weights 10 times less:
http://www.corning.com/specialtymate...ror_blank.aspx
"Corning has a rich history in space exploration, producing mirrors for the Hubble, Gemini, and Suburu telescopes; along with providing window glass for all of NASA's manned spacecraft missions and the Internationl Space Station. These mirrors are made of Corning's ultra low expansion ULEŽ glass, a material that exhibits virtually no dimensional changes over extreme temperature variations and is generally considered the best in the world for astronomical optics.
The "lightweighting" techniques used in the Hubble mirror have been extended to other space program and ground-based astronomy applications. Reducing the weight of these large mirrors - sometimes down to one eighth the weight of an equal sized solid mirror - allows changes to the superstructure for ground-based telescopes, helping to reduce costs and extending functionality. Corning has continued to develop and expand the use of this technology.
Current and future space-based telescopes may contain Corning mirrors that are less than 10 percent of the weight of an equal-sized solid mirror. The technology has been adapted for use in the GOES Weather Satellites, which are key U.S. resources for weather, hydrologic and climate forecasts. "
cheers
bob