According to National Geographic:

Three of today's largest telescopes-Gemini North, Subaru, and Keck-stand within hailing distance of one another atop the nearly 14,000-foot peak of Hawaii's Mauna Kea, an inactive volcano. The altitude puts them above 40 percent of Earth's atmosphere-and most of its water vapor, which is opaque to the infrared wavelengths the astronomers like to study-but also makes it difficult for the astronomers and engineers who work there to breathe and think. Many wear clear-plastic oxygen tubes in their nostrils as routinely as we might wear eyeglasses. Others rely on the body's ability to adapt but worry about making what they call a CLM, or "career-limiting mistake." "At altitude, we don't improvise; that would be a disaster," says Gemini astronomer Scott Fisher. "We're kind of trained monkeys up here. The real thinking goes on at sea level."

These amazing images trace one telescope's production, from glass chunks to completed mirror.

via National Geographic (thanks, Marilyn Terrell!)

Glass is spread over this vast mold, melted, and then slowly spun to create an even, parabolic surface.

These are the 10-pound chunks of the lightweight glass up close.

Now techs must polish the glass until they're read to give it a final coat of highly reflective aluminum.
The polished glass is used in the bodies of super-giant telescope arrays like this one, atop Mauna Kea.
The Large Binocular Telescope on Mount Graham. Images will appear to come from a 22.8-meter telescope. It can also be manipulated to point in any direction you like.

The telescopes in this gallery include Jantar Mantar, a famous observatory built in 1724 in India. But there's also the European Southern Observatory's mega-telescope in Chile (pictured above), and a huge South African observatory. Possibly the most glamorous, though, is the Griffith Observatory, which is always getting gussied up for some formal event and has a whole sassy museum thing going on.

[Eric Africa]