http://www.space.com/22555-giant-magellan-telescope-mirror-casting.html?cmpid=51463011278974
The surface of each GMT primary mirror must be perfect within 20 nanometers, or about the thickness of a single glass molecule. To put that in perspective: If one of these mirrors were spread out to cover the entire continental United States, GMT officials said, there could be no bump on it more than 0.5 inches (1.75 centimeters) high.