I heard on sunday during a visit at the NOT telescope on La Palma an old story:
The astronomer there explained that a mirror of the telescope can not break like glass. As an illustration (and because people like those stories) he told the story (with the comment he is not sure wether thats really true) there was once in Texas an astronomer that was so angry about his employer and his life that he shot at the telescope mirror to destroy it.
The result was that the bullets just hit the mirror and bumped back after leaving a hole in the glass.
The idea behind this is that the glas of a mirror cools down so slowly that there is no more tension in it and it wont shatter. The slow cooling is also the trick if you want to produce unbreakable cups. These also dont shatter into pieces when they fall on the floor, but are very expensive.
I now wondered if this is a urban legend and googled a little.
I found the website of Bill Keel (http://astronomy.ua.edu/keel/telescopes/mcdonald.html) and of Jason Ellis (http://vagabond92.wordpress.com/2011/01/22/mcdonald-observatory/).
Bill Keel reports how he tried to find out who was the person shooting and Jason shows pictures of the bullet holes in the mirrors.
It seems that the event was not recorderd very clear and it is reported by Jasen Ellis that the guy who shot was actually not from Texas but from Ohio and he also was no astronomer but some technician. Bill Keel notes the original text from Big and Bright: A History of the McDonald Observatory (by David Evans and J. Derral Mulholland, Univ. of Texas Press, 1986).
Firearms are very common in Texas. Astronomer Brian Warner had his tongue only slightly in cheek when he remarked that “Jeff Davis County is about the size of Israel – and slightly better armed.” The prophecy inherent in the aphorism came to pass. The full precipitating causes may never be known, but one February night in 1970 a McDonald Observatory employee (not a Texan, but an Ohioan newly hired from another observatory!) suffered a breakdown and carried a pistol to the observing floor of the 107-inch telescope. He fired a shot at his supervisor, and then unloaded the rest of the clip into the primary mirror. Happily, fused silica is more resilent than ordinary glass, and the big mirror did not break. The craters have been bored out and painted black to reduce any light-scattering effect, and the end result is simply a slight reduction in the efficiency of the telescope. It is now the equivalent of a 106-inch telescope. The incident made the national television news, with Walter Cronkite describing it before a projection showing the wrong telescope upside down.
So it seems to be true!