Hale Telescope mirror.. It was completed in 1948 and is still in active use. The Hale Telescope represented the technological limit in building large optical telescopes for over 30 years. It was the largest telescope in the world until the Russian BTA-6 was built in 1976, and the second largest until the construction of the Keck Observatory Keck 1 in 1993. Hale supervised the building of the telescopes at the Mount Wilson Observatory with grants from the Carnegie Institution of Washington: the telescope in 1908 and the telescope in 1917. These telescopes were very successful, leading to the rapid advance in understanding
Telescope Observatory]] to search for extraterrestrial life. [[File:Heao b.jpg|thumb|200px|[[Einstein Observatory]] was a space-based focusing optical X-ray telescope from 1978. ]] [[X-ray]] telescopes can use [[X-ray optics]], such as a [[Wolter telescope]]s composed of ring-shaped 'glancing' [[mirror]]s made of [[heavy metals]] that are able to reflect the rays just a few [[degree (angle)|degrees]]. The mirrors are usually a section of a rotated [[parabola]] and a [[hyperbola]], or [[ellipse]]. In 1952, [[Hans Wolter]] outlined 3 ways a telescope could be built using only this kind of mirror. Examples of an observatory using this type of telescope are the [[Einstein Observatory]], [[ROSAT]], and the