History of the telescope a wider field of view than a classic Cassegrain. During the period 1850–1900, reflectors suffered from problems with speculum metal mirrors, and a considerable number of "Great Refractors" were built from 60 cm to 1 metre aperture, culminating in the Yerkes Observatory refractor in 1897; however, starting from the early 1900s a series of ever-larger reflectors with glass mirrors were built, including the Mount Wilson 60-inch (1.5 metre), the 100-inch (2.5 metre) Hooker Telescope (1917) and the 200-inch (5 metre) Hale telescope (1948); essentially all major research telescopes since 1900 have been reflectors. A number of 4-metre class (160 inch)
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