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)
William Herschel Telescope 1984). The WHT saw first light on 1 June 1987; it was the third largest optical telescope in the world at the time. The total cost of the telescope, including the dome and the full initial suite of instruments, was £15M (in 1984, equivalent to £M in ); within budget once inflation is taken into account. The telescope consists of a f/2.5 primary mirror made by Owens-Illinois from Cervit, a zero-expansion glass-ceramic material, and ground by Grubb Parsons. The mirror blank was produced in 1969 as one of a set of four, along with those for the AAT, CFHT and