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)
Large Synoptic Survey Telescope of view, but at the expense of light-gathering power as the large second mirror blocks much of the aperture of the telescope. The LSST primary mirror (M1) is in diameter, the secondary mirror (M2) is in diameter, and the tertiary mirror (M3), inside the ring-like primary, is in diameter. The secondary mirror is expected to be the largest convex mirror in any operating telescope, until surpassed by the ELT's 4.2 m secondary . The second and third mirrors reduce the primary mirror's light-collecting area to , equivalent to a telescope. Multiplying this by the field of view produces an etendue