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
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