<<The Large Synoptic Survey Telescope (LSST) is a wide-field survey reflecting telescope with an 8.4-meter primary mirror, currently under construction, that will photograph the entire available sky every few nights.
The telescope uses a novel 3-mirror design, a variant of three-mirror anastigmat, which allows a compact telescope to deliver sharp images over a very wide 3.5-degree diameter field of view. Images will be recorded by a 3.2-gigapixel CCD imaging camera, the largest digital camera ever constructed. The telescope is located on the El Peñón peak of Cerro Pachón, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.
The LSST is the latest in a long line of improvements giving telescopes larger fields of view. The earliest reflecting telescopes used spherical mirrors, which although easy to fabricate and test, suffer from spherical aberration; a very long focal length was needed to reduce spherical aberration to a tolerable level. Making the primary mirror parabolic removes spherical aberration on-axis, but the field of view is then limited by off-axis coma. Such a parabolic primary, with either a prime or Cassegrain focus, was the most common optical design up through the Hale telescope in 1949. After that, telescopes used mostly the Ritchey–Chrétien design, which uses two hyperbolic mirrors to remove both spherical aberration and coma, leaving only astigmatism, giving a wider useful field of view. Most large telescopes since the Hale use this design—the Hubble and Keck telescopes are Ritchey–Chrétien, for example. LSST will use a three-mirror anastigmat to cancel astigmatism: three non-spherical mirrors. The result is sharp images over a very wide field 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 8.4 meters in diameter, the secondary mirror (M2) is 3.4 meters in diameter, and the tertiary mirror (M3), inside the ring-like primary, is 5.0 meters in diameter. The second and third mirrors reduce the primary mirror's light-collecting area to 35 square meters, equivalent to a 6.68-meter-diameter telescope.
The primary and tertiary mirrors (M1 and M3) are designed as a single piece of glass, the "M1M3 monolith". Placing the two mirrors in the same location minimizes the overall length of the telescope, making it easier to reorient quickly. Making them out of the same piece of glass results in a stiffer structure than two separate mirrors, contributing to rapid settling after motion.
The optics includes three corrector lenses to reduce aberrations. These lenses, and the telescope's filters, are built into the camera assembly. The first lens at 1.55 m diameter is the largest lens ever built, and the third lens forms the vacuum window in front of the focal plane.>>