Sitall
Encyclopedia
Sitall aka Sitall CO-115M or Astrositall, is a crystalline glass ceramic material with ultra low coefficient of thermal expansion (CTE). It was originally manufactured in the former Soviet Union
and was used in the making of primary mirror
s for the Russian Maksutov telescope
s, but since dissolution has diminished in quality.
Sitall, with a CTE of only 0±1.5 x 10−7/°C in the temperature range -60 to 60°C, placing it in a rather small group of transparent materials with low CTE such as Pyrex
, Zerodur
, Cervit and fused quartz
.
Materials of low coefficient of thermal expansion are critical in the manufacture of optical elements for telescopes. In segmented mirror
telescopes, it is desirable to have this coefficient as near zero as possible, and to have a high degree of homogeneity in the material. The Southern African Large Telescope
(SALT) selected Sitall for the manufacture of its 91 primary mirror segments by Lytkarino
Optical Glass Factory (LZOS). Using this company was a direct result of increased scientific collaboration between Russia and South Africa since 1994.
Sitall was used for the primary and secondary mirrors of the VLT Survey Telescope
.
Sitall has been ballistically tested by The Pentagon
for use as a complex composite armour system, intended to resist chemical and kinetic assault.
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
and was used in the making of primary mirror
Primary mirror
A primary mirror is the principal light-gathering surface of a reflecting telescope.-Description:The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective metal , or in later telescopes, glass or other material coated with a reflective layer...
s for the Russian Maksutov telescope
Maksutov telescope
The Maksutov is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus lens in a design that takes advantage of all the surfaces being nearly "spherically symmetrical". The negative lens is usually full diameter and placed at the entrance pupil of the...
s, but since dissolution has diminished in quality.
Sitall, with a CTE of only 0±1.5 x 10−7/°C in the temperature range -60 to 60°C, placing it in a rather small group of transparent materials with low CTE such as Pyrex
Pyrex
Pyrex is a brand name for glassware, introduced by Corning Incorporated in 1915.Originally, Pyrex was made from borosilicate glass. In the 1940s the composition was changed for some products to tempered soda-lime glass, which is the most common form of glass used in glass bakeware in the US and has...
, Zerodur
Zerodur
Zerodur®, a registered trademark of Schott Glass Technologies, is a lithium aluminosilicate glass-ceramic produced by Schott AG since 1968. It has been used for a number of very large telescope mirrors including Keck I and Keck II. With its very low coefficient of expansion it can be used to...
, Cervit and fused quartz
Fused quartz
Fused quartz and fused silica are types of glass containing primarily silica in amorphous form. They are manufactured using several different processes...
.
Materials of low coefficient of thermal expansion are critical in the manufacture of optical elements for telescopes. In segmented mirror
Segmented mirror
A segmented mirror is an array of smaller mirrors designed to act as segments of a single large curved mirror. The segments can be either spherical or asymmetric . They are used as objectives for large reflecting telescopes...
telescopes, it is desirable to have this coefficient as near zero as possible, and to have a high degree of homogeneity in the material. The Southern African Large Telescope
Southern African Large Telescope
The Southern African Large Telescope is a 66m2 area optical telescope with a nominally 9.2 meter aperture but up to about 11.1m x ~9.8 m diameter aperture, and designed mainly for spectroscopy. It is located close to the town of Sutherland in the semi-desert region of the Karoo, South Africa...
(SALT) selected Sitall for the manufacture of its 91 primary mirror segments by Lytkarino
Lytkarino
Lytkarino is a town in Moscow Oblast, Russia, located on the left bank of the Moskva River southeast of Moscow. Population: Lytkarino was founded in the first half of the 15th century as a village of the same name and granted town status in 1957....
Optical Glass Factory (LZOS). Using this company was a direct result of increased scientific collaboration between Russia and South Africa since 1994.
Sitall was used for the primary and secondary mirrors of the VLT Survey Telescope
VLT Survey Telescope
The VLT Survey Telescope is the latest telescope to be added to ESO’s Paranal Observatory in the Atacama Desert of northern Chile. It is housed in an enclosure immediately adjacent to the four Very Large Telescope Unit Telescopes on the summit of Cerro Paranal. The VST is a wide-field survey...
.
Sitall has been ballistically tested by The Pentagon
The Pentagon
The Pentagon is the headquarters of the United States Department of Defense, located in Arlington County, Virginia. As a symbol of the U.S. military, "the Pentagon" is often used metonymically to refer to the Department of Defense rather than the building itself.Designed by the American architect...
for use as a complex composite armour system, intended to resist chemical and kinetic assault.