Sitall (also known as Sitall CO-115M or Astrositall) is a crystalline

glass-ceramic Glass-ceramics are polycrystalline materials produced through controlled crystallization of base glass, producing a fine uniform dispersion of crystals throughout the bulk material. Crystallization is accomplished by subjecting suitable glasses to ...

with ultra-low

coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape A shape or figure is a graphics, graphical representation of an object or its external boundary, outline, or external Surface (mathematics), surface, as opposed to other pro ...

(CTE). It was originally manufactured in the former

Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen national ...

and was used in the making of

primary mirror A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective metal ...

s for the Russian

Maksutov telescope The Maksutov (also called a "Mak") 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 ...

s, but since dissolution has diminished in quality. It has a CTE of only 0 ± 1.5 °C⁻¹ in the temperature range −60 to 60 °C, placing it in a small group of transparent materials with low CTE such as Vycor,

Zerodur Zerodur (notation of the manufacturer: ZERODUR®), registered trademarkof Schott AG, 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 GTC, Keck ...

CerVit Cer-vit is a family of glass-ceramic materials that were invented by Owens Illinois in the mid-1960s. Its principle ingredients are the oxides of lithium, aluminum and silicon. It is melted to form a glass which is then heat treated to nucleate and ...

and

fused quartz Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses in which other ingredients are added which change ...

. Materials of low coefficient of thermal expansion are critical in the manufacture of optical elements for telescopes. In

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 (if they are part of a larger parabolic reflector). They are used as objectives for ...

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 The Southern African Large Telescope (SALT) is a 10-metre class optical telescope designed mainly for spectroscopy. It consists of 91 hexagonal mirror segments each with a 1-metre inscribed diameter, resulting in a total hexagonal mirror of 11.1 ...

(SALT) selected Sitall for the manufacture of its 91 primary mirror segments by

Lytkarino Optical Glass Plant Lytkarino Optical Glass Plant (russian: Лыткаринский завод оптического стекла) is a company based in Lytkarino, Russia and established in 1934. It is part of the Shvabe Holding of the state-owned Rostec corporation ...

. The choice arose out of increased scientific collaboration between Russia and South Africa since 1994. Sitall was also used for the primary and secondary mirrors of the

VLT Survey Telescope The VLT Survey Telescope (VST) is a telescope located at 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 (VLT) Unit Telescopes on the summit o ...

References

Further reading