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 t ...

with ultra-low

coefficient of thermal expansion Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions). Substances usually contract with decreasing temp ...

(CTE). It was originally manufactured in the former

Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...

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, parabolic, or hyperbolic shaped disks of polished ...

s for the Russian Maksutov telescopes, 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, CerVit 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, such as soda-lime glass, lead glass, or borosi ...

. 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 Plant. 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 of ...

References

Further reading