Low-expansion glass

thumb|A PYREX 1-quart [[measuring cup manufactured after 1940, featuring graduations in United States customary units]]

glass consisting of pure silica

thumb|300px|Opening of the European Extremely Large Telescope|ELT secondary mirror Zerodur blank mold containing the glass at first annealing at the [[Schott AG 4-meter blank annealing facility in Mainz, Germany.]] Zerodur is a lithium-aluminosilicate glass-ceramic manufactured by Schott AG. Zerodur has a near zero coefficient of thermal expansion (CTE), and is used for high-precision applications in telescope optics, microlithography machines and inertial navigation systems.

Sitall (also known as Sitall CO-115M or Astrositall) is a crystalline glass-ceramic 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 mirrors for the Russian Maksutov telescopes. 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.