Sitall aka 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, but since dissolution has diminished in quality. Sitall has 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. Sitall was used for the primary and secondary mirrors of the VLT Survey Telescope.