With the Extremely Large Telescope (ELT), even a millionth of a millimeter (0.000001 mm) counts. The material the mirrors are made of is the key to the perfect view into space. Ten kilometers of steel railroad track expand two meters when the temperature rises 20° C. The same conditions would cause ZERODUR® glass-ceramic to only expand by one millimeter. For an ELT mirror segment (1.40 meters in diameter), glass-ceramic’s low coefficient of thermal expansion means deviation only in the millionth of a millimeter range – in other words, sharpness guaranteed.
August 18th, 2017