The 949th ELT mirror segment is cast and ready to take shape

The primary mirror of ESO’s Extremely Large Telescope (ELT), known as M1, will be by far the largest mirror ever made for a telescope. With a diameter of more than 39 metres, M1 is too large to be made from a single piece of glass and will instead consist of 798 hexagonal segments, each about five centimetres thick and 1.5 metres across, working together to collect tens of millions of times as much light as the human eye. An additional 133 segments have been produced to facilitate the maintenance and recoating of the segments once the telescope is operational. ESO has also procured 18 spare segments, bringing the total number to 949. Now, German company SCHOTT has successfully cast the blank for the last of the 949 segments, seen in this photo. The M1 blanks, shaped pieces of material that are later polished to become the mirror segments, are made from ZERODUR©, a low-expansion glass-ceramic material developed by SCHOTT and optimised for the extreme temperature ranges at the ELT’s site in the Atacama Desert. The 949th segment is seen in this image before being cut into its hexagonal shape and polished — steps that will be performed by French company Safran Reosc.