The first six hexagonal segments for the main mirror of the European Southern Observatory's (ESO's) Extremely Large Telescope (ELT) have been successfully cast by Schott (Mainz, Germany) at their facility in Mainz. These segments will form parts of the ELT's 39 m main mirror, which will have 798 segments in total when completed. The ELT will be the largest optical telescope in the world when it sees first light in 2024.
The 39 m diameter primary mirror of ESO's ELT will be by far the largest ever made for an optical-infrared telescope. Such a giant is much too large to be made from a single piece of glass, so it will consist of 798 individual hexagonal segments, each measuring 1.4 m across and about 5 cm thick. The segments will work together as a single huge mirror to collect tens of millions of times as much light as the human eye. SEE VIDEO:
As with the telescope's secondary mirror blank, the ELT main mirror segments are made from the low-expansion ceramic material from Schott. ESO has awarded this German company with contracts to manufacture the blanks of the first four ELT mirrors (known as M1 to M4, with M1 being the primary mirror).
The first segment castings are important as they allow the engineers at Schott to validate and optimize the manufacturing process and the associated tools and procedures.
The casting of the first six segments is a major milestone, but the road ahead is long--in total more than 900 segments will need to be cast and polished (798 for the main mirror itself, plus a spare set of 133). When fully up to speed, the production rate will be about one segment per day.
After casting, the mirror segment blanks will go through a slow cooling and heat treatment sequence and will then be ground to the right shape and polished to a precision of 15 nm across the entire optical surface. The shaping and polishing will be performed by Safran Reosc (Saint Pierre du Perray, France), which will also be responsible for additional testing.
SOURCE: ESO; http://www.eso.org/public/news/eso1801/