Free Astronomy Magazine March-April 2018

49 MARCH-APRIL 2018 SPACE CHRONICLES T he SCHOTT team that successfully casted the first ELT main mirror segments. [SCHOTT/ESO] tracts to manufacture the blanks of the first four ELT mirrors (known as M1 to M4, with M1 being the pri- mary mirror). The first segment castings are im- portant as they allow the engineers at SCHOTT to validate and optimise the manufacturing process and the sent at the first castings: “It was a wonderful feel- ing to see the first segments being successfully cast. This is a major milestone for the ELT!” As with the telescope’s sec- ondary mirror blank, the ELT main mirror seg- ments are made from the low-ex- pansion ceramic material Zerodur© from SCHOTT. Zerodur© was originally developed for astronomical telescopes in the late 1960s. It has an extremely low coefficient of thermal expansion, meaning that even in the case of large temperature fluctuations, the material does not expand. Chemi- cally, Zerodur© is very resistant and can be polished to a high standard of finish. The actual reflective layer, made of aluminium or sil- ver, is usually vaporised onto the extremely smooth surface shortly before a telescope is put into oper- ation and at regular inter- vals afterwards. Many well-known tele- scopes with Zerodur© mir- rors have been operating reliably for decades, in- cluding ESO’s Very Large Telescope in Chile. ESO has awarded this Ger- man company with con- 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 mir- ror itself, plus a spare set of 133). When fully up to speed, the pro- duction rate will be about one segment per day. After casting, the mirror segment blanks will go through a slow cooling and heat treat- ment sequence and will then be ground to the right shape and polished to a precision of 15 nanometres across the entire optical surface. The shaping and polish- ing will be performed by the French company Safran Reosc, which will also be responsible for additional testing. T his video shows the fusion of the first ELT main mirror segments. [SCHOTT AG] !