ESO has signed a contract with the French optics company Reosc, a subsidiary of Sagem, Safran group, to manufacture the deformable shell mirrors that will comprise the fourth mirror (M4) of the European Extremely Large Telescope (E-ELT) These mirrors will be mounted and supported in the adaptive mirror unit (ann15045) to form the largest ever adaptive mirror unit ever made.

The contract was signed by representatives of Reosc and ESO at a ceremony at ESO Headquarters on 8 July 2015.

The M4 deformable mirror system forms a key part of the E-ELT. Measuring 2.4 metres in diameter, it will be made up of six thin segment mirrors, each only 1.95 millimetres thick and made of ceramic glass. Each shell is one of six 60-degree petal sections that form the circular segmented M4 mirror.

SCHOTT will deliver pieces of ceramic to Reosc, then Reosc will manufacture and polish the thin shells. The AdOptica consortium in Italy - ADS International and Microgate, partnered with INAF (Istituto Nazionale di Astrofisica) as subcontractors - will produce the shell assemblies and build the entire support unit (ann15045).

The prototyping of this challenging mirror has been ongoing for several years. Reosc produced the first flat thin-shell mirror for the E-ELT programme already in 2008. This 1.95-millimetre thick, 1-metre diameter mirror was then cut into two petals and used for the demonstration prototype [1]. In 2010, Reosc successfully manufactured a 2.7-metre thin mirror made of borofloat glass and cut it into segments.

In 2011, Reosc then delivered the first thin shell for the deformable mirror of the Very Large Telescope (VLT). This aspheric mirror (ann12015) was coated and integrated in the support unit and has been under testing at ESO since the end of 2012 (potw1307a). A second thin-shell mirror for the VLT deformable secondary mirror (DSM) was delivered at the end of 2013 (ann14010), and will be tested at ESO in early 2016.

Reosc will manufacture twelve mirror shells for the E-ELT M4, planning to produce the first six mirrors within five years and to complete the manufacturing of the twelve mirrors by 2023.