ESO signs contracts for giant ELT telescope optics

24 Jan 2017

Schott, Sener and FAMES charged with manufacturing some of the biggest, high-spec mirrors and sensors ever designed.

At a ceremony last week at the headquarters of the European Southern Observatory, in Garching, near Munich, four contracts were signed for major components destined for the Extremely Large Telescope, which ESO is building.

These were for: the casting of the telescope’s giant secondary and tertiary mirrors, awarded to Schott; the supply of mirror cells to support these two mirrors, awarded to Sener Group; and the supply of the edge sensors that form a vital part of the ELT’s giant segmented primary mirror control system, awarded to the FAMES consortium. The secondary mirror will be largest ever deployed on a telescope and the largest convex mirror ever produced.

The construction of the 39-metre ELT, the largest optical/near-infrared telescope in the world, is moving forward. The giant telescope employs a complex five-mirror optical system that has never been used before and which requires optical and mechanical elements that test modern technology to its limits. Contracts for the manufacture of several of these challenging telescope components were signed by ESO’s Director General, Tim de Zeeuw, and representatives of three industrial contractors in the ESO member states. The value of the deals was not revealed.

de Zeeuw commented, “These four contracts today, each for advanced components at the heart of the ELT’s revolutionary optical system, underline how the construction of this giant telescope is moving ahead and is on target for first light in 2024. At ESO, we look forward to working with Schott, Sener and FAMES — three leading industrial partners from our Member States.”

The first two contracts, signed with Schott cover the casting of the ELT’s largest single mirrors — the 4.2-metre secondary and 3.8-metre tertiary mirror — to be constructed from Schott’ low-expansion ceramic material Zerodur, developed for astronomical telescopes in the late 1960s.

This material has almost no thermal expansion, which means that even in the case of large temperature fluctuations, it does not lose its precise shape. Chemically, the material is resistant and can be polished to a high finish. Its reflective layer, made of aluminum or silver, is usually vaporized onto the surface shortly before the telescope is put into operation.

Mirror, mirror

Suspended upside-down at the top of the telescope structure, high above the 39-metre primary mirror, the secondary mirror will be largest ever employed on a telescope and the largest convex mirror ever produced. The concave tertiary mirror is also an unusual feature of the telescope. The ELT secondary and tertiary mirrors will rival in size the primary mirrors of many modern research telescopes and weigh 3.5 and 3.2 tonnes respectively. The secondary mirror is to be delivered by the end of 2018 and the tertiary by July 2019.

The third contract was signed with the Sener Group. This covers the provision of the sophisticated support cells for the ELT secondary and tertiary mirrors and the associated complex active optics systems that will ensure these massive, but flexible, mirrors retain their correct shapes and are correctly positioned within the telescope.

The fourth contract was signed on behalf of the FAMES consortium, which is composed of Fogale and Micro-Epsilon. This covers the fabrication of 4608 edge sensors for the 798 hexagonal segments of the ELT’s primary mirror.

These sensors are the most accurate ever used in a telescope and can measure relative positions to an accuracy of a few nanometers. They form a fundamental part of the complex system that will continuously sense the locations of the ELT primary mirror segments relative to their neighbors and allow the segments to work together to form a perfect imaging system.