ESO has signed a contract with the engineering and construction company Abengoa Chile to build the ELT Technical Facility at ESO’s Paranal Observatory in northern Chile. This new facility will host the assembly and maintenance facilities for the mirrors of ESO’s Extremely Large Telescope (ELT), as well as all the coating, washing and stripping facilities needed to coat and maintain the reflectivity of the mirrors.
The primary mirror of the ELT will consist of 798 separate reflective segments . At the ELT Technical Facility, the polished segment assemblies will be coated and integrated with the edge sensors on the sides of each segment, to detect the position of each segment relative to its neighbours .
Once the telescope is complete and in operation, each segment of the primary mirror will need recoating every 1.5 years to ensure its cleanliness and reflectivity. As there are 798 segments, the most efficient way to do this is to remove, clean, recoat, and replace two or three segments every single day.
The ELT Technical Facility will also house the coating unit for the secondary and tertiary mirrors of the ELT, each of which measures around four metres in diameter.
The facility will also be used for the initial assembly, integration and verification tasks of the mirrors, as well as for future recoating activities.
After around three months of detailed design, construction of the building will begin in the second half of 2018 and will last 11 months. The building should be completed by July 2019.
The signature event was held at the ESO Vitacura offices in Santiago, Chile, and the contract was signed by Sergio Adrian Morrone from Abengoa Chile and the ESO Director General, Xavier Barcons, in the presence of Willy Benz, ESO Council President, and Ambassador Gabriel Rodríguez, Director of Energy, Science and Technology and Innovation at the Chilean Ministry of Foreign Affairs. Also participating were ESO’s Representative in Chile, Fernando Comerón, as well as other officials from ESO and Abengoa.
 The primary mirror of the ELT will measure 39 metres in diameter and will consist of 798 segments. The segments are hexagonal in shape, which permits the use of a common support structure for all segments. The maximum corner to corner dimension for the segments is about 1.45 metres. Each segment will be able to move individually, forming an active optics system that ensures optimal image quality at all times.
 These sensors are the most accurate ever used in a telescope and can measure relative positions to an accuracy of a few nanometres. They form a fundamental part of the very complex system that will continuously sense the locations of the ELT primary mirror segments relative to their neighbours and allow the segments to work together to form a perfect imaging system.
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and by Australia as a strategic partner. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its world-leading Very Large Telescope Interferometer as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. ESO is also a major partner in two facilities on Chajnantor, APEX and ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will become “the world’s biggest eye on the sky”.