From ESO: “ESO Awards Contract to Polish the ELT Tertiary Mirror”

ESO 50 Large

European Southern Observatory

15 February 2017
Marc Cayrel
Garching bei München, Germany
Tel: +49 89 3200 6685

Peter Grimley
Assistant Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6383

ESO’s Extremely Large Telescope (ELT), scheduled to see first light in 2024, is at the cutting edge of telescope technology. Its optical system will consist of no fewer than five separate mirrors, each of them a significant engineering challenge.

ESO has now awarded the contract to polish the third mirror in the light path, known as M3, to the French company Reosc [1], a subsidiary of Safran Electronics & Defense. They will receive the blank from SCHOTT, design the mirror and its mounting interfaces, polish the surface, and complete all necessary optical tests before delivery [2]. Reosc were also awarded the contracts to design, polish and test the telescope’s secondary mirror in July 2016, and to manufacture the deformable shell mirrors that will comprise the ELT’s fourth mirror (M4).

M3 will be a giant 3.8-metre concave mirror — as big as the primary mirror of many of today’s world-class telescopes. It will be an unusual feature, as most current large telescopes such as ESO’s Very Large Telescope and the NASA/ESA Hubble Space Telescope use only two curved mirrors, sometimes using a flat tertiary mirror to redirect light to a convenient focus. The curved surface of M3 will work together with the primary and secondary mirrors to deliver a better image quality over a large field of view.

The structural element of the mirror, before the reflective coating is applied, will be made of a sophisticated material called Zerodur™ from SCHOTT [3]. It will then need to be shaped and polished to a precision of 15 nanometres.


[1] Reosc, a subsidiary of Sagem, a Safran high-technology company, is a world leader in the design, production and integration of high-performance optics, including for astronomy, space, high-energy lasers and the semiconductor industry. Reosc develops and produces high-performance optics for satellites, large telescopes and high-energy lasers, as well as the semiconductor industry. The company also built the single-piece 8-metre mirrors for ESO’s Very Large Telescope and the Gemini international telescopes, the 11-metre mirror for the Gran Telescopio de Canarias, mirrors for Europe’s Nirspec instrument on NASA’s James Webb Space Telescope, and mirrors for ESA’s GAIA astronomy satellite.

[2] The contract to cast the M2 mirror blank was awarded on 18 January 2017.

[3] Zerodur™ was originally developed for astronomical telescopes in the late 1960s. It has almost no thermal expansion even in the case of large temperature fluctuations, is highly chemically resistant, and can be polished to a high standard of finish. The actual reflective layer, made of aluminum or silver, is usually vapourised onto the extremely smooth surface shortly before the telescope is put into operation. Many well-known telescopes with Zerodur mirrors have been operating reliably for decades. They include, for example, ESO’s Very Large Telescope in Chile.

Further information about the ELT

See the full article here .

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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. 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, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

ESO LaSilla
ESO/Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres

VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

ESO Vista Telescope
ESO/Vista Telescope at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

ESO/NTT at Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres

ESO VLT Survey telescope
VLT Survey Telescope at Cerro Paranal with an elevation of 2,635 metres (8,645 ft) above sea level

ALMA Array
ALMA on the Chajnantor plateau at 5,000 metres

ESO/E-ELT to be built at Cerro Armazones at 3,060 m

APEX Atacama Pathfinder 5,100 meters above sea level, at the Llano de Chajnantor Observatory in the Atacama desert