Multiple E-ELT Mirror Segments Tested Together for the First Time

24 February 2012

The next pieces of the jigsaw for the European Extremely Large Telescope (E-ELT) — the world’s biggest eye on the sky — are coming together. At ESO’s Garching facility in Germany, a full-size mock-up of a small section of the E-ELT primary mirror and its support structures provides a first test of multiple segments of the E-ELT’s main mirror. Four of the 1.45-metre segments — the final mirror will have 798 — are being put through their paces to see how these hexagonal mirrors and their complex support units behave in real conditions, and whether they match the exacting requirements needed for the telescope.

The mirror segments are made from different types of material from different manufacturers and are not yet coated to make them reflective. The current tests focus on the mechanical support systems, whose advanced sensors and actuators are used to keep the separate segments in the right position and shape, so that together they can form a perfect, giant mirror.

Two of the four segments can be tilted by position actuators to adjust the shape of the mirror. The actuators move and support the segments — each with a weight of 245 kg — with nanometre precision. Shape actuators enable minute deformations of each segment to correct for tiny errors in the surface shape, caused by inevitable variations in manufacturing and assembly, as well as mechanical stresses or temperature shifts. Edge sensors constantly measure the relative positions of each segment and its neighbours to ensure that the overall surface shape remains unchanged when the whole assembly is tilted or disturbed by external factors such as wind, temperature changes or vibrations.

During tests, the whole assembly can be tilted at an angle of up to 45 degrees. Standing some two metres above the floor, the four test mirror segments have been placed on their supports to see how they behave together and when tilted.

The assembly is also being used to check accessibility for the engineers and technicians on-site to conduct maintenance and tests. Each day when the telescope is in operation, two of the 798 segments will be lifted out to be recoated, as the entire mirror will need recoating every 18 months to maintain its high reflectivity.

The E-ELT is ESO’s most ambitious project so far. With start of operations planned for early in the next decade, the E-ELT will tackle the biggest scientific challenges of our time, and aim for a number of notable firsts, including tracking down Earth-like planets around other stars in the habitable zones where life could exist. Dubbed E-ELT for European Extremely Large Telescope, this revolutionary new ground-based telescope concept will have a 40-metre-class main mirror and will be the largest optical/near-infrared telescope in the world: “the world’s biggest eye on the sky”.


Each E-ELT primary mirror segment is hexagonal, measures 1.45 metres across, is 50 mm thick and weighs 245 kg. Mounted on its support structure, it weighs 325 kg.

The primary mirror of the E-ELT will contain 798 segments, measure more than 39 metres across, and weigh some 259 tonnes.




Richard Hook
ESO, La Silla, Paranal, E-ELT and Survey Telescopes Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591

About the Announcement



Assembled E-ELT mirror segments undergoing testing
Assembled E-ELT mirror segments undergoing testing