The ELT's M5 mirror blank is finalised

15 May 2024

M5, the fifth mirror on the light path of ESO’s Extremely Large Telescope (ELT), has reached an important milestone: its blank, the shaped piece of material that is later polished to become the mirror, has just been finalised. The ELT will be equipped with five mirrors in total, and M5 is the smallest of them — but building its blank was no small feat. The French company Mersen Boostec manufactured this remarkable piece in their facilities near Tarbes, in France’s southwest. 

M5 is a flat, elliptical mirror measuring 2.7 by 2.2 metres, constructed from six segments brazed together. It might not seem much in comparison to the ELT’s enormous 39-metre primary mirror, but in reality, it is the largest tip-tilt mirror in the world.

Together with M4, M5 is a crucial component of the ELT’s adaptive optics system: their unique synergy will allow the ELT to take extremely sharp images, compensating for perturbations caused by the telescope mechanisms, wind vibrations, and atmospheric turbulence. To stabilise the images, M5 will adjust its position 10 times per second, without bending. 

This challenge required Mersen Boostec to make the M5 blank out of a very special material: silicon carbide, which is simultaneously very stiff and very lightweight. On top, a thin layer of silicon carbide was deposited atom by atom, to ensure the surface of the blank can be polished to an accuracy of less than a hundredth of the thickness of a human hair.   

The French company Safran Reosc, which has now received the blank, will inspect it and later integrate it with its support, polish the assembled blank, and supply the equipment required for its handling, transport, operation, and maintenance. The Spanish company SENER Aeroespacial is carrying out the design, construction, and verification of the mirror cell (its support system), as well as its control system and auxiliary equipment.  

ESO’s Extremely Large Telescope, the world’s biggest eye on the sky, is poised to tackle the most profound astronomical challenges of our time, promising groundbreaking discoveries once it sees first light later this decade. The construction of its individual components, like the M5, is pushing technology to the limit. 



Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Tel: +49 89 3200 6670

About the Announcement



A person bending over a large, flat, oval structure, using a bright light to inspect it very closely. A schematic drawing on a piece of paper close to the person shows the structure’s shape; pens and other tools rest on the paper.
The M5 blank being inspected at Mersen Boostec
A 3D rendering of an oval mirror standing on top of a mechanical structure, in front of a black background and illuminated by a blue light from below. The mirror reflects the ceiling of a white room with construction equipment and bright blue lights.
ELT's M5 mirror (artist's impression)


The smallest ELT mirror is a big challenge | ELT Updates
The smallest ELT mirror is a big challenge | ELT Updates
Artist’s animation showing the tip-tilt motion of the M5 mirror
Artist’s animation showing the tip-tilt motion of the M5 mirror
ELT M5 blank video compilation
ELT M5 blank video compilation