Announcement

The ELT’s M5 mirror cell is now complete

4 September 2024

The unique five-mirror design of ESO’s Extremely Large Telescope (ELT) has been driving innovation in optics and engineering throughout its inception, and today is no exception: the cell of the ELT’s fifth mirror, M5, has been completed. M5 is the largest tip-tilt mirror in the world, and its impressive support cell will be able to steer the light gathered by the telescope up to 10 times per second to stabilise the images collected by the ELT.

The M5 cell is a support mechanism that will be fixed to the ELT, uniting the M5 mirror to the larger ELT framework. It was designed, constructed, and tested by the Spanish company Sener at their facilities near Barcelona, Spain; Sener are also responsible for the corresponding control system and auxiliary equipment. ESO’s Director General Xavier Barcons and ESO’s ELT Programme Manager Roberto Tamai visited the company's premises today to celebrate the provisional acceptance of the M5 cell.

The challenge achieved by Sener was to develop a mechanism able to steer the M5 mirror at high speed and very high accuracy. They succeed in this by mounting a specific actuator system — produced by Cedrat Technologies in France — onto a very stiff structure, as well as by coupling very fast and accurate position sensors with a powerful, fast digital processor to achieve an extremely precise control of the mirror position.

At 2.7 x 2.2 metres, M5 is the smallest of the five mirrors on the ELT, but is still the largest tip-tilt mirror ever constructed for a telescope. The cell’s tip-tilt mechanism will adjust the angle of the mirror to within a few tens of milli-arcsecond accuracy up to 10 times per second, all without bending the mirror. The miniscule movements will compensate for perturbations from wind, atmospheric turbulence, or the telescope mechanisms, which affect image quality. When partnered with M4, the fourth mirror on the ELT’s light path, this forms an adaptive optics system that stabilises images, allowing the ELT to take the sharpest, highest-resolution images seen yet by an optical telescope.

While the M5 cell has been finalised, the M5 mirror blank has been completed by the French company Mersen Boostec and is now being polished by Safran Reosc, also based in France. Both mirror and cell will eventually be shipped to Chile, to join the structure of the “world’s largest eye on the sky”, currently under construction in Chile’s Atacama Desert.

ESO’s ELT is expected to see first light later this decade, and is poised to take on some of the biggest scientific challenges of our time, probing the furthest reaches of the cosmos and tracking down Earth-like planets around other stars.