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First segments of world’s largest telescope mirror have a shiny new surface

14 Marzo 2024

The construction of ESO’s Extremely Large Telescope (ELT) in the Chilean Atacama Desert has reached a new milestone: a team at ESO’s Paranal Observatory have added a reflective, shiny layer, as well as special sensors, to the first segments of the telescope’s primary mirror. This “coating” means the segments are now essentially ready to start observing the skies, once installed at the heart of the world’s largest optical telescope later this decade.

The 39-metre primary mirror of ESO’s ELT, known as M1, will be by far the largest mirror ever made for a telescope. Too large to be made from a single piece of glass, it will consist of 798 glass-ceramic hexagonal segments, each about five centimetres thick and 1.5 metres across. The mirror segments are manufactured in Europe in a multi-step, multinational process. The first 18 made their way across the ocean to Paranal earlier this year, and coating is the next step on their journey — a milestone that three segments have now reached.

Coating an M1 mirror segment is a complex process that takes about two hours. In addition to a reflective layer, which uses 1.7 grams of silver, the coating includes additional layers of nickel chromium and silicon nitride to improve adhesion to the mirror blank and protect the silver from tarnishing. Overall, the coating is around 120 nanometres thick, or about one thousand times thinner than a human hair.

To make sure that all segments can work together as a single mirror, they are equipped with sensors to detect misalignments. Besides adding a coating to the first M1 segments, ESO engineers have also installed these so-called edge sensors, two per side of each segment, and integrated the electronics and mechanical supports needed for them to work. Finally, they have performed inspections and health checks to ensure that the segments are ready to be installed on ESO’s ELT. The entire process takes place inside a ‘clean room’ — where the number of particles in the air is carefully controlled to avoid contamination — in the ELT Technical Facility in Paranal.

The coating and integration procedures will be repeated for all other M1 segments. Meticulously testing and documenting the process for the first three segments was therefore key to ensure the operation can be ramped-up once new segments arrive at Paranal.

Once the telescope is operational, the coating process will be repeated on each segment every 18 months, to ensure the best reflectivity and sensitivity. In practice, this means that two segments will need to be recoated every day for the entire lifetime of the telescope. To facilitate this with minimal disruption to the scientific observations, an extra 133 segments are being manufactured, in addition to the 798 needed for the mirror. 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.