ESO ELT: Surfaces to discover the secrets of the universe!

The European Southern Observatory, ESO, is constructing the world’s largest telescope, named the Extremely Large Telescope (ELT), in Chile. The telescope will be completed in 2024 and is a unique technological and engineering task.

In order to achieve the highest level of performance the mirror coating must reflect the optimal amount of light. As metallic coatings degrade a little over time, a periodic maintenance is planned, where the mirror coating is stripped away and a new coating is applied.

The primary mirror is 39m in diameter and consists of 798 hexagonal segments with 133 individual segment geometries. Each segment is irreplacable and has a  surface that is polished within few nanometers precision (peak to valley). Additional to this, the mirror segments have delicate sensors and actuators on the back side, which are not to be harmed during a process of coating removal.

Together with Professor Per Møller from DTU, IPU has conducted a project for ESO, developing a chemical process and an automation concept to remove the mirror coating on the primary mirror of ELT. The project had to validate how the removal of the mirror coating affects the outermost atomic layers of the ceramic glass surface.

ELT segment prototype. Crucial for the success of the project, was that the chemical process should be integrated into an automation concept that allows a large number of segments to be processed. The mirror maintenance process must be carried out near the telescope’s location on Cerro Armazones in the Chilean desert, which is at approximately 3 km altitude. The remote location of this site added an additional dimension to the project, as resources are scarce, earthquakes occur relatively frequent and disposal of chemical waste is complicated. The project developed an automation concept that minimizes the risk of damage to mirror surfaces and to the instruments on the back of the segments. The automation concept and process chemistry were adapted to the difficult conditions in the desert and to the ultimate requirements for the safety of mirrors and operators.

The companies Polyteknik and Danish Fundamental Metrology have been subcontractors to IPU and have contributed to a successful project.

Read more about the ESO ELT project

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