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Large Synoptic Survey Telescope (LSST): Safran Reosc delivers the anti-reflection coating of world’s largest precision lens


World leader in high-performance optics, Safran Electronics & Defense subsidiary Safran Reosc has just successfully applied the anti-reflection coating for the three lenses of the refractive corrector on the LSST, the most powerful telescope in the world. It was a genuine technical achievement to work on the largest of the three lenses, as one this big had never been built before.

A world first! At the end of November 2018, the teams at the Safran Reosc Thin Films lab delivered the anti-reflection coating of the world's largest optics system, mounted on the LSST.  This high-power telescope will come online on a mountain in the North of Chile starting in 2022. From its location, it will photograph the whole sky several times a week, providing a high-cadence 3D readout of unrivalled precision.

True craftsmanship

To achieve a perfect image quality over the enormous focal plane of the LSST, this refractor system is formed by three lenses, including the largest ever built, measuring 1.57 m across.  Once the lenses were manufactured and polished, it was up to Safran Reosc to apply the anti-reflective coating. The challenge this represented was multi-faceted, especially in terms of the equipment required, as the vacuum chamber used - unique in Europe - usually only accepts optical components no bigger than 1.10 m in diameter. Several months of preparatory work were carried out by the Safran Reosc Thin Films team to validate the equipment used to move and secure the lens, adjust process parameters and rehearse each operation prior to starting the work.

Producing an anti-reflective coating is a very delicate operation, consisting in the application of several films of vitreous materials with specific optical properties. Each film is barely a few hundred nanometers thick (i.e. a thousand times thinner than a human hair). Not only must the application must be completed in a vacuum, at high temperature and uniformly along all the surface curves, it also requires conditions of cleanliness approaching those of surgery.  Due to the dimensions and weight of the lens (200 Kg), two days were required to heat it to the required temperature and two further days to allow it to cool down.

Unrivalled expertise

To achieve these exploits, the Safran Reosc specialists drew on skills deployed on similar projects, such as the reflective coating of large mirrors on several European satellites - Gaia the space surveyor, the recently launched Aeolus carrying a lidar, the Euclid space telescope hunting for dark matter (under construction), but also the lenses for the refractor on the CFHT (Canada-France-Hawaii Telescope) or other defense programs. Yet none of these previous projects came close to the feat required to build the LSST.

This successful outcome is the fruit of the audacity, professionalism and expertise of Safran Reosc teams. It represents not just a new benchmark in the array of achievements and innovations that have marked the company's history over 80 years or more, it has also enabled us to consolidate globally-renowned know-how for future projects in the fields of astronomy, space, lasers and defense.