Manufacturing the ELT M5 blank mirror: a world of challenges

Le résumé fourni par la source

The Extremely Large Telescope (ELT), a 40-m class optical ground-based telescope led by the European Southern Observatory (ESO), will be the world’s largest optical telescope upon completion. Its adaptive optics system relies on two key mirrors: the M4, a 2-mm-thick deformable glass sheet actuated by 5,000 actuators, and the M5, a 2.2 × 2.7 m² flat, fast tip-tilt mirror made of silicon carbide (SiC). The M5 will be the largest tip-tilt mirror ever built for a telescope, designed to stabilize images by compensating for wind, atmospheric turbulence, and telescope-induced perturbations. It also directs the beam toward the Nasmyth platforms, where scientific instruments are housed. We will focus on the unique manufacturing challenges of the M5 blank mirror. Producing even a single petal required the largest raw SiC block ever made. But the main technical difficulty was combining chemical vapor deposition (CVD) of SiC - essential for optical performance - with brazing technology, necessary to achieve the mirror’s dimensions. This combination demanded innovative improvements and rigorous qualification processes. The development was a collaborative effort involving ESO, Safran Reosc, Mersen Gennevilliers, and Mersen Boostec, ensuring consistent polishing behavior across all petals. Finally, we will discuss upcoming developments for the second M5 blank mirror, the spare one, which will benefit from an improved SiC CVD layer and enhanced brazing material, both specifically qualified for this application.

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