ELT M5 manufacturing and testing status

The ELT M5 mirror, measuring 2.7m×2.2m, will be the world's largest tip-tilt mirror and one of the largest silicon carbide with chemical vapor deposition (SiC-CVD) brazed mirror —while also meeting high optical quality requirements. Serving as the folding flat mirror in the ELT telescope, its purpose is to compensate for wind vibrations, atmospheric turbulence, and telescope mechanisms, utilizing its dedicated fast tip-tilt function.

The manufacturing of the blank mirror, composed of six brazed segments, was completed in June 2024. Subsequently, pads and mechanical interfaces were bonded and integrated onto the mirror. Optical fabrication and testing have been ongoing since January 2025.

While manufacturing a flat mirror might appear straightforward, M5 presents unique and significant challenges due to its large size, reduced edge margins, complex thickness management, reproducibility of SiC-CVD layer deposition, susceptibility to bending and quilting effects, the presence of brazed joints, and stringent wavefront error (WFE) specifications. These factors combine to make both polishing and metrology highly demanding.

To address these challenges, we have implemented an integrated approach combining conventional and innovative tools and processes, aimed at ensuring efficient and safe production within a reduced timeline. A dedicated polishing station with optimized tools and processes has been established, along with a specialized WFE test bench that enables rapid and automatic measurements to nanometer-level precision. Moreover, in-situ metrology tools have been deployed to monitor the mirror at several intermediate stages, further enhancing the efficiency of correction steps.