Real-time advanced ultrasonic imaging of assemblies of glass and SiC mirrors of the extremely large telescope

The Extremely Large Telescope of the European Southern Observatory (ESO) is currently under construction and is expected to become the world’s largest optical/near-infrared terrestrial astronomical observatory. Its optical design incorporates five mirrors of varying shapes, sizes, and materials. The primary mirror, with a diameter of 39 m, is perhaps the most remarkable. The fifth mirror (M5), made of SiC, ranks among the largest tip-tilt mirrors globally. Various structural joining techniques, such as bonding and brazing, are applied to these optical components to enable nanometric relative positioning between the assembled parts. This study reviews advanced real-time ultrasonic imaging based on Total Focusing Methods (TFM) for their inspection. For the M1 mirror, TFM is utilized in a tandem configuration for regular industrial use and is also applied once with Rayleigh waves to inspect the surface and subsurface of the M5 mirror.