World's Largest Convex Mirror Blank Ready for Final Touches
Despite being the ELT’s secondary mirror, M2 still has an impressive diameter of 4.25 meters — larger than the primary mirror of many astronomical telescopes operating today. Following sixteen months of precision manufacturing at SCHOTT, M2 will now receive its final touches — more precisely, fine polishing — at Safran Reosc. The French company will polish the mirror to a precision of 15 nanometres across its entire optical surface, and a final layer of reflective silver and a wafer-thin protective layer of silicon oxide will be applied by ESO at a coating facility at ESO’s Paranal Observatory in Chile.
M2 will be the largest ever secondary mirror employed on a telescope, as well as the largest convex mirror ever produced. Fabricating this highly convex, aspherical mirror is a considerable challenge — and the result will be a truly remarkable example of pioneering optical engineering. The secondary mirror and its support system — weighing 12 tonnes — will hang upside-down high above the 39-metre primary mirror.
See: https://www.eso.org/public/announcements/ann19003/