Grinding the ELT’s M3 mirror

The Extremely Large Telescope’s (ELT's) tertiary mirror, M3, is four metres in diameter and weighs in at a whopping three tonnes. Here, the M3 mirror is in the grinding phase at Safran Reosc in France, which is designed to improve the surface quality to an accuracy of a few micrometres. Following this stage, the mirror will be polished to an accuracy of a few nanometres.