The ELT M2 mirror in metrology

At 4.25 metres across, the Extremely Large Telescope’s (ELT's) secondary mirror, M2, will be the largest optical secondary mirror ever used on a telescope. Here, the Safran Reosc team in France is measuring the accuracy of the M2’s surface; it needs to be accurate to a few nanometres, about 20 000 times thinner than a human hair.