The moment of truth

In December 2023, the first 18 segments of ESO’s Extremely Large Telescope (ELT) main mirror embarked on a whirlwind 10 000 km journey from Europe to the Chilean Atacama Desert. This Picture of the Week captures the moment of truth when ESO engineers at the ELT Technical Facility unveiled one of the segments to ensure it had arrived safely at ESO’s Paranal Observatory.

Due to the sheer size and fragility of the mirror segments —each mirror is five centimetres thick, 1.5 metres wide and shaped to nanometre accuracy— they were each shipped in a private, protective cocoon. Each box was equipped with humidity, temperature and shock sensors to track the internal conditions during the long journey. When loaded into the transport vehicles, the boxes were cushioned by air bags. Upon arrival at the technical facility, the segments were carefully inspected to ensure they hadn’t been damaged during delivery.

With the first segments safely stored at Paranal, the ELT is coming ever closer to its first light. More than 100 segments have already left the production line and are being checked by ESO engineers before they can be shipped to Chile. With a total 798 mirror segments comprising an over 39-metre main mirror, the ELT is a marvel of both engineering and astronomy. It will push the limits of our observable Universe leading to yet unfathomable discoveries.