Extremely Large Telescope, Ground telescope in Cerro Armazones, Chile

The Extremely Large Telescope is a ground telescope on Cerro Armazones in Chile and consists of hundreds of hexagonal mirror segments that together form an enormous reflective surface. The facility sits high in the Atacama Desert and includes a rotating dome and several instruments designed to observe infrared light.

The European Southern Observatory began construction in 2014 on Cerro Armazones because of the stable atmospheric conditions there. The facility was planned to push the limits of optical and infrared astronomy.

The name was shortened from European Extremely Large Telescope to Extremely Large Telescope in 2017, reflecting the growing number of international partners and the Chilean site. The facility is run by the European Southern Observatory, and its 798 hexagonal mirror segments are produced by Schott AG in Mainz, one per day at full capacity.

The high elevation provides clear views but visitors should prepare for thin air and extreme temperature swings. Access to the facility is typically restricted and organized tours are usually required.

The dome's two halves retract sideways to open, and the telescope inside uses five mirrors rather than the usual two or three. Its 798 hexagonal primary segments, each 1.45 m across and only 5 cm thick, are made of the glass-ceramic Zerodur, produced at a rate of about one segment per day.