ELT site Chile: Land of extremes

Covering more than 756,000 square kilometers, Chile stretches almost the entire length of the West coast of South America and is often referred to as an “elongated” country. After several years of research and comparative studies at various potential locations, the European Southern Observatory (ESO) has decided to erect the world’s largest optical telescope here. “Cerro Armazones” is the name of the barren site, ideally suited for astronomy, in the north of the country where construction work for the ELT (Extremely Large Telescope) began in June 2014.
Flattened mountaintop of Cerro Armazones - ELT site Chile
The flattened mountaintop of Cerro Armazones is seen from the air. The peak will host ESO's 39-metre Extremely Large Telescope (ELT), the world's biggest eye on the sky.
Why Chile?

It’s no simple matter finding a site large enough for a telescope roughly the size of the Colosseum in Rome. Add to that the need for optimal conditions for astronomical observation, including the astronomical quality of the atmosphere, the number of clear nights per year, the atmospheric water vapor content and the stability of the air above the observation site. Economic factors such as construction and operating costs also play a role, as does the creation of synergies by proximity to existing observatories.

Optimal conditions for observation are almost always found only in high and dry regions. Experts have been examining numerous sites in Argentina, Chile, Morocco and Spain for years, conducting comparative studies on air and weather conditions in their search for a suitable location.

The Cerro Armazones mountain

Next to La Palma in Spain, high elevation sites in Chile were identified as most appropriate: Armazones, Ventarrones, Tolonchar and Vizcachas made the shortlist. They all offer superb conditions for astronomical observations. The ESO’s selection board ultimately decided on the Armazones location because of its balance of favorable conditions.

The 3,060-meter-high mountain is located in the central part of the Chilean Atacama Desert. It is about 130 kilometers south of the city of Antofagasta and about 20 kilometers away from the Cerro Paranal, where the ESO’s Very Large Telescope (VLT) is located. Similar to that mountain, the Cerro Armazones also exhibits ideal observational conditions for astronomical research, especially its more than 320 clear nights per year.
Desert construction work

In support of the ELT project and its construction of the 39-meter telescope, the Chilean government secured a vast area of about 189 square kilometers around Cerro Armazones for the ESO. Since it is directly adjacent to the VLT site, both observatories enjoy long-term protection from direct, negative influences such as mining activities and light pollution.

Construction of the ELT on Armazones is currently underway. The construction of an access road and the leveling of the summit have already been completed. Next is the construction of the dome which will soon house mirror substrates made of ZERODUR® glass-ceramic.
Artist's impression of the Extremely Large Telescope (ELT) on Cerro Armazones
Artist's impression of the Extremely Large Telescope (ELT) on Cerro Armazones, a 3060-metre mountaintop in Chile's Atacama Desert. The design for the E-ELT shown here was published in 2011 and is preliminary.