Construction begins on the first and largest gamma-ray observatory in Chile

Republished from ESO, the European Southern Observatory, where it first appeared as a press release on 2025-12-18. ESO press-release texts and images are released under the Creative Commons Attribution 4.0 International licence; the text is reproduced here unchanged apart from formatting. Credit: ESO. Read the original for the latest version and any updates.

Yesterday, a groundbreaking ceremony for the CTAO’s southern array facility took place at the European Southern Observatory’s (ESO’s) Paranal site in Chile, marking the beginning of construction of the telescope foundations. The CTAO, or Cherenkov Telescope Array Observatory, will be the world’s largest and most powerful gamma-ray observatory, providing new insights into the high-energy Universe. Its southern array will be the first gamma-ray observatory to be built in Chile.

“We are happy to welcome this innovative facility to ESO’s family. It’s our pleasure to see the start of construction of the southern site of this powerful observatory here at Paranal in Chile’s Atacama Desert — a place with the most pristine skies on Earth. This groundbreaking is a huge milestone for both the CTAO and ESO, but also for Chile as this new facility will strengthen the country’s position as a global hub for astronomy,” said ESO Director General Xavier Barcons in his welcoming speech during the groundbreaking ceremony.

The CTAO is an international project, in which ESO is a founding partner and host to its southern array; several ESO Member States are also involved in the project and scientific teams around the world, including in Chile, are preparing to observe with this facility in the coming years. The start of the construction of the foundations, work being led by a consortium of Chilean companies, paves the way for the first telescopes to be deployed at Paranal by the end of 2026.

For the celebration, representatives of CTAO, ESO, the Chilean government and local authorities gathered at ESO’s Paranal Observatory. In addition to ESO’s Director General, participants at the ceremony included Andreas Kaufer, ESO Director of Operations; Thomas Klein, ESO Director of La Silla Paranal Observatory; Volker Heinz, CTAO Construction Programme Manager; Stuart McMuldroch, CTAO Director General; Francisco Colomer, Chair of the CTAO ERIC Council; Ricardo Díaz, Governor of the Antofagasta Region; Valeska Molina, Regional Secretary of the Ministry of Science, Technology and Innovation for Antofagasta Region; and Alejandra Pizarro, Director of the Chilean National Agency for Research and Development (ANID). Following a welcome and speeches, a time capsule was buried next to the future array area. It was filled with elements from Chile and CTAO partners, as well as scientific items, representing good wishes and goals for the telescopes now under construction.

“Thanks to the commitment of our partners from around the world and the support of ESO as our hosts here in Chile, we are now turning a vision into reality as construction begins on what will be the most advanced gamma-ray observatory on Earth,” said McMuldroch.

“We are proud to host the CTAO’s southern array and operate it right here at ESO’s Paranal Observatory, together with ESO’s Very Large Telescope and ESO’s Extremely Large Telescope,” said Klein. “This revolutionary facility will transform our understanding of the Universe, opening a new window onto the most energetic phenomena in the cosmos”.

The CTAO is designed to detect very high-energy gamma-rays emitted by the most violent and powerful events in the Universe. It will comprise over 60 telescopes at two sites: CTAO-South and CTAO-North [1] — one in each hemisphere — with a total collection area of over 1 million square metres. The southern site alone will have more than 50 telescopes, designed to capture a broad energy range — from 20 GeV to 300 TeV, billions of times more energetic than visible light [2].

The CTAO will detect high-energy radiation with unprecedented accuracy and precision, far outstripping current gamma-ray telescopes. When an energetic gamma photon hits Earth’s atmosphere, it produces a cascade of particles that cause the emission of what is known as Cherenkov radiation — a characteristic faint blue visible-light flash. This flash lasts only a few billionths of a second so it must be imaged with super-fast and sensitive cameras, with telescopes of enormous light-gathering power operating under pristine dark skies.

By pinpointing the sources of these gamma-rays, CTAO will provide deeper insights than ever before into the most extreme events and objects in our Universe, focusing on key areas like: understanding the origin and role of relativistic cosmic particles; probing extreme environments such as black holes and neutron stars; and exploring the frontiers of physics by searching for dark matter and testing the limits of Einstein’s theory of relativity.

In 2018 the CTAO, ESO and Chilean authorities signed agreements to have the CTAO southern array hosted at ESO’s Paranal Observatory, less than ten kilometres southeast of the location of ESO’s Very Large Telescope. This region in the Chilean Atacama Desert provides the clearest and darkest skies of any astronomical observatory on Earth, thanks to Chile’s geography and the country’s commitment to the preservation of its night skies.

“By building such a powerful and important facility in this area, ESO and CTAO hold deep trust that Chilean authorities will safeguard this extraordinary site for generations to come, and secure the huge value that astronomical facilities like CTAO generate locally and globally,” says Barcons.

“Paranal is a unique place in the world to study the Universe,” highlighted Heinz. "The Atacama Desert now welcomes another world-leading facility, and, in just one year, we expect to have here CTAO telescopes providing the first-ever observations of the gamma-ray sky from Chile.”

Notes

[1] The northern hemisphere site is located at the Instituto Astrofísica de Canarias Observatorio del Roque de los Muchachos on the island of La Palma, Spain.

[2] GeV and TeV stand for giga-electronvolts and tera-electronvolts, respectively. For comparison, visible light has an energy of just a couple of electron-volts.