Construction begins on ESO’s gamma-ray observatory in Chile

31 Dec 2025

Cherenkov Telescope Array Observatory to be world’s largest and provide new insights into high-energy Universe.

A groundbreaking ceremony for the CTAO’s southern array facility took place last week 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, designed to provide new insights into the high-energy Universe. Its southern array will be the first gamma-ray observatory to be built in Chile.

ESO Director General Xavier Barcons commented, “We are happy to welcome this innovative facility to ESO’s family and to see the start of construction of the southern site of this powerful observatory at Paranal. 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.”

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. Participants 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; and Alejandra Pizarro, Director of the Chilean National Agency for Research and Development.

Time capsule

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 goals for the telescopes now under construction.

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 — 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.

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.

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.