Paranal, Chile, 17 December 2025 — Representatives from the Cherenkov Telescope Array Observatory (CTAO), the European Southern Observatory (ESO), and governmental authorities gathered today to celebrate the official groundbreaking of the CTAO’s southern site, CTAO-South. After years of successful site preparations, the event marked the beginning of construction on the telescope foundations, paving the way for the first telescopes to be completed by the end of 2026. The CTAO will be the world’s largest and most powerful observatory for gamma-ray astronomy, and the first to be built in the Southern Hemisphere. With it, Chile will open a new observational window, exploring the Universe at the highest energies.
The ceremony began at ESO’s Paranal Observatory with opening remarks from Thomas Klein, ESO Director of La Silla Paranal Observatory, followed by speeches made by Stuart McMuldroch, CTAO Director General; Xavier Barcons, ESO Director General; Francisco Colomer, Chair of the CTAO ERIC Council, as well as political authorities, including 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 National Agency for Research and Development (ANID). The event also brought together international partners from the Chilean scientific community and industry, along with CTAO and ESO staff that joined the celebration of this major milestone in the project’s development.
During his remarks, McMuldroch expressed his excitement for this moment, a culmination of years of dedication and international collaboration. “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.”
“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 CTAO and ESO, but also for Chile as this new facility will strengthen the country’s position as a global hub for astronomy,” said Barcons in his speech.
Following the ceremony, participants moved to the CTAO-South site, located 10 kilometres southeast of Paranal in the Atacama Desert, for a symbolic onsite celebration. There, Volker Heinz, CTAO Construction Programme Manager, welcomed the attendees to the site and then representatives buried a time capsule containing items from Chile and partner countries around the world, symbolising how the work undertaken in Chile will contribute to scientific progress on a global scale. The capsule also included scientific items representing the ultimate goal of the telescopes now under construction: to advance our understanding of the Universe and expand human knowledge. A commemorative plaque, set upon nearby stones, now marks the location of the buried capsule, beside the future telescope area.
The CTAO, ESO, and Chile signed agreements to have the CTAO southern array hosted at ESO's Paranal Observatory in 2018. “Paranal is a unique place in the world to study the Universe,” highlighted Heinz during his welcoming speech at CTAO-South, explaining that Paranal is already home to ESO’s VLT — a key instrument in the discoveries recognised by the 2020 Nobel Prize in Physics — and the ESO’s Extremely Large Telescope that can be seen under construction in the distance from CTAO-South. "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.”
To cover its broad energy range — from 20 GeV to 300 TeV, billions of times more energetic than visible light — the CTAO will employ three types of telescopes: the Large-Sized Telescopes (LSTs), the Medium-Sized Telescopes (MSTs), and the Small-Sized Telescopes (SSTs). The current configuration for CTAO-South includes more than 50 telescopes. To give the CTAO a complete view of the night sky, it will have two arrays of telescopes located in both the northern and southern hemispheres. The CTAO-South site in Chile, in tandem with the CTAO-North site in La Palma, Spain, will revolutionize our view of the high-energy Universe.
With its unprecedented sensitivity and precision, the CTAO will help address some of the most fundamental questions in astrophysics. Its research will focus on three key areas: 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 addition, the CTAO will play a key role in multi-wavelength and multi-messenger astronomy in the coming decades, providing essential gamma-ray data to complement observations across the entire electromagnetic spectrum and from other cosmic messengers such as neutrinos and gravitational waves.
The CTAO is also a Big Data project, expected to generate hundreds of petabytes of data each year (around 12 PB after compression). Committed to the principles of Open Science, it will be the first gamma-ray observatory to operate as an open, proposal-driven facility, providing public access to its high-level scientific data and software products. Ten per cent of the observing time at CTAO-South is reserved for Chilean scientists, guaranteeing that Chile, as host country, gains direct scientific returns and strengthens its own research capacities through the Observatory’s presence.
As construction begins in the Atacama Desert, today’s ceremony symbolises not only a technological milestone but also a shared commitment from Chile and the international community to push the boundaries of what we know about the most energetic and mysterious corners of the Cosmos.
About the CTAO
The CTAO ERIC (also known as CTAO Central Organisation) is in charge of the construction and operation of the CTAO. Thus, it is made up of the groups and people dedicated to the management and administration of the Observatory’s development and the overall project, science, computing and systems engineering activities. The high-level organisational structure is broken into five main groups: Director’s Office, On-Site Construction, Project Office, Project Science Office, and Administration Office. The Central Organisation is responsible for managing the CTAO’s four sites: the Headquarters hosted by the Istituto Nazionale di Astrofisica (INAF) in Bologna (Italy), the Science Data Management Centre hosted by the Deutsches Elektronen-Synchrotron DESY in Zeuthen (Germany) and the two telescope arrays, CTAO-North at the Instituto de Astrofísica de Canarias’ (IAC’s) Roque de los Muchachos Observatory on La Palma (Spain), and CTAO-South, at the ESO’s Paranal Observatory in the Atacama Desert (Chile).
This group works in close cooperation with partners from around the world toward the development of the Observatory. Major partners include In-Kind Contribution Collaborations that are developing essential hardware and software, in addition to the CTAO Consortium, an international group of researchers who works on the scientific exploitation of the Observatory.
The CTAO ERIC members include Austria, Croatia, the Czech Republic, the European Southern Observatory (ESO), France, Germany, Italy, Poland, Slovenia, Spain, and Switzerland. Further countries — Australia, Brazil, Japan, South Africa, and the United States — are engaged in the process of joining the CTAO ERIC as Strategic Partners or Third Parties.
About ESO
The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO’s headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal, ESO will host and operate the south array of the Cherenkov Telescope Array Observatory, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building “the world’s biggest eye on the sky” — ESO’s Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society.
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