On Friday, 16 October, the European Southern Observatory (ESO) and the Cherenkov Telescope Array Observatory ERIC (CTAO ERIC) signed an updated hosting agreement for the CTAO’s southern hemisphere array, CTAO-South, located at ESO’s Paranal Observatory in the Atacama Desert, Chile. The document, signed by CTAO Director General Stuart McMuldroch and ESO Director General Andreas Kaufer, formalises the continuation of the partnership with the definitive CTAO ERIC legal entity and under its framework. 

The signing took place on La Palma, Spain, following a CTAO ERIC Council meeting held on the island to coincide with the inauguration of the Large-Sized Telescope (LST) sub-array. The ceremony marked Andreas Kaufer’s first official engagement with the CTAO as the new Director General of ESO, following his appointment on 1 September 2026, during which he also participated as the head of ESO’s delegation in the Council meeting. 

The document updates the original 2018 agreement, which was signed by the CTAO’s interim legal entity. By concluding the agreement directly between ESO and the CTAO ERIC, the two organisations secure their collaboration under the Observatory’s permanent legal structure. The updated text further reflects the evolution of the project and its operational model, as well as the long-term vision for the Observatory. 

“It is a pleasure to see CTAO-South taking shape and its infrastructure being built,” said Andreas Kaufer, ESO Director General. “Eight years after the first hosting agreement, this update reflects all the developments and new frameworks in place to ensure a successful project in the southern hemisphere. I would like to thank Stuart and his team for their collaboration on this agreement; we are now looking forward to its implementation.”  

“Observatories are magical places,” added Stuart McMuldroch, CTAO Director General. “By signing this hosting agreement, we expand our ability to study the Universe, and we deeply appreciate ESO’s continued support in making this a reality.”  

The ESO-CTAO Partnership 

The CTAO ERIC, established in January 2025, is in charge of the construction and operation of the Observatory. In addition to ESO, its members include Austria, Croatia, the Czech Republic, France, Germany, Italy, Poland, Slovenia, Spain, and Switzerland. The Netherlands is also part of the CTAO ERIC as an Observer. 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. 

ESO is not only a founding member of the CTAO ERIC, but it also serves as a hosting partner for the CTAO-South site. Additionally, the CTAO ERIC, ESO, and the Chilean government—represented by the National Agency for Research and Development (ANID)—collaborate through the CTAO-South Coordination Committee. This body advises on and monitors the construction and operation of the southern array. 

Progress at CTAO-South 

Located at ESO’s Paranal Observatory in Chile’s Atacama Desert, CTAO-South is one of the CTAO ERIC’s four facilities. The southern array will cover three square kilometres and will consist of 37 Small-Sized Telescopes (SSTs) designed to detect the highest-energy gamma rays, 14 Medium-Sized Telescopes (MSTs) to observe the core energy range. Under an upcoming, enhanced configuration, two Large-Sized Telescopes (LSTs) will also join the array to observe at low energies.  

Led by the Central Organisation’s CTAO-South team, the site is rapidly taking shape. Key preparation work has already been completed, including the construction of 17 kilometres of on-site roads, and the foundations for both the SSTs and MSTs. Together, these milestones ensure the site is ready for the arrival of its first telescope: an SST that was bid farewell in Italy in mid-September before beginning its journey to CTAO-South.