One month to the inauguration of the four Large-Sized Telescopes at the CTAO observatory in La Palma

On 15 October 2026, the LST Collaboration of the Cherenkov Telescope Array Observatory (CTAO) will celebrate the inauguration of the sub-array of four Large-Sized Telescopes (LSTs) that will operate at the CTAO-North site, located in La Palma, in the Canary Islands. These new infrastructures are driven by the CTAO LST Collaboration, an international consortium responsible for the design, construction, and operation of the observatory’s large telescopes—a state-of-the-art facility intended to boost very high-energy gamma-ray astronomy and explore the most extreme phenomena in the universe.

The Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) participates in the development of the CTAO and in the design of the Large-Sized Telescopes. Four of its researchers—Josep Maria Paredes, Marc Ribó, David Gascón and Pol Bordas—all of them from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB), have been involved in documenting the process, from the initial design phase to technological development and the scientific goals of the project. IEEC researchers at the Institute of Space Sciences (ICE-CSIC) and the Universitat Autònoma de Barcelona (UAB) are also participating in the CTAO.

The inauguration of the four telescopes marks a key milestone for the development of the CTAO observatory, as it signifies the completion of the construction of all LSTs planned for this site. The first of them, the LST-1, was inaugurated in 2018 and has since been undergoing commissioning under the responsibility of the LST Collaboration. Three new LSTs now join it, which will eventually be accompanied by the Medium-Sized Telescopes (MSTs).

However, the work does not end with this milestone. The LSTs must undergo a thorough commissioning phase by the LST Collaboration before being formally accepted by the CTAO Central Organisation (CTAO ERIC) and begenning operations and the exploitation of so-called “early science”. LST-1 is expected to be the first to be accepted during 2027, bringing closer the start of the Observatory’s major scientific output. The design and construction of these telescopes have been carried out by the LST Collaboration, acting as an in-kind contributor to CTAO ERIC and complying with strict technical requirements.

Spain plays a strategic and fundamental role in the CTAO. Together with Japan, it is the country contributing the most to the development of the LSTs, accounting for 32% of the project. This, together with its role as host country of CTAO-North, gives the Spanish scientific community privileged access to observation time at these facilities, generating an important hub for attracting talent and scientific production in the country.

Under the name CTAO-Spain, the group of institutions dedicated to hardware, software, and science development activities for the observatory are brought together. These groups, distributed across Andalusia, the Canary Islands, Catalonia, and Madrid, include (in alphabetical order):

• Centre for Energy, Environmental and Technological Research (CIEMAT)
• Complutense University of Madrid (UCM)
• Institute of Astrophysics of Andalusia (IAA-CSIC)
• Institute of Cosmos Sciences of the University of Barcelona (ICCUB)
• Institute of Space Sciences (ICE-CSIC)
• Institute of Space Studies of Catalonia (IEEC)
• Institute for High Energy Physics (IFAE)
• Instituto de Astrofísica de Canarias (IAC)
• University of Alcalá (UAH)
• University of Jaén (UJA)

In addition to this scientific-technological contribution from the aforementioned entities within the LST Collaboration, industry plays a critical part. Spanish companies have manufactured essential components of the telescopes, such as drive systems, camera mechanics, and infrastructure.

Discover the CTAO

Marc Ribó (ICCUB-IEEC) took part in the first CTAO science-case meeting in 2006 and has formally led ICCUB’s contribution to the CTAO since 2010. CTAO represents a paradigm shift because, unlike previous instruments operating in the same energy range, it will function as an observatory open to the scientific community. 

Ribó explains: “Previous Cherenkov telescopes have been used within the framework of large international collaborations that operated in a relatively closed manner with respect to the wider scientific community. The large number of scientific cases that CTAO will enable, as well as the major financial investment required, have led CTAO to be conceived as an observatory open to the international scientific community. While some observations of strategic scientific targets will be carried out by the CTAO Consortium, there will also be numerous projects led by independent teams that will likely address scientific questions not previously explored.”

David Gascón, director of the Technology Unit at ICCUB and member of the IEEC, has worked in the engineering area of the LST since 2008. His work, along with that of his team, has focused on designing specific components for the telescope cameras, including three integrated circuits developed to measure high-speed photon signals.

In collaboration with other Spanish institutions, the team participated in developing the trigger system. This electronic system operates in real time, analysing millions of images per second to filter out background noise or spurious information (such as ambient light or light from other stars) and search for patterns corresponding to gamma rays. As Gascón explains, these electronics are what allow signals of interest to be isolated and identified in fractions of a nanosecond.

Researcher Pol Bordas (ICCUB-IEEC) who has co-led the scientific programme of LST-1 over the last two years, explains how these telescopes operate. Bordas describes how the LSTs use the Earth’s atmosphere as part of the detector: when high-energy photons hit the atmosphere, they generate particle cascades that produce Cherenkov light—a very fast, bluish light captured by telescopes on the ground.

The LSTs, equipped with 23-metre-diameter mirrors, are designed to capture these emissions at the Observatory’s lowest energy range. This capability will be dedicated to studying transient phenomena, defined as fast and not necessarily periodic events. Among the study targets mentioned by the researcher are: gamma-ray bursts (GRBs), fast radio bursts (FRBs), and the potential electromagnetic gamma-ray counterpart associated with gravitational waves.

“The future operational start of the LSTs as part of the CTAO will serve both to confirm current physical predictions and to potentially detect new astrophysical events undocumented to date,” Bordas concludes.