The Institute of Cosmos Sciences of the University of Barcelona (ICCUB) takes part in the development of the Cherenkov Telescope Array Observatory (CTAO) and in the conception of large-sized telescopes designed to explore the most extreme phenomena in the Universe.
On 15 October 2026, the sub-array of four Large-Sized Telescopes (LSTs) that will operate at the Cherenkov Telescope Array Observatory (CTAO) site on the island of La Palma (CTAO-North) will be inaugurated. This infrastructure is driven by the CTAO LST Collaboration, the international consortium responsible for designing, building and operating these large-sized telescopes, a next-generation facility aimed at advancing very-high-energy gamma-ray astronomy and exploring the most extreme and violent phenomena in the universe.
The Institute of Cosmos Sciences of the University of Barcelona (ICCUB) has participated in the development of CTAO and in the conception of these four large telescopes. Four of its researchers, Josep Maria Paredes, Marc Ribó, David Gascón and Pol Bordas, have been involved in documenting the process, from the initial design phase through technological development and the scientific goals of the project.
The inauguration of the four telescopes marks a fundamental milestone in the development of CTAO, as it completes the construction of all the large-sized telescopes planned for CTAO-North. The first telescope, LST-1, was inaugurated in 2018 and has since been undergoing commissioning under the responsibility of the LST Collaboration. It is now joined by three additional large-sized telescopes, which will eventually be accompanied by medium-sized telescopes (MSTs).
However, this work does not end with this milestone. The LSTs must undergo an extensive commissioning phase carried out by the LST Collaboration before being formally accepted by the CTAO Central Organization (CTAO ERIC) and beginning operations and the exploitation of so-called “early science”. LST-1 is expected to be the first telescope accepted, during 2027, bringing closer the start of the Observatory’s major scientific output.
The design and construction of these telescopes has been carried out by the LST Collaboration, which has acted as an in-kind contributor to CTAO ERIC and has complied with strict technical requirements.
Spain’s key role in the project
Spain plays a strategic and fundamental role in 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 observing time at these facilities and makes Spain an important hub for talent attraction and scientific production.
Under the name CTAO-Spain, the institutions in Spain dedicated to hardware, software and scientific activities for the Observatory are grouped together. These institutions, located in Andalusia, the Canary Islands, Catalonia and Madrid, are:
- Institute of Cosmos Sciences of the University of Barcelona (ICCUB)
- Institute of Space Studies of Catalonia (IEEC)
- Centre for Energy, Environmental and Technological Research (CIEMAT)
- Institute of Astrophysics of Andalusia (IAA-CSIC)
- Institute of Astrophysics of the Canary Islands (IAC)
- Institute of Space Sciences (ICE-CSIC)
- Institute for High Energy Physics (IFAE)
- University of Alcalá (UAH)
- Complutense University of Madrid (UCM)
- University of Jaén (UJA)
In addition to the scientific and technological contribution of Spanish teams within the LST Collaboration, the industrial sector also plays an important role. Spanish industry has manufactured key telescope components, including motion systems, camera mechanics and infrastructure.
The project timeline and Spanish participation
Professor Josep Maria Paredes (ICCUB-IEEC) traces the origins of the CTAO project back more than 20 years. The initial need was to overcome the sensitivity limitations of earlier instruments such as the H.E.S.S., MAGIC and VERITAS Cherenkov telescopes, as well as the earlier HEGRA experiment.
Paredes explains: “The scientific and technological experience acquired by Spanish groups through participation in these previous collaborations served as the basis for their current technical contribution to CTAO and, specifically, to the LST project. Hosting CTAO-North in the Canary Islands guarantees the Spanish scientific community 10% of the observing time, a factor that will enable the execution of its own projects and serve as motivation for new generations of researchers.”
From international collaborations to an open observatory
Professor Marc Ribó (ICCUB-IEEC) took part in the first CTAO science-case meeting in 2006 and has formally led ICCUB’s contribution to 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.”
Technological development: electronics and the data filtering system
David Gascón, director of the ICCUB Technology Unit and member of the IEEC, has been working in the LST engineering area since 2008. His work, together with that of his team, focuses on designing specific camera components, including three integrated circuits developed to measure photon signals at high speed.
In collaboration with other Spanish institutions, the team participated in the development of the data filtering system. This electronic system operates in real time, analysing millions of images per second to filter out spurious or background information (ambient light or other stars), and identify patterns corresponding to gamma rays. As Gascón notes, this electronics system is what makes it possible to isolate and identify signals of interest within fractions of a nanosecond.
Scientific goals: studying transient events
Researcher Pol Bordas (ICCUB-IEEC) explains how these telescopes operate. Bordas describes how the LSTs use the Earth’s atmosphere as part of the detector: when high-energy photons strike the atmosphere, they generate particle cascades that produce Cherenkov light, a very fast bluish light detected by the telescopes on the ground.
The LSTs, equipped with mirrors 23 metres in diameter, are designed to detect these emissions in the Observatory’s lowest energy range. This capability will be used to study transient phenomena, defined as rapid and not necessarily periodic events. Among the objects of study mentioned by the researcher are gamma-ray bursts (GRBs), fast radio bursts (FRBs), and the possible gamma-ray electromagnetic counterpart associated with gravitational waves.
“The future operation of the LSTs as part of CTAO will serve both to confirm current physical predictions and, potentially, to detect new astrophysical events not documented to date,” notes Bordas.
About CTAO
CTAO will be the world’s largest and most powerful gamma-ray astronomy observatory. Its unprecedented precision and broad energy range (from 20 GeV to 300 TeV) will help answer some of the most exciting questions in astrophysics, grouped into three main themes: understanding the origin and role of cosmic relativistic particles; studying extreme environments such as black holes and neutron stars; and exploring the frontiers of physics in the search for dark matter and deviations from Einstein’s theory of relativity. In addition, thanks to its enhanced performance, CTAO will play a key role in the coming decades in the fields of multi-messenger and multi-wavelength astronomy, providing crucial information about gamma rays in the most extreme environments.
To cover its broad energy range, CTAO will use three types of telescopes: Large-Sized Telescopes (LSTs), Medium-Sized Telescopes (MSTs) and Small-Sized Telescopes (SSTs). More than 60 telescopes will be distributed between two sites: CTAO-North, in the Northern Hemisphere, located at the Roque de los Muchachos Observatory of the Institute of Astrophysics of the Canary Islands (IAC) on La Palma; and CTAO-South, in the Southern Hemisphere, at the European Southern Observatory’s Paranal Observatory in Chile’s Atacama Desert. The CTAO headquarters are hosted by Italy’s National Institute for Astrophysics (INAF) in Bologna and by the Science Data Management Centre (SDMC), located on the campus of the German Electron Synchrotron (DESY) in Zeuthen.
CTAO is a big-data project. It will generate hundreds of petabytes (PB) of data per year, approximately 12 PB after compression. In line with its commitment to open science, CTAO will be the first ground-based gamma-ray observatory to operate as an open, proposal-driven observatory, providing public access to high-level scientific data and software products.
The CTAO Central Organization is responsible for constructing and operating the Observatory. It manages the four CTAO sites (the headquarters in Bologna, the SDMC in Zeuthen, and the two telescope arrays in La Palma and Atacama) and works closely with partner groups worldwide on Observatory development. Its main partners include the In-Kind Contribution Collaborations (IKCs), which are developing essential hardware and software, as well as the CTAO Consortium, an international group of researchers dedicated to the scientific exploitation of the Observatory. Among the IKCs is the CTAO LST Collaboration, responsible for developing the LSTs.
The members of CTAO ERIC are Austria, Croatia, Czech Republic, France, Germany, Italy, Poland, Slovenia, Spain, Switzerland and the European Southern Observatory. The Netherlands also participates in CTAO ERIC as an observer. Other countries, including Australia, Brazil, Japan, South Africa and the United States, are strategic partners or third parties.
About the LSTs
The LSTs are one of the three telescope types that CTAO will use to cover its broad energy range from 20 GeV to 300 TeV. When gamma rays interact with the Earth’s atmosphere, they generate particle cascades that produce Cherenkov light. Because lower-energy gamma rays generate only very small amounts of this light, telescopes with large collecting areas are required to detect it. The LST, with its 23-metre-diameter reflector, will provide CTAO’s unique sensitivity in the low-energy range (between 20 and 150 GeV).
Despite standing 45 metres high and weighing 100 tonnes, each LST can be repositioned in less than 20 seconds to point to any location in the sky. Both this rapid repositioning capability and the low energy threshold of the LSTs are essential for CTAO studies of galactic transient phenomena, high-redshift active galactic nuclei and gamma-ray bursts.
The CTAO LST Collaboration is responsible for designing and building these telescopes. It consists of more than 500 scientists and engineers from 25 institutions across 11 countries: Brazil, Bulgaria, Croatia, Czech Republic, France, Germany, Italy, Japan, Poland, Spain and Switzerland.
The LST Collaboration is making rapid progress at the CTAO-North site on La Palma. In 2018, the LST prototype, LST-1, was inaugurated and has been undergoing commissioning ever since. On 15 October 2026, the sub-array of four LSTs will be inaugurated in La Palma by the LST Collaboration, marking the completion of the construction of these telescopes at CTAO-North.