Four large telescopes will be inaugurated on La Palma on 15 October 2026, completing construction of all the large telescopes planned for CTAO-North and bringing the Cherenkov Telescope Array Observatory closer to its first scientific operations. LST-1 has been undergoing tests at the Observatorio del Roque de los Muchachos since its inauguration in 2018; three more telescopes are now joining it at the site.
The instruments are not yet ready for full scientific use. Before CTAO ERIC, the central organisation responsible for the observatory, can formally accept each telescope, the LST Collaboration must complete an extensive commissioning process. CSIC expects LST-1 to be the first accepted telescope, during 2027. That decision would open the way to CTAO's early science programme and broader scientific output.
The telescopes detect faint flashes produced when high-energy gamma rays collide with Earth's atmosphere. The collisions create particle cascades and extremely brief blue Cherenkov light. From the ground, the instruments capture that light and reconstruct events that cannot be observed directly in the same way.
Each LST has a 23-metre reflector, stands 45 metres high and weighs 100 tonnes. Despite its size, it can be repositioned in less than twenty seconds to point anywhere in the sky. That speed matters when tracking transient phenomena such as gamma-ray bursts and fast radio bursts, and when searching for possible gamma-ray counterparts to gravitational-wave events.
The project gives Spain a strong position within CTAO. Alongside Japan, Spain is the largest contributor to the LST programme, with a 32% share. Hosting CTAO-North at the Observatorio del Roque de los Muchachos on La Palma also guarantees the Spanish scientific community 10% of observing time. Researchers will have direct access to an instrument designed to operate as an open observatory, rather than a facility reserved for a single closed collaboration.
Spanish participation covers both research and engineering. The ICCUB contributed to CTAO's development and to the conception of the four LSTs. Josep Maria Paredes and Marc Ribó followed the project from its early design through technology development. David Gascón and Pol Bordas worked on scientific planning. Spanish companies also manufactured basic components, including movement systems and camera mechanics, along with infrastructure.
Gascón's team at the ICCUB and IEEC worked on camera components and developed three integrated circuits to measure photon signals at high speed. Spanish institutions also helped build the data-filtering system. It analyses millions of images per second in real time, removing background signals such as ambient light and other stars, while isolating patterns associated with gamma rays within fractions of a nanosecond.
The Spanish network includes institutions in Andalusia and the Canary Islands, as well as Catalonia and Madrid. Its members include ICCUB, IEEC, CIEMAT, IAA-CSIC, IAC, ICE-CSIC, IFAE, UAH, UCM and UJA. CTAO puts observing capacity directly within researchers' reach.
CTAO will combine LST, medium-sized and small-sized telescopes across La Palma and the Atacama Desert in Chile. La Palma will host the northern part of the planned observatory. The complete infrastructure is expected to include more than sixty instruments across CTAO-North and CTAO-South, covering energies from 20 GeV to 300 TeV.
The observatory is expected to generate hundreds of petabytes of data annually, or approximately 12 PB after compression. It will provide public access to high-level scientific data and software products.
Its scientific agenda includes cosmic relativistic particles and extreme environments around black holes and neutron stars. Researchers will also study dark matter and possible departures from Einstein's theory of relativity. A published CTAO scientific study describes the facility as the next generation of ground-based Cherenkov telescopes, with projected sensitivity up to an order of magnitude greater than that of current instruments.
In October 2026, LST-1 and the MAGIC telescope were used to observe the distant blazar OP 313. Its gamma-ray emission travelled for about eight billion years before reaching Earth and being detected. The observations show that LST-1 can contribute to research on extremely distant high-energy objects while it remains in commissioning. The full northern configuration has yet to complete formal acceptance.
