La Palma, Spain, 15 October 2026 – Two hundred and fifty scientists and representatives from leading research institutions around the world celebrated the inauguration of the Large-Sized Telescope (LST) sub-array at the CTAO-North site at the Roque de los Muchachos Observatory in La Palma, Spain. The completion of the four telescopes marks a significant milestone for the global astrophysics community and represents a crucial step toward the full scientific operation of the Cherenkov Telescope Array Observatory (CTAO).

Each telescope features a 23-meter-diameter reflector and is equipped with state-of-the-art ultra-fast cameras. Designed to detect the faint Cherenkov light flashes when very high-energy gamma rays interact with the Earth’s atmosphere, the LSTs are the most sensitive instruments in the world for observing gamma rays at energies between approximately 20 gigaelectronvolts (GeV) and several teraelectronvolts (TeV).

This exceptional sensitivity is crucial for detecting distant and transient gamma-ray sources. As very high-energy gamma rays are increasingly absorbed as they travel across the Universe, the LSTs’ low-energy threshold enables scientists to probe sources at cosmological distances, to look further back in cosmic history, and to see the very short variability of dynamically evolving gamma-ray sources with greater statistical accuracy.

A productive pioneer: the LST-1

Even during its commissioning phase, the Large-Sized Telescope prototype (LST-1) has produced groundbreaking results since 2020. These include observations of pulsars, studies of active galactic nuclei at cosmological distances, and rapid follow-up observations of transient phenomena such as novae and gamma-ray bursts.

With the three new LSTs (LST-2, LST-3, and LST-4) joining the existing LST-1, the four telescopes will become a true array. By observing the same gamma-ray showers simultaneously from multiple directions, the sub-array will provide substantially improved sensitivity, angular resolution, and energy reconstruction, opening up a new level of precision in the study of the gamma-ray Universe.

“The completion of the LST sub-array is the culmination of more than fifteen years of international collaboration and technological innovation,” said Prof. Masahiro Teshima, the Principal Investigator of the LST Collaboration and Spokesperson of the CTAO Consortium. “These telescopes will allow us to explore some of the most extreme environments in the Universe, including supermassive black holes, neutron stars, the possible origins of cosmic rays, and the nature of dark matter.”

Reaching out to the early Universe

The LSTs form the low-energy core of CTAO-North and will provide a unique opportunity to probe the Universe at an epoch roughly 12 billion years ago, when it was less than two billion years old. Their sensitivity to the lowest-energy gamma rays will enable the detection and study of bright VHE gamma-ray bursts at high redshift, potentially extending gamma-ray observations to sources at redshifts of around z∼4, and providing new insights into the early Universe.

Over the coming years, the LSTs will undergo commissioning and integration into CTAO operations. Alongside the other CTAO telescopes, planned for both of its arrays, they will address fundamental questions in astrophysics, particle physics, and cosmology, such as the nature of dark matter, the origin of cosmic rays, and the physics of extreme cosmic accelerators.