Four 23-metre Cherenkov telescopes now stand complete at the Roque de los Muchachos Observatory on La Palma, Spain, roughly 2,300 metres above the Atlantic. Some 250 scientists and officials attended a ceremony today to mark the finish of the Large-Sized Telescope (LST) sub-array, the low-energy core of the future Cherenkov Telescope Array Observatory (CTAO).

The instruments detect the faint flashes of Cherenkov light that very high-energy gamma rays produce when they strike the Earth's atmosphere. According to the LST collaboration, the telescopes are the most sensitive instruments in the world for observing gamma rays at energies between approximately 20 GeV and 3 TeV.

The first telescope, LST-1, was built in 2020. It has already delivered 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. The three new telescopes now join it, and the four will observe the same gamma-ray air showers simultaneously from multiple directions. The array's designers expect this multiplicity to deliver improved sensitivity, angular resolution and energy reconstruction compared with a single dish.

What does the new sensitivity buy?

The array's reach into the lowest-energy gamma rays carries the most scientifically consequential claim in the announcement. The collaboration projects that it will enable astronomers to detect and study bright gamma-ray bursts at high redshift, potentially extending gamma-ray observations to sources at redshifts of around z~4 and opening a window onto the early universe. That figure is a projection grounded in instrument capability, not a measured result; the commissioning phase over the coming years will test it.

"The completion of the LST sub-array is the culmination of more than fifteen years of international collaboration and technological innovation," said Masahiro Teshima, principal investigator of the LST collaboration and a Japanese astrophysicist who also serves as 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 and the nature of dark matter."

Where does the array sit in CTAO's portfolio?

The LSTs form one of three telescope classes within CTAO, which the announcement says will become the world's most powerful observatory for gamma-ray astronomy when it begins operations in 2028, covering 20 GeV to 300 TeV:

• Six LST telescopes, sensitive from roughly 20 GeV to 3 TeV
• 23 Medium-Sized Telescopes (MST), operating between 80 GeV and 50 TeV
• 37 Small-Sized Telescopes (SST), working between 2 TeV and 300 TeV

The full set of 66 telescopes will be split between two sites: CTAO-North at La Palma and CTAO-South at the Paranal Observatory in the Atacama Desert in Chile. The LSTs anchor the low-energy capability of the northern site.

Over the coming years, the four completed instruments will undergo commissioning and integration into wider CTAO operations — the step that will determine whether the promised gains in sensitivity and energy reconstruction materialise across the full array.