Four Giant Telescopes Just Switched On, and They Can See 12 Billion Years Into the Past

On October 15, 2026, on a volcanic ridge high above the clouds in La Palma, Spain, some 250 scientists gathered to celebrate the completion of four enormous telescopes that do not quite do what telescopes are supposed to do. They never look at stars. They never take pictures of galaxies. Instead, for a few billionths of a second at a time, they catch a faint blue flash in Earth's own atmosphere: the dying whisper of the most violent events in the universe.

The telescopes are the Large-Sized Telescopes, or LSTs, of the Cherenkov Telescope Array Observatory (CTAO). Four of them, each with a 23-meter reflector, now form the low-energy core of what will become the world's largest and most powerful gamma-ray observatory. The inauguration marks the end of more than 15 years of international collaboration, and the beginning of a new era in astronomy.

Catching light that isn't there

Gamma rays are the highest-energy form of light, born in the most extreme places imaginable: the edges of supermassive black holes, the surfaces of spinning neutron stars, the detonation of entire stars. But they never reach the ground. When a very high-energy gamma ray strikes Earth's atmosphere, it triggers a cascade of secondary particles moving faster than light travels through air, which emit a brief blue glow called Cherenkov light. The whole event lasts nanoseconds.

The LSTs are built to catch exactly that flash. Each 23-meter reflector funnels the faint blue glow onto an ultra-fast camera, and software reconstructs the direction and energy of the original gamma ray. It is an absurdly difficult thing to do, which is why the LSTs are the most sensitive instruments in the world in their energy band: gamma rays between roughly 20 gigaelectronvolts and several teraelectronvolts.

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. (Prof. Masahiro Teshima, LST Collaboration)

Why the low end matters

The 20 GeV threshold is the whole point. Space-based gamma-ray telescopes top out around that energy, while older ground-based instruments started higher. The LSTs bridge the gap, covering the transition between the two. That matters because the highest-energy gamma rays get absorbed as they travel across the cosmos: they collide with background starlight and annihilate before reaching us. The lower the energy an observatory can reach, the further back it can see.

Because of their low threshold, the LSTs can probe sources at cosmological distances, potentially out to a redshift of around z ~4. That is light emitted when the universe was less than two billion years old, roughly 12 billion years ago, from an era when the first galaxies were still assembling. The universe is foggy for high-energy gamma rays, and the LSTs push the boundary of visibility further back than any ground-based gamma-ray instrument before them.

Built for speed

Despite weighing 100 tonnes each and standing 45 meters tall, every LST can swing to any point in the sky in less than 20 seconds. That is not an engineering flex. Gamma-ray bursts, the brightest explosions in the universe, can flare and fade in seconds. The LSTs exist to catch them in the act, and to chase possible gamma-ray counterparts to gravitational-wave events and fast radio bursts.

The prototype proved the concept works. LST-1 has been operating since 2020, even during commissioning: it has observed pulsars, studied active galactic nuclei at cosmological distances, and performed rapid follow-ups on novae and gamma-ray bursts. In October 2026, LST-1 worked together with the MAGIC telescope to catch gamma rays from the distant blazar OP 313, light that had traveled for about eight billion years before hitting La Palma.

One flash, four eyes

Until now, LST-1 worked alone. The three new telescopes, LST-2, LST-3, and LST-4, turn it into a true array. By observing the same atmospheric shower simultaneously from multiple directions, the four telescopes together deliver substantially improved sensitivity, angular resolution, and energy reconstruction. Four eyes on the same flash see far more than one.

The science targets are the open questions of modern astrophysics. Where do cosmic rays, the mysterious high-energy particles constantly raining down on Earth, actually come from? What is dark matter, and does it leave a gamma-ray fingerprint when it annihilates? What powers the jets of supermassive black holes, which accelerate particles to energies no human machine can match? The answers, if they come, will come from the extreme universe. This is the machine built to look at it.

A 25-country machine that runs on petabytes

The LSTs were developed under Japanese leadership within an international collaboration spanning roughly 25 countries, with institutional partners in Brazil, Bulgaria, the Czech Republic, Croatia, France, Germany, Italy, Japan, Poland, Spain, and Switzerland. The collaboration pushed forward optics, photodetectors, mechanics, and high-speed electronics to reach the lowest energy threshold ever achieved by a ground-based gamma-ray observatory.

The scale is hard to overstate. CTAO is a Big Data project: the full observatory will generate hundreds of petabytes of raw data per year, about 12 petabytes after compression. And it will be the first gamma-ray observatory of its kind run as an open, proposal-driven facility, with high-level science data and software products released publicly. Anyone with a good idea and a good proposal can point the most sensitive gamma-ray eyes on Earth at their target.

The full CTAO plan calls for more than 60 telescopes of three sizes (large, medium, and small), split between CTAO-North on La Palma and CTAO-South at the Paranal Observatory in Chile's Atacama Desert, covering energies from 20 GeV to 300 TeV. The LST sub-array is the low-energy core of the northern site, and its inauguration is the crucial step toward full scientific operation.

What happens next

The telescopes are built, but the science has not started in earnest. Over the coming years the LSTs will go through commissioning and integration into CTAO operations, with formal multi-telescope observations targeted around 2027. LST-1 is expected to be the first telescope formally accepted by the observatory's central organization that year.

The timing matters for another reason. Astronomy is entering the multi-messenger era: gravitational-wave detectors catch the ripples of colliding neutron stars, neutrino observatories catch particles from cosmic accelerators, and gamma-ray telescopes catch the light. The LSTs, with their 20-second slew, are designed to be the rapid-response arm of that network. When gravitational-wave observatories detect a merger, the LSTs can be looking at the right patch of sky before the afterglow fades.

When the array is fully online, it will watch for the briefest, most energetic signals in nature. Every gamma-ray burst it catches, every distant blazar it resolves, every non-detection that tightens the bounds on dark matter, sharpens the map of the extreme universe. The telescopes on La Palma do not take pretty pictures. They take something better: measurements of the most violent physics the cosmos has to offer.