Four 23-meter telescopes on La Palma mark the completion of construction for CTAO-North\u2019s Large-Sized Telescope sub-array.Their large mirrors and rapid response are designed to capture faint atmospheric flashes from energetic cosmic sources.Commissioning and formal acceptance remain ahead, while the first telescope has already produced scientific results.<\/p>\n
A brief flash high above a mountain can carry evidence of an explosion billions of years ago. Catching that faint signal requires enormous mirrors, fast cameras and the ability to turn toward a changing sky. Four telescopes on La Palma are bringing that combination to a new stage of development.<\/p>\n
The milestone completes construction of the four instruments, but does not mean the entire observatory has begun full operations. Technical testing, commissioning and formal acceptance remain necessary. Three new telescopes join LST-1, a prototype that has already demonstrated its scientific value during commissioning.<\/p>\n<\/p>\n
Reading gamma rays through the atmosphere<\/p>\n
The telescopes do not capture cosmic gamma rays directly at ground level. When a high-energy gamma ray enters Earth\u2019s atmosphere, it creates a cascade of secondary particles. Those particles produce a faint flash of Cherenkov light that the mirrors collect and focus onto a camera.<\/p>\n
Researchers use those images to infer the original gamma ray\u2019s<\/a> direction and energy. The atmosphere therefore becomes part of the detection system, rather than simply an obstacle between the telescope and space. The cameras must record a fleeting optical signal instead of making a conventional photograph of a distant object.<\/p>\n Lower-energy gamma rays generate less Cherenkov light, making large collecting areas particularly important. The LSTs are designed to give CTAO sensitivity from approximately 20 gigaelectronvolts to around 3 teraelectronvolts. In that context, \u201clow energy\u201d means the lower end of ground-based gamma-ray astronomy, rather than weak radiation by everyday standards.<\/p>\n Each parabolic reflector provides roughly 400 square meters of reflecting surface. Its camera uses 1,855 photomultiplier tubes to convert arriving light into electrical signals. Together, the optics and electronics allow the instruments to investigate sources too faint or short-lived for less sensitive observations.<\/p>\n Four viewpoints sharpen the picture<\/p>\n LST-1 has operated as a single telescope while the other three instruments were being built. Adding LST-2, LST-3 and LST-4 changes how researchers can observe an atmospheric particle shower<\/a>. Several telescopes can record the same event from different positions on the ground.<\/p>\n When gamma rays from the universe enter Earth’s atmosphere, they produce particle showers. These trigger very brief flashes of blue light that last only fractions of a second. The CTAO telescopes can detect this Cherenkov radiation. (CREDIT: Pekic \/ CTAO) <\/p>\n Combining those views helps reconstruct where the shower developed and which direction its initiating gamma ray came from. It also improves estimates of energy and the separation of gamma-ray events from unwanted backgrounds. The collaboration expects substantially better sensitivity and angular resolution than a single instrument can provide.<\/p>\n Speed matters alongside precision. Each telescope<\/a> stands about 45 meters tall and weighs approximately 100 tonnes, yet can reposition across the sky within 20 seconds. That agility helps it respond when another instrument announces a sudden cosmic event.<\/p>\n Gamma-ray bursts are particularly demanding targets because their brightness can change rapidly. Delayed observations may miss the most informative phase. Rapid movement and a low energy threshold therefore address two different problems: getting to the source quickly and collecting its faintest accessible gamma-ray signal.<\/p>\n The prototype has already delivered results<\/p>\n The first LST was inaugurated in 2018 and began astronomical observations during commissioning in 2019. Its published performance assessment used observations of the Crab Nebula<\/a> and its pulsar, familiar reference sources in gamma-ray astronomy. The work showed that the prototype reached its expected performance under the conditions studied.<\/p>\n That assessment also illustrates why an instrument\u2019s advertised energy threshold needs context. Researchers estimated a threshold around 20 GeV when events triggered the instrument, rising to approximately 30 GeV after analysis. Useful scientific performance depends on event selection and observing conditions, alongside the telescope\u2019s design.<\/p>\n The CTAO will cover a very wide energy range and provide excellent angular resolution and sensitivity in comparison to any existing gamma-ray detector. With its ability to detect energies between 20 GeV and 300 TeV and its unprecedented resolution, the CTAO will be able to observe further than ever before and push the CTAO to the edge of the known electromagnetic spectrum, providing a completely new view of the sky. (CREDIT: CTAO) <\/p>\n More distant targets have demonstrated its reach. LST-1 detected very-high-energy emission from the quasar OP 313 during a bright flare in December 2023. Subsequent work, published in Astronomy & Astrophysics<\/a> in 2026, combined observations with the nearby MAGIC telescopes to investigate the source and its emission.<\/p>\n OP 313 has a redshift of 0.997, and the detected radiation traveled for roughly eight billion years. Those observations provide a demonstrated example of the prototype\u2019s ability to investigate distant activity. They also offer a foundation for the more ambitious observations planned with the complete LST sub-array.<\/p>\n Why a lower threshold opens a deeper view<\/p>\n Distance creates a particular difficulty for gamma-ray astronomy. High-energy gamma rays can interact with background light between galaxies and be absorbed before reaching Earth. The effect increasingly limits which distant sources remain observable at very high energies.<\/p>\n Access to lower energies helps astronomers work around that barrier. The LST collaboration hopes to detect sufficiently bright gamma-ray bursts at redshifts<\/a> around four. Such sources would probe an era roughly 12 billion years ago, when the universe was less than two billion years old.<\/p>\n That reach is a scientific goal, rather than a guaranteed distance limit or a completed observation. Success will depend on a burst\u2019s brightness, spectrum, timing and visibility from La Palma. It would nevertheless extend studies of energetic explosions into a much earlier period of cosmic history.<\/p>\n The LST sub-array at CTAO-North at the \u201cRoque de los Muchachos\u201d Observatory on La Palma. (CREDIT: Beedron Record \/ CTAO\/LST) <\/p>\n Other targets include supermassive black holes, neutron stars and environments that accelerate cosmic particles. The wider observatory will also search for possible signals associated with dark matter. Those searches test hypotheses about unseen matter rather than assuming that a detectable gamma-ray signature must exist.<\/p>\n Commissioning comes before routine observations<\/p>\n The next stage is to verify the instruments and integrate them into CTAO operations. The observatory\u2019s central organization must formally accept the telescopes after the required technical work. Their inauguration marks progress toward that process rather than its completion.<\/p>\n