What is LST?

The Large-Sized Telescopes (LSTs) are the largest telescopes in the future Cherenkov Telescope Array Observatory (CTAO). They are designed to detect very high-energy gamma rays, an extremely energetic form of radiation that originates from the most distant and violent corners of the universe. Thanks to this capability, the LSTs enable the study of cosmic phenomena that are difficult to observe using other instruments.

In technical terms, the LSTs cover an energy range of approximately 20 GeV to 3 TeV, placing them at the low-energy end of the CTAO spectrum. This range is key to detecting extragalactic sources at cosmological distances — such as active galactic nuclei — as well as galactic sources with lower acceleration capabilities, such as pulsars. Furthermore, their rapid response time makes them essential tools for studying transient phenomena such as gamma-ray bursts or other short-lived stellar events.

Scientific objectives

LSTs help to answer some of the big questions in astrophysics and fundamental physics, addressing problems that link the universe on a large scale with the most basic laws of nature.

In scientific terms, their main objectives are:

• The origin of cosmic rays: to study how and where the most energetic particles in the universe are accelerated, and how they interact with the interstellar and intergalactic medium.
• Impact on the constituents of the Universe: to analyse the role of gamma rays in the evolution of galaxies, magnetic fields and background radiation.
• Fundamental physics and beyond the Standard Model: to investigate processes that could reveal new particles or interactions not described by the Standard Model of particle physics.
• Search for dark matter: detecting possible indirect signals of annihilation or decay of dark matter particles.
• Effects of quantum gravity: exploring possible deviations in the propagation of light at extreme energies that could indicate a quantum structure of space-time.

Dimensions and design: large yet agile

At first glance, LSTs are striking for their size: they stand around 45 metres tall, with a mirror 23 metres in diameter and a weight of nearly 100 tonnes. This large size is no accident, but rather a necessity for capturing the faint Cherenkov light generated by low-energy gamma rays as they interact with the Earth’s atmosphere.

However, despite their size, LSTs are designed to be extremely light and agile. Their structure uses advanced materials such as carbon fibre reinforcement, which reduces weight without compromising stability. This lightness is essential for the telescope to reposition itself quickly: it is capable of pointing at any point in the sky in less than 20 seconds, a crucial feature for observing short-lived transient events.

Optics and light collection

At the heart of the LST is its large parabolic reflector, 23 metres in diameter, with a reflective surface area of approximately 400 m². This mirror collects the faint Cherenkov light produced in the atmosphere and focuses it onto the telescope’s camera, using a technique known as the Cherenkov effect.

From an optical point of view, the choice of a parabolic geometry minimises aberrations and improves the quality of the collected signal. The system’s focal length is 28 metres, which optimises angular resolution and efficiency in detecting low-energy events. As these events produce only a few dozen photons, maximising the collecting area is essential for their detection.

The camera: speed and sensitivity

The LST’s camera is one of its most advanced components. Designed to be compact and lightweight (weighing less than two tonnes), it features 1,855 light sensors based on photomultiplier tubes (PMTs), capable of detecting even single photons with great efficiency.

Technically speaking, the camera covers a field of view of approximately 4.3 degrees and is optimised to operate in very low-signal conditions. Its high readout speed allows it to distinguish the extremely brief flashes of Cherenkov light — which last barely a few nanoseconds — from the background noise of the night. This capability is key to correctly identifying events produced by low-energy gamma rays.

Mechanical system and repositioning capability

To achieve its extraordinary speed of movement, the LST incorporates a mechanical system based on circular rails 23 metres in diameter and six bogies (wheeled carriages). This design allows the telescope to be rotated with great precision and speed.

From a technical perspective, one of the main challenges has been to balance lightness with structural strength. The combination of advanced materials and an optimised design allows the telescope to maintain its stability even in adverse conditions such as wind, without compromising its rapid repositioning capability.

Thanks to this combination, the LST is capable of repositioning itself and pointing at any location in the sky in less than 20 seconds, enabling a rapid response to external alerts to observe short-lived transient phenomena such as gamma-ray bursts (GRBs) or to search for the counterparts of gravitational waves, amongst other things.

Stereoscopic observation and scientific output

LSTs do not operate in isolation. CTAO-North, located at the Roque de los Muchachos Observatory on the island of La Palma (Spain), will have four LST telescopes that will work together using the stereoscopic observation technique. This means that several telescopes simultaneously observe the same event, allowing for a more precise reconstruction of the properties of the atmospheric cascade generated by the gamma-ray burst.

This technique significantly improves angular resolution, energy determination and the ability to distinguish between gamma-ray signals and the charged-particle background. As a result, the LSTs are essential for extending the scientific reach of the CTAO to fainter and more distant sources.

The LST Collaboration

The development of the LSTs is the result of a broad international collaboration. More than 500 scientists and engineers from 85 institutions in 11 countries, including Spain, Germany, Japan, Italy, France and Brazil, are involved in this project.

From an organisational perspective, this multidisciplinary collaboration brings together experts in astrophysics, engineering, electronics and data analysis, enabling the technological and scientific challenges associated with the detection of very high-energy gamma rays to be addressed. For more information on the LST collaboration, visit their website.

Project status

The LST prototype, LST-1, was inaugurated in October 2018 and is currently in the commissioning phase, already producing scientific results. To date, the collaboration has published numerous studies on galactic and extragalactic sources, as well as transient phenomena.

As for the full deployment, the mechanical components and mirrors of the four telescopes were completed in 2025. The LST-4 camera was also integrated that same year, followed by the LST-3 camera the following year. The installation of the final camera, for LST-2, is scheduled for June 2026, with the inauguration of the complete array planned for October of that same year.