The LST-4 is one of the three telescopes currently under construction at the Roque de los Muchachos Observatory, La Palma (Photo: Alice Donini)

The LST-4 is one of the three telescopes currently under construction at the Roque de los Muchachos Observatory, La Palma (Photo: Alice Donini)

Under construction: New Cherenkov telescopes on La Palma

The LST-1 is getting company. The prototype of the Large-Sized Telescope (LST) was previously the sole representative of this type of telescope at the Roque de los Muchachos Observatory on La Palma. Construction of the three remaining LST-2, LST-3 and LST-4 began in 2023. The builder is the LST collaboration within the CTA Observatory, in which the Max Planck Institute for Physics (MPP) is significantly involved. On 3 May, the project reached an important milestone: the mirror dish of the LST-4 was successfully placed on the lower structure.

During this complicated process, the 18-tonne mirror dish was first hoisted to a height of 18 meters above the ground. A 200-tonne crane then lifted it safely into position. Thanks to the perfect weather and the experience of the companies involved, everything went smoothly and the work was completed within a few hours.

The elegant mechanical design of the telescope is the result of a collaboration between MPP, LAPP Annecy, IFAE Barcelona and several industrial partners. IAC, the operator of the Roque de los Muchachos observatory on La Palma, is in charge of the construction and installation.

Parallel to LST-4, the consortium is also pushing ahead with the construction of LST-2 and LST-3 as well as LST-4. The construction and assembly of the upcoming LSTs are on schedule and are expected to be completed in early 2026. LST-1 was inaugurated in 2018, has completed its commissioning and is already producing scientific results.

Easily manoeuvrable heavyweights

The mirror diameter of the telescopes is an impressive 23 meters, with a reflective surface area of 400 square meters. The telescopes measure light that objects such as supernovae or black holes emit as gamma radiation. The radiation is absorbed by the earth's atmosphere and reaches the earth's surface as so-called Cherenkov light. The mirrors collect and focus the light onto the camera, where ultra-modern photomultiplier tubes convert it into electrical signals. Thanks to the large mirror surface, the LSTs can also measure and process low-energy light.

Despite their substantial size - the telescopes are 45 meters high and weigh around 110 tonnes - the LSTs are easy to manoeuvre. Tests carried out on the LST-1 in February 2024 show that the instrument can be aligned to any point in the sky in less than 20 seconds. This makes the LST ideal for measuring transient gamma ray signals in the low-energy range.

Parallel to the observatory on the Canary Island of La Palma, the CTAO is continuing work at its site in the southern hemisphere in Paranal, Chile. Two further LSTs are being built there, financed by the Italian Resilience and Recovery Plan (PNRR).