Until recently, the prototype of the Large-Sized Telescope (LST), the LST-1, has stood alone on the CTAO’s northern hemisphere array site at the Instituto de Astrofísica de Canarias’ (IAC’s) Roque de los Muchachos Observatory in Villa de Garafía on La Palma, Spain. But, in 2023, the CTAO LST Collaboration began laying the foundations and constructing the three remaining LSTs planned for the site – the LST-2, LST-3 and LST-4. It was on 3 May that an important milestone was achieved when the 18-tonne mirror dish structure of the LST-4 was successfully lifted onto its lower structure.

This intricate operation involved raising the dish to a height of 18 metres above the ground, where it was securely positioned with the assistance of a 200-tonne crane. The installation process presented its challenges, particularly as the large structure must be lifted and installed in a single procedure. However, thanks to the perfect weather conditions and the expertise of contractor MERO-TSK and subcontractor CaSanaTec, the operation was smoothly executed within a few hours. The telescope’s elegant mechanical design is a product of a cooperation between MPP Munich, LAPP Annecy, IFAE Barcelona and several industrial partners. The construction and installation contract is being executed largely by the IAC, together with the LST Collaboration members.

Left: The LST-4 dish structure is installed. Credit: Alice Donini

Concurrently, construction is progressing for the LST-2 and LST-3, alongside the LST-4, with more key milestones planned for each telescope. Construction and assembly of the upcoming LSTs remain on schedule with an expected completion by early 2026.

The LSTs are procured and constructed by the LST Collaboration for the CTAO. The LST-1 was inaugurated in 2018 and is completing its commissioning and is already demonstrating scientific results. The LSTs have a reflective surface of 400 m2 that collects and focuses the Cherenkov light into the camera, where 1855 state-of-the-art photomultiplier tubes convert the light into electrical signals. Thanks to the large reflective surface, they have an unrivalled collection area that improves sensitivity at the lowest energies. This leads to a significant improvement in sensitivity over the previous generation of IACTs for energies below 50 GeV. Despite their substantial size, weighing approximately 110 tonnes and standing 45 metres tall, the LSTs boast impressive manoeuvrability. Recent tests conducted with the LST-1 in February by MPP, IFAE, and LAPP confirm their capability to swiftly position to any point in the sky in less than 30 seconds, facilitating the capture of transient, low-energy gamma-ray signals.

In addition to the four LSTs on the CTAO’s northern site, an enhancement plan funded by the Italian Resilience and Recovery Plan (PNRR), which was proposed by INAF and INFN, includes the implementation of two LSTs for the CTAO’s southern hemisphere array in Paranal, Chile. More news from the LST Collaboration and the progress of both sites are still to come, so stay tuned for further updates. 

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Northern Hemisphere Array Begins to Take Shape with Installation of LST-4 Dish Structure - CTAO