On 15 April, the CTAO LST Collaboration successfully completed the installation of all 198 mirror facets on LST-3, one of the Large-Sized Telescopes (LSTs) currently under construction at CTAO-North on La Palma, Spain. The mirror installation process, which began on 31 March, is a meticulous and collaborative effort involving multiple research institutions and companies. Each mirror is installed individually, requiring careful alignment and coordination.
Mirrors are a critical component of Cherenkov telescopes, as they reflect the faint Cherenkov light produced when high-energy gamma rays interact with Earth’s atmosphere onto the telescope’s camera. In the case of the LSTs—the largest class of CTAO telescopes—198 hexagonal mirrors are required to cover its massive 400 m² reflective surface (the equivalent of around two tennis courts).
Each mirror facet is coated with a multi-layer material designed to maximise reflectivity while ensuring long-term durability. Mounted on a lightweight carbon-fiber structure, each mirror segment weighs just 50 kg. This balance between strength and weight allows the LST to maintain structural integrity while enabling rapid repositioning to capture brief, transient gamma-ray events.
The mirrors are positioned 28 metres from the camera, forming a 23-metre diameter parabolic dish. This design creates a so-called isochronous surface, meaning that the reflected light coming from any point of the large dish arrives at the camera simultaneously—an essential feature for accurately reconstructing the particle showers caused by gamma-ray interactions and determining their origin in the sky.
With the installation now complete, the LST Collaboration will proceed to equip each mirror with cables for precision actuators as part of the Active Mirror Control system. This system enables micron-level adjustments to each mirror, ensuring optimal pointing accuracy and observational performance.
The LST Collaboration will continue working on the LST-3, along with the other two LSTs currently under construction at the site, with the next major milestone being the installation of one of the cameras. All three telescopes are expected to be completed by spring next year, joining the LST-1, inaugurated in 2018 and currently under commissioning.
The companies and research groups from the LST Collaboration involved in the mirror installation are: CaSana (company and main work centre), Institute of Physics of the Czech Academy of Sciences (FZU), ICRR, University of Tokyo, Chiba University and shifters from INFN. For the cable work: FZU, Palacký University Olomouc, and MPP.