On 22 May, the CTAO LST Collaboration reached another major milestone with the successful installation of the camera for LST-4, one of the three Large-Sized Telescopes (LSTs) under construction at the CTAO-North site on La Palma, Spain. The camera installation represents the finalisation of the telescope’s construction and marks its transition to the commissioning phase. 

The camera installation was a complex and carefully coordinated procedure. After extensive preparation and alignment work, the nearly two-tonne camera was lifted by crane and secured onto the telescope structure with millimetre precision. The process involved teams from multiple institutions working in synchrony to ensure safety and protect the delicate instrumentation. 

The LST-4 camera features a wide field of view of 4,3 degrees and is composed of 1,855 photomultiplier tubes (PMTs). These highly sensitive light detectors convert the faint flashes of Cherenkov light—emitted when high-energy gamma rays interact with Earth’s atmosphere—into digital signals for scientific analysis. Each PMT is paired with a specially designed light guide that enhances efficiency by directing photons toward the detector. 

The camera’s internal electronics perform real-time signal analysis using advanced algorithms to identify the characteristic signatures of gamma-ray events. To capture the fleeting Cherenkov flashes associated with these, which last only a few nanoseconds or billionths of a second, the camera rapidly digitises and records the signals at gigahertz (GHz) sampling rates, processing up to a billion data points per second—orders of magnitude faster than conventional cameras. 

With the camera now in place, the LST-4 enters its commissioning phase, joining the LST-1, the prototype and the first telescope built on a CTAO site. During commissioning, the telescope will undergo rigorous testing to verify that it meets the CTAO’s scientific and technical requirements. In parallel, the LST Collaboration will continue the construction of the northern hemisphere site’s remaining two LSTs.

Congratulations to all the teams in the LST Collaboration involved in this milestone!