From 22 to 29 July, the CTAO SST Collaboration conducted a successful test campaign on the new camera prototype design for the Small-Sized Telescope (SST). The camera prototype was shipped to Tenerife, Spain, and integrated into the ASTRI-2 telescope, a pathfinder for the SST, for testing under real observational conditions. With a quarter of its full array of pixels installed, the camera recorded its first signals from the night sky just hours after installation, marking an important technical step in the SST’s ongoing development. 

When high-energy gamma rays interact with the Earth’s atmosphere, they produce a brief, faint flash of bluish Cherenkov light, lasting only a few nanoseconds (billionths of a second). Though invisible to the naked eye, this light can be captured by the ultra-fast, sensitive cameras of the CTAO telescopes. Depending on the telescope type, the CTAO uses different kinds of cameras, but all of them are composed of hundreds of detectors, or pixels. 

For the SST, the smallest of the CTAO’s three telescope types, the camera consists of 2048 silicon photomultipliers (SiPMs) that convert Cherenkov light into digital signals, enabling scientists to analyse and study gamma rays. As a complex and critical component, the camera must be rigorously tested at various stages of its development. 

In July, the SST Collaboration conducted a comprehensive test campaign on the new SST camera prototype design, known as QCAMi. To evaluate its performance, the camera, equipped with a quarter of the total 2048 SiPMs, was installed on the ASTRI-2 telescope in Tenerife, in a collaborative effort with the ASTRI Mini-Array project. ASTRI was designed not only as a stand-alone experiment, but also as a pathfinder for the SST. 

Within hours of installation, the prototype camera successfully recorded signals from the night sky, including muon rings and cosmic-ray showers moving across the camera in the nanosecond timescale. These tests are part of the planned prototyping phase, and the excellent results further validate the expectations of strong performance from the final camera. Left: The left panel shows a Cherenkov cascade detected by the QCAMi, while the right panel shows a muon ring. Credit: SST Collaboration

The SSTs are optimised to detect the most energetic gamma rays within CTAO’s range, from 5 to 300 TeV, and will be installed at CTAO-South, the Observatory’s southern hemisphere array site. Standing nine metres tall and weighing 17,5 tonnes, each SST features a compact dual-mirror Schwarzschild–Couder optical system. This system includes a segmented 4,3-metre primary mirror made up of 18 hexagonal elements, which reflects light onto a 1,8-metre monolithic secondary mirror. This dual-mirror design provides excellent spatial resolution across a wide field of view while maintaining a compact focal length ideal for lightweight cameras.  

The SST Collaboration is an In-Kind Contribution team for the CTAO, responsible for developing the SSTs. It is coordinated by the INAF institute from Italy and comprises research institutions and universities from Australia, Brazil, France, Germany, Italy, Japan, the Netherlands, Switzerland, and UK. The SST cameras will be built by a team of institutes from Germany, the Netherlands, Japan, UK, and Australia, led by the Max-Planck-Institute für Kernphysik in Heidelberg, Germany. 

The Italian participation in this campaign was supported by the Next Generation EU funds within the National Recovery and Resilience Plan (PNRR), Mission 4 – Education and Research, Component 2 – From Research to Business (M4C2), Investment Line 3.1 – Strengthening and creation of Research Infrastructures, Project IR0000012 – “CTA+ – Cherenkov Telescope Array Plus.”