Our group participates in instrumentation development and observations for the next-generation very-high-energy gamma-ray observatory, the Cherenkov Telescope Array Observatory (CTAO). CTAO observes gamma rays over a wide energy range using telescopes with large, medium, and small mirror diameters. Among them, the 5-300 TeV energy band is covered by the 4 m Small-Sized Telescopes (SSTs). A major science goal of the SSTs is the discovery of yet-unidentified “PeVatrons,” astrophysical sources that accelerate Galactic cosmic rays up to the PeV range.
We have been developing the focal-plane camera for this SST for more than ten years. In February 2025, together with international collaborators, we assembled a prototype close to the final camera design at Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg, Germany (Fig. 1). In July 2025, we transported this focal-plane camera to the Teide Observatory on Tenerife, Spain, and installed it on an SST optical system that had already been constructed at the site (Figs. 2 and 3).
During about one week in July, we carried out test observations of cosmic-ray air showers. As shown in Fig. 4, we successfully detected atmospheric Cherenkov emission induced by high-energy cosmic rays, as well as characteristic Cherenkov-ring images emitted by cosmic-ray muons passing near the telescope.
The SST project is now entering mass production of about 40 telescopes. Starting in 2026, they are expected to be installed sequentially at the CTAO site in Paranal, Chile, with full scientific operations anticipated in the latter half of the 2020s.