On 5 June, a delegation from the CTAO SST Collaboration and the CTAO Central Organisation’s Telescope team visited the facilities of the Italian company Dal Ben, located in the Veneto region, to inspect the progress of the Small-Sized Telescope (SST) electromechanical structures’ production. In a significant step forward, the SST structures have now officially entered serial production following the approval of their final design. 

The approval to enter the new phase came two months ago with the successful completion of the Critical Design Review (CDR), a rigorous evaluation confirming that the structure meets all technical and scientific requirements set by the Observatory. The review, carried out by a panel of CTAO experts and external professionals, took place in February 2025 and resulted in a broadly positive outcome, with only minor adjustments required. These have since been resolved through close cooperation between the SST Collaboration and the CTAO Central Organisation’s Telescope team, allowing the serial production phase to start. 

Now, production is underway for the first batch of 10 SST structures, as part of a contract awarded by INAF (Istituto Nazionale di Astrofisica) to the Dal Ben company for the construction of a total of 25 units. At least one is expected to be fully assembled later this year for testing at Dal Ben’s facilities. Meanwhile, the CNRS (Centre National de la Recherche Scientifique) has issued a call for bids to assign a second contract for the construction of an additional 12 SST structures, with the contract award expected in the coming months. According to the current schedule, the first SSTs are expected to be ready at the CTAO’s southern hemisphere array site, CTAO-South, in Chile by the middle of next year, where the approved Alpha Configuration foresees a total of 37 SSTs. 

The SSTs represent the smallest class of telescopes for the CTAO, optimised to detect the most energetic gamma rays in the Observatory’s energy range, from 5 to 300 TeV. Standing nine metres tall and weighing 17.5 tonnes, each SST employs a compact, dual-mirror Schwarzschild–Couder optical system. This design enables excellent spatial resolution across a wide field of view while maintaining a small focal ratio suitable for compact cameras. 

Each telescope includes a segmented 4.3-metre primary mirror, made up of 18 hexagonal elements, which reflects light onto a monolithic 1.8-metre secondary mirror. The Cherenkov light is then focused into a camera where it is digitised and processed. This dual-reflector system differentiates the SSTs from the Large-Sized Telescopes (LSTs) and Medium-Sized Telescopes (MSTs), which use single-mirror designs. 

The SST Collaboration is an In-Kind Contribution team for the CTAO, responsible for developing the SSTs. It comprises research institutions and universities from Australia, Brazil, France, Germany, Italy, Japan, the Netherlands, Switzerland, and the United Kingdom. 

The production of the aforementioned first 10 telescopes are carried out with contribution of 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”.