On 2–3 December, the CTAO Central Organisation’s Telescope team visited the facilities of the Italian company Dal Ben, located in the Veneto region, to conduct the Test Readiness Review (TRR) for the electromechanical structure of the Small-Sized Telescope (SST). After the CTAO SST Collaboration completed the integration of the first SST telescope at the facility, which includes a mechanical and electrical model of the camera, this review represents a key milestone, determining the telescope’s readiness for the start of factory testing.
With the positive TRR outcome, the SST Collaboration team immediately proceeded to factory testing, which will run until the end of January 2026. At the end of the testing program a Pre-Shipment Review (PSR) will be held to clear the shipment of the first telescope to the CTAO-South site in Chile, which is expected between February and March 2026. If all goes as planned, the telescope will be integrated on site between late May and early June.
This accomplishment is the culmination of the SST Collaboration’s steady progress over the past year. After completing the telescope’s Critical Design Review in February 2025, which confirmed that the structure meets the technical and scientific requirements set by the Observatory, the serial production of 14 SST structures was initiated. This is the first 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. The approved Alpha Configuration foresees a total of 37 SSTs.
The CNRS (Centre National de la Recherche Scientifique) has assigned a contract for the construction of the remaining 12 SST structures to an industry group led by Dal Ben SpA, including its French subsidiary Dal Ben SAS. Activities started in September 2025, and telescope production will occur in two phases through early 2027: the first telescope structure, as a qualification model for the tender, then the additional eleven structures. Preparations for procurement and the manufacturing is under way, which will start after the current factory testing of the first SST.
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 the camera, which is being developed by an international team led by the MPIK (Max-Planck-Institut für Kernphysik), 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, the United Kingdom and the USA.
The production of the aforementioned first 14 telescopes are carried out with contribution of the Next Generation EU funds within the National Recovery and Resilience Plan (NRRP), 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;” Cup: C53C22000430006.