The Medium-Sized Telescope (MST) is a key element of the Cherenkov Telescope Array Observatory (CTAO), a global project to improve our understanding of high-energy gamma rays from space. The CTAO consists of a network of telescopes designed to detect cosmic gamma rays. The MST plays a key role in CTAO, covering the mid energy range, specifically between 150 GeV and 5 TeV (giga-electronvolts to tera-electronvolts). Gamma rays in this energy range are relatively more frequent and easier to capture than the lower and higher end of the spectrum. This is one of the reasons for using the MSTs at both observatory sites. The initial observatory layout, designated as the alpha configuration, comprises 14 MSTs in the southern hemisphere (located at Paranal, Chile) and 9 in the northern hemisphere (located at La Palma, Spain).

The MST is an Imaging Atmospheric Cherenkov Telescope based on a modified Davies-Cotton optics. It has a reflector radius of 19.2 m, a diameter of 12 m, and a focal length of 16 m. Two different camera designs have been developed for the telescope. Its large field of view of 7 degrees will allow comprehensive surveys of the gamma-ray sky.

The MST project is an international collaboration of research institutes and universities from Germany, France, Brazil, Poland, United States, Italy, Austria and Spain. DESY, the project lead institue, is also responsible for the design of the telescope structures. The telescope structure consists mainly of steel components. The interior of the telescope’s tower, which is 9 m high and 2 m in diameter, houses all the electrical cabinets and some of the components of the drive system. Automatic lubrication of the azimuth and elevation drives and a novel structural and drive monitoring system allow remote fault diagnosis, reducing regular maintenance and potential downtime. The mirrors are constructed as spherical facets of hexagonal shape, with a side-by-side length of 1.2 m and a radius of curvature of 32.14 m. The mirrors are manufactured using cold slumping technology. The reflector is made up of 86 identical segments, which are fixed to the mirror support structure using two actuators and a fixed point. Active mirror control allows the tilt of each individual mirror segment, to be adjusted and aligned to form a uniform reflector surface.

A full-scale prototype of the telescope structure was built in Berlin in 2012 and operated until February 2020. The performance of the prototype was evaluated and optimisations were made, including simplifying the assembly process, mass production capabilities and reducing overall manufacturing costs. The prototype telescope structure was used as a testbed for control software development and debugging. The Critical Design and Manufacturing Readiness Review (CDMR) of the telescope structure was performed in 2022. The review by the CTAO project office and external experts was completed by the end of 2023, a milestone that allowed the procurement of the first three Pathfinder telescopes to be installed in Chile and La Palma to begin. The first three telescopes will be installed at the observatory sites in early 2026.