Medium-Sized Telescope

»The MST will be the CTAO’s “workhorse" with sensitivity in its core energy range, from about 150 GeV to 5 TeV.«

The Medium-Sized Telescope (MST) is being built by an international collaboration of institutes and universities from Austria, Brazil, France, Germany, Italy, Poland, Spain and Switzerland. This will be the CTAO’s “workhorse,” optimized to detect gamma rays in its core energy range, from about 150 GeV to 5 TeV. The planned Alpha Configuration for the CTAO includes 23 MSTs – 14 in the southern hemisphere and nine in the northern hemisphere. The MST will have 86 hexagonal-shaped mirrors that are aligned with an active mirror control assembly to create a uniform reflector of about 12 metres in diameter. 


The MST is a modified Davies-Cotton telescope with a reflector size of 88 and a focal length of 16 m. It will have two different camera designs that use photomultiplier tubes (PMTs). The cameras have a field of view of about 8 degrees, enabling the MSTs to take rapid surveys of the gamma-ray sky. 

Credit: G. Pérez, IAC, SMM

The key specifications are listed in the table below.

MST Specifications Table

MST Cameras

The MST cameras will have a large field of view of about 8 degrees, enabling observation of gamma-ray sources that may be concentrated in one area of the sky or widely spread apart. Two camera designs will be used for the MSTs: NectarCAM and FlashCam. In 2020, an agreement was reached to use NectarCAM on the CTAO-North site and FlashCam on the CTAO-South site.



NectarCAM is composed of 265 individual removable modules, each one composed of seven PMTs. The analog signal detected by the PMTs is preamplifiered and sent to the trigger and a readout electronics system in charge of differentiating and recording gamma-ray signals, respectively. To sample the arriving signal the NectarCAM uses the so-called ‘Nectar’ ASIC, an integrated circuit capable of sampling at a high rate of GSamples/s, to take one billion samples in just one second. NectarCAM shares many design features and components with the Large-Sized Telescope (LST) camera.



FlashCam is the first Cherenkov camera concept whose readout system follows a fully digital approach, with a high performance ethernet transmission to the camera server. Its design follows a horizontal architecture with the photon detector plane (PDP), the readout electronics and the data acquisition system as key building blocks. The PDP contains PMTs arranged in a hexagonal structure and combined in modules of twelve PMTs each. From there, the analog signals are transmitted via cables to the readout electronics, where it is continuously digitized by the so-called Fast Analog-to-Digital Converters (FADCs) and sampled at a frequency of 250 MSamples/s. Thus, instead of working with analog signals as the NectarCam, the FlashCam readout follows a fully digital approach.

>> The dual-mirrored Schwarzschild-Couder Telescope (SCT) is being developed and proposed as an alternative type of medium telescope. Read more <<

MST Prototype

An MST prototype operated in Berlin between 2012 and 2020 to validate the design of the individual components, test the interfaces between the mating assemblies and to define the assembly process of the product. Extended test campaigns with FlashCam and NectarCAM prototypes were performed on the telescope during this period in order to verify their interfaces and overall integrity. 


Preparations to install MST pathfinders on both CTAO sites has significantly progressed in recent years: In 2020, NectarCAM passed the Critical Design & Manufacturing Review (CDMR) and a technical revision was successfully completed for the MST structure in November 2022. Permits for the construction of the first MST on La Palma have already been granted by local governments and detailed site studies and characterization (including definition of logistics, location, geotechnical studies, etc.) have been carried out in Chile.

Photo of MST prototype in Berlin

MST Contacts:

Structure Coordinator: Markus Garczarczyk

FlashCam Coordinator: German Hermann

NectarCAM Coordinator: Jean-Francois Glicenstein