Small-Sized Telescope

The Small-Sized Telescope (SST) will be built by an international collaboration that includes research institutes and universities from Australia, Brazil, France, Germany, Italy, Japan, Netherlands, Switzerland and UK. The CTAO’s southern array will include 37 SSTs according to the “Alpha Configuration,” largely outnumbering the other CTAO telescopes.

The SSTs will be optimized to observe celestial gamma rays in the range of 5 TeV to 300 TeV, covering the highest end of CTAO’s sensitivity range. These very high-energy gamma-ray showers produce a large amount of Cherenkov light but are less frequent than lower–energy showers. Thus, compared to the other CTAO telescopes, the SSTs have smaller reflectors and a larger field of view to increase their capability to detect these energetic events. Additionally, to increase the probability of detection, there will be a higher number of telescopes spread out over several square kilometers.

The SST relies on a modified Schwarzschild-Couder dual-mirror optical design, providing a good spatial resolution over a large field of view. They are compact telescopes with small focal ratios allowing the use also of compact cameras. Each SST has a primary mirror based on 18 hexagonal segments with a total aperture of 4.3 m and a monolithic secondary mirror of 1.8 m in diameter.  

Credit: G. Pérez, IAC, SMM

The key specifications are listed in the table below:

SST Camera

The SST small plate scale permits the use of a novel compact camera based on silicon photomultipliers (SiPM) sensors. The SST camera contains 32 SiPM tiles. Each tile is an array of 64 (6 mm x 6 mm) active pixels, for a total of 2048 pixels, covering a field of view of approximately 9o. The camera trigger and readout electronics are based around TARGET ASICs that capture Cherenkov light with 128-frame movies, with each frame lasting one billionth of a second. 

The SST-CAM is an evolution of the CHEC-S camera.  It is very compact and housed in an aluminium enclosure, resulting in a total size of 570 mm x 570 mm x 500 mm and a mass of ~90kg. It is hermetically sealed with an external chiller that circulates chilled liquid through the camera to control the temperature. An entrance window and external door system protect it from the environmental elements. The entrance window features a multi-layer coating that acts as a bandpass between 290-550 nm to minimise the night-sky background while maximising the transmission of Cherenkov light, which peaks at ~320 nm. The camera includes an illumination system to provide calibration via fast, variable-intensity LED flashes.

SST Status

The current SST concept was validated, both for the structure and camera, by developing the prototype dual-mirror ASTRI-Horn Cherenkov telescope and the CHEC-S SiPM camera, respectively. The improved ASTRI-Horn telescope serves as a precursor for the SST’s structure, which takes the baseline from this telescope. Further improvements on the ASTRI-Horn telescope structure have been implemented with the development of the ASTRI mini array telescopes. The first of nine telescopes is now mounted in Tenerife (see figure). The M1 dish will be further modified for the SST following a different structural optimization that provides an improved mechanical behavior, while the sizes and the interfaces will be kept exactly the same. 

In February 2023, the project underwent the Product Review, in which the baseline design of the telescope was presented. This was the formal conclusion of the Design Verification and Engineering Review that the telescope underwent in July 2020. The project is now in the consolidation phase that will end with the Critical Design Review, after which, it will begin the production phase.