Two Cherenkov telescopes help us understand extreme cosmic phenomena through very high-energy gamma rays; they are currently the most sensitive devices of their kind in the world. The prototypes, which were developed by an international team involving Czech scientists, are now in Ondřejov. This is an exceptional opportunity for Czech astrophysicists, as the telescopes may be moved abroad in the future.
Cherenkov telescopes are typically part of large global observatories at high altitude. The fact that we have two such devices in the Czech Republic opens up new possibilities for Czech astronomy, and even more so, as it has now been proven that they are extraordinarily sensitive to the highest energies of gamma rays. The telescope development started in 2012 and was carried out by an international team consisting of 15 institutions in the Czech Republic, Poland, and Switzerland. Their goal was to create a new instrument for detecting high-energy gamma radiation that would use modern technology and overcome the limitations of previous detectors.
The devices, known as SST-1M, or single mirror small-sized telescopes, have been tested over the past two years at the Ondřejov Observatory, where their operation have been contributed to by the Institute of Physics of the Czech Academy of Sciences (FZU), the Astronomical Institute of the Czech Academy of Sciences, and Palacký University in Olomouc. Despite being called "small," they are telescopes with the largest mirrors in the Czech Republic. The total diameter of the telescope mirror, composed of smaller segments, is 4 meters.
Cherenkov radiation is a faint blue glow that occurs when a charged particle moves faster than the speed of light in a given environment. This phenomenon occurs, among other things, when a gamma ray photon enters the Earth's atmosphere, collides with the nucleus of one of the atoms in the atmosphere, and creates a shower of secondary particles that travel through the atmosphere at high speeds. If Cherenkov radiation from these particle showers is captured, we can determine the properties of the original gamma photon, i.e., where it came from and what its energy was. With this information, we can better understand the extreme cosmic phenomena in which gamma radiation is produced, such as in jets of matter in active galaxies or in neutron star collisions.
"Our goal was to develop a modern Cherenkov telescope that would overcome some of the limitations of previous telescopes of the same type – namely, a small field of view and the limiting effect of moonlight," explains astroparticle physicist Jakub Juryšek from FZU, adding: "SST-1M telescopes are equipped with a sensitive camera based on SiPM technology, which, unlike conventional photomultipliers, allows gamma photons to be observed even during a full moon. At the same time, the large 9-degree field of view allows us to capture even extensive atmospheric showers caused by gamma-ray photons of the highest-energy. We currently operate the most sensitive Cherenkov telescopes in the world for gamma-ray photons with energies above 50 TeV.“
The team built two SST-1M prototypes at the Ondřejov Observatory, 155 meters apart, which allows atmospheric showers to be observed from different angles and thus helps to better determine the energy and direction of arrival of the primary gamma-ray photon.
Over the past two years, scientists from FZU have been developing complex software for reconstructing the properties of atmospheric showers and calibrating the signal measured by the telescopes. The telescopes were thoroughly tested by observing the Crab Nebula, which is not only a beautiful object often sought after by amateur astronomers, but also the brightest stable source of gamma radiation in the sky. "We observed the Crab Nebula in stereoscopic mode for a total of 33 hours. We verified that the reconstructed spectrum in the gamma range corresponds to the results of other observatories. The angular resolution in the gamma-ray energy range also corresponds to our simulations, which means that we understand the telescopes and the data obtained very well, and we can thus expand the list of observed targets to include other astrophysically interesting sources. Thanks to the aforementioned highest sensitivity to the high-energy gamma photons, we have a unique opportunity to look into the physical processes that take place in cosmic particle accelerators in our Galaxy," adds Jakub Juryšek. The results were published in the prestigious journal Astronomy & Astrophysics.
Having been tested successfully, the telescope prototypes will now continue to collect data at the Ondřejov Observatory until a decision is made on their future location. Since the telescope potential has been proven, the international team plans to move them to a location with atmospheric conditions that are more favourable for observing cosmic gamma-ray photons.
C. Alispach et al. (from FZU: A. Araudo, J. Blažek, A. Christov, J. Chudoba, P. Čechvala, P. Dědic, P. Janeček, J. Juryšek, D. Mandát, S. R. Muthyala, A. L. Müller, V. Novotný, M. Palatka, M. Pech, M. Prouza, P. Schovánek, T. Tavernier, P. Trávníček, J. Vícha): Observation of the Crab Nebula with the Single-Mirror Small-Size Telescope stereoscopic system at low altitude. Astronomy & Astrophysics 699 (2025) A255, doi: 10.1051/0004-6361/202555292