The first telescope of a Cherenkov Telescope Array

  • A multinational effort develops the Cherenkov Telescope Array to study high-energy galactic radiation sources
  • The array will take takes advantage of the Cherenkov effect that takes place when gamma rays hit the atmosphere
  • The inaugurated telescope was to be the first one to be operated by the Cherenkov Telescope Array Observatory

On Wednesday October the 10th 2018, in the Canary Island of La Palma more than 200 people from around the World met at the north headquarters of the Cherenkov Telescope Array (CTA). They met there on an unique occasion: the inauguration of the first Large-Sized Telescope (LST) of the Cherenkov Telescope Array.

Cherenkov radiation is emitted when a charged particle travels through a dielectric (i.e. an electrically insulating) medium above a characteristic speed threshold. This effect can be seen, for instance, around underwater nuclear reactors, which emit a blue glow as a result of this effect. In an astronomical context, high-energy gamma rays can impact on the atmosphere, generating showers of charged particles able to surpass the required speed threshold. Cherenkov telescopes detect the particle showers released by gamma rays. They do so indirectly, by using taking advantage of the released Cherenkov radiation, the faint light radiated by the charged particles in the particle showers.

The Cherenkov Telescope array

The Cherenkov Telescope Array project was conceived for the building of a next-generation ground-based gamma-ray detection telescope. The CTA was planned to consist of two arrays of Imaging Atmospheric Cherenkov telescopes (IACTs), split between both hemispheres of the Earth. The Northern Hemisphere array located in La Palma would put an emphasis on the study of extragalactic objects of the lowest possible energies, while the Southern Hemisphere array, located at Cerro Paranal (Chile) would cover the full range of energies, concentrating on sources of radiation inside our galaxy.

High-speed charged particles crossing the water around an underwater nuclear reactor generate Cherenkov radiation
High-speed charged particles crossing the water around an underwater nuclear reactor generate Cherenkov radiation

The CTA will study in detail the spatial structure, light curve and energy spectra of close to a thousand astronomical sources, via the study of very high-energy gamma ray astronomy on basis of Cherenkov radiation observation. The CTA builds on top of previous successes and experience sucha as that of the High Energy Stereoscopic System (H.E.S.S.) and the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC telescopes).

On the one hand, CTA uses telescope arrays and stereoscopic analysis, improving the sensitivity dramatically. On the other, it exploits the use of large telescopes to attain the lowest possible energy/radiation threshold. Taken together, this facilitates the detailed study of the universe in connection to the strongest energy radiations: gamma rays. By the observation of the results of their impact on the atmosphere, it will be possible to study the most extreme fundamental physics and astrophysical phenomena.

The Telescope

The telescope inaugurated in October 2018, called LST-1, was the first of four Large-Sized Telescopes (LST) at the north part of the CTA Observatory at the Roque de los Muchachos Observatory (island of La Palma, Canary Islands) managed by the Instituto de Astrofísica de Canarias. Once the north telescope array is fully finished, the north network will have fifteen Medium-Sized Telescopes (MSTs) installed.

On October the 9th, 2015, an act celebrated the placement of the first stone of the LST-1. Once the foundations of the telescope were complete, in January 2017, the team continued to complete construction milestones such as the installation of the rail system (September 2017) or the installation of the mirrors (December 2017). In February 2018, the structure of the LST-1, and the camera support were installed in June of that year. The camera was finally installed on September 25th 2018.

The LST team comprises over 200 scientists of ten countries including Brazil, Croatia, France, Germany, India, Italy, Japan, Poland, Spain and Sweden. In such an international context, design and management was undertaken jointly by the Annecy Laboratory of Particle Physics (LAPP); the Max Planck Institute for Physics at Munich; the National Institute for Nuclear Physics of Italy (INFN); The Institute for Cosmic Ray Research (ICRR) of the University of Tokyo; the Institute of High Energy Physics (IFAE) at Barcelona and the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT) in Madrid. Among the institutions who participate of the construction also are the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Sciences (IEEC).

Major Atmospheric Gamma Imaging Cherenkov Telescope II (MAGIC II), another Cherenkov-effect based telescope
Major Atmospheric Gamma Imaging Cherenkov Telescope II (MAGIC II), another Cherenkov-effect based telescope

IFAE and local contributions to the LST-1 project

Of the contributing institutions, together with IGFAE, two other catalan research institutions had an important participation in the technological development of the LST-1. The Institute for High Energy Physics (IFAE) was responsible for the coordination, control and assembly of the mechanical system for the ground anchoring and rotation of the telescope. The Institute of Cosmos Sciences of the University of Barcelona (ICCUB) contributed to the design of one of the devices for the amplification of the signal. Finally, the Institute of Space Sciences participated with the development of the control software and scheduler. All three institutions contributed to the definition of the scientific objectives of the project.

Gamma ray detection

Other than the LST, two more types of telescopes were needed to be able to study the energy range between 20 gigaelectronvolt (GeV) and 300 teraelectronvolt (TeV): the telescopes of small and medium size. As the low-energy gamma rays produce little Cherenkov light, the telescopes need to have large mirrors to capture the images. As a result, four LST telescopes will be located both at the North and South of the CTA observatory, to cover the 20 – 150 GeV low-energy sensitivity range of the CTA.

The LST, with a parabolic reflecting surface and a diameter of 23 metres, is held in place by a tubular structure made of carbon fibre and steel pipes. The reflecting surface of 400 square metres captures and focuses the Cherenkov effect light towards the camera, where the photomultiplier tubes convert the light into electric signals that are processed by the camera electronic systems. Even though the LST-1 is 45 metres tall and weighs about 100 tonnes, it still is extremely fast and can reposition in only 20 seconds in order to be able to acquire the brief signals of low-energy gamma rays.

Artist representation of all three classes of CTA telescopes planned for the southern hemisphere at Paranal Observatory representation of all three classes of CTA telescopes planned for the southern hemisphere at Paranal Observatory

The LST is to increase the reach of science to cosmological distances and fainter sources with soft energy spectra. Both the speed of reorientation of the telescopes and the low energy threshold they provide are key elements for the study of transitory gamma ray sources in our galaxy, or for the study of active galactic nuclei and the explosions of gamma rays shifting towards the “high-red” part of the radiation spectrum.

This inaugurated telescope was expected to be the first LST of the CTA, and the first telescope in one in the array to be operated by the CTA observatory (CTAO). However, as every technical delivery in the CTA multinational project, a stringent process or revision would await the LST-1 in order to verify that the design accomplishes the scientific objectives of the CTA, its exploitation needs, security standards, and so forth, before being approved by CTAO.

The website of the inauguration of the LST-1 can be visited here to find information in other languages and to obtain supplementary materials. In the meantime, the project has continued its advance. Find online the current status of this project, which is to provide astrophysics with a richness of new cosmological data in the future to come.

Image credits:

Cherenkov radiation from underwater reactor was downloaded from Wikipedia and licensed via a Creative Commons Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0) license.

Major Atmospheric Gamma Imaging Cherenkov Telescope II (MAGIC II) was downloaded from Wikimedia Commons and licensed via an Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) license.

Artist representation of South CTA telescopes was downloaded from Wikimedia Commons and licensed via an Attribution 4.0 International (CC BY 4.0) license.