The CTA observatory: the future of gamma-ray astronomy

Currently under construction at two sites in the Northern and Southern Hemispheres, the Cherenkov Telescope Array Observatory (CTA) will be the world's largest array of telescopes capable of detecting high-energy gamma rays. The Laboratoire Leprince-Ringuet (LLR*) is heavily involved in this project.

Artistic rendering of the CTAO's northern hemisphere site in the Canary Islands. Credit: CTAO.

06 Mar. 2025

Research

February 2025 saw the inaugural meeting of the European Research Infrastructure Consortium (ERIC) in Bologna, Italy. When completed, CTA will be the world's largest observatory sensitive to gamma rays, the highly energetic part of electromagnetic radiation emitted by a multitude of cosmic phenomena, for example in star-forming regions, supernova remnants or the active nuclei of galaxies housing massive black holes. “This provides a legal and governance framework for this major observatory project,” explains Mathieu de Naurois, Vice- Chair of the ERIC CTAO council and CNRS researcher at the École polytechnique's Laboratoire Leprince-Ringuet.

Today, these gamma rays are detected by satellites like Fermi or ground-based telescopes like H.E.S.S., an observatory in Namibia where LLR also plays an important role. While these instruments have helped to open a new “window” on the Universe and map the first sources of gamma rays, CTA will enable us to continue this exploration with greater sensitivity, better resolution and a wider field of view.

More than 25 countries and 150 laboratories and institutes (including 13 in France) are working together on this major project, currently under construction at two complementary sites: in Chile and on the Canary Islands. These two sites will cover the entire sky. Gamma rays will not be detected directly; when they reach the Earth's upper atmosphere, they interact with the atoms in it, creating a cascade of subatomic particles that produce a brief flash of bluish light, a phenomenon known as the Cherenkov effect. This light is captured by CTA, which stands for Cherenkov Telescope Array.

The array of telescopes will be arranged with different telescope sizes: large instruments (LST) will be sensitive to frequent, relatively low-energy phenomena; small telescopes (LST) can be spread over a wide area, to improve the chances of capturing the rarer, high-energy phenomena; medium-sized telescopes (MST) will cover the intermediate energy range. LLR is currently involved in the development of NectarCAMs, one of the camera types that will equip the latter.

graph showing the size of the telescopes at the CTA observatory: large (45 m), medium (27 m) and small (9 m). — Size of the different telescope models at the CTA observatory.

*LLR: a joint research unit CNRS, École Polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France

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