The Telescopes

The CTAO is the world’s largest and most powerful ground-based observatory for gamma-ray astronomy at very-high energies. The CTAO’s telescopes will detect high-energy gamma rays with the imaging air Cherenkov technique, which includes the Earth’s atmosphere as a critical step in the detection process (go to How CTAO Works). When the gamma rays interact with the Earth’s atmosphere, they create particle cascades that generate extremely fast flashes of light. This “Cherenkov light” is what CTAO’s telescopes capture so that scientists can study the gamma rays indirectly and give us a completely new view of the extreme Universe.

To make this possible, three classes of telescope are required to cover the CTAO’s broad energy range (20 GeV to 300 TeV). Following the Alpha Configuration, the CTAO is planning four Large-Sized Telescopes in the northern hemisphere for its low-energy range (20 to 150 GeV), up to 23 Medium-Sized Telescopes distributed over both array sites for its core energy range (150 GeV to 5 TeV), and up to 37 Small-Sized Telescopes in the southern hemisphere to extend the energy range above 5 TeV.

Our Telescopes in Numbers

The CTAO will cover a very wide energy range and provide excellent angular resolution and sensitivity in comparison to any existing gamma-ray detector. With its ability to detect energies between 20 GeV and 300 TeV and its unprecedented resolution, the CTAO will be able to observe further than ever before and push the CTAO to the edge of the known electromagnetic spectrum, providing a completely new view of the sky.

Explore Our Telescopes

The Details

Why Three?

Each class of telescope is tasked to cover a different energy range, thus the designs and distribution of telescopes on the two CTAO array sites are important. Since the showers generated by the highest energy gamma rays are rare but produce a large amount of Cherenkov light, many SSTs spread over a large area improve our chances of detecting them. On the other hand, the LSTs are less in number and have massive reflectors to capture the more faint but frequent lower-energy flashes. The MSTs capture the range in between.


It may look like our telescopes have one solid monolithic mirror, but, if you look closer, you’ll see each telescope’s reflector is comprised of many highly reflective hexagonal mirror facets. This design, invented by Guido Horn D’Arturo in the 1930’s, allows us to build very large reflectors more easily and more economically. The CTAO will use computer-controlled optics systems to actively align approximately 3,500 of these highly polished mirror facets to focus light into the telescopes’ cameras.


To detect the short flashes of Cherenkov light, the telescopes’ cameras must be about a million times faster than a common camera. To do this, they will use high-speed digitisation and triggering technology capable of reading shower images at a rate of one billion frames per second and sensitive enough to resolve single photons. Depending on the camera design, photomultiplier tubes (PMTs) or silicon photomultipliers (SiPMs) will convert the light into an electrical signal that is then digitised and transmitted to record the image of the cascade.

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