How CTAO Works

How the CTAO will detect Cherenkov light

Detecting Cherenkov Light

The gamma rays that the CTAO will detect don’t make it all the way to the Earth’s surface. When gamma rays reach the Earth’s atmosphere they interact with it, producing cascades of subatomic particles. These cascades are also known as air or particle showers. Nothing can travel faster than the speed of light in a vacuum, but light travels 0.03 percent slower in air. Thus, these ultra-high energy particles can travel faster than light in air, creating a blue flash of “Cherenkov light” (discovered by Russian physicist Pavel Cherenkov in 1934) similar to the sonic boom created by an aircraft exceeding the speed of sound. Although the light is spread over a large area (250 m in diameter), the cascade only lasts a few billionths of a second. It is too faint to be detected by the human eye but not too faint for the CTAO. The CTAO’s large mirrors and high-speed cameras will detect the flash of light and image the cascade generated by the gamma rays for further study of their cosmic sources. Learn more about gamma rays and their cosmic sources.

Telescope Arrays

These cascades are so rare (one gamma-ray photon per m2 per year from a bright source or one per m2 per century from a faint source), that the CTAO will be using more than 60 telescopes spread between two array sites in the northern and southern hemispheres to improve the chance of capturing them.


While the northern hemisphere array will be more limited in size and will focus on the CTAO’s low- and mid-energy ranges from 20 GeV to 5 TeV, the southern hemisphere array, with its prime view of the rich central region of our Galaxy, will span the mid- to high-energy range of the CTAO, covering gamma-ray energies from 150 GeV to 300 TeV. Three classes of telescope will be distributed in the northern and southern hemisphere based on their sensitivity: the Small-Sized Telescope (SST), Medium-Sized Telescope (MST), and Large-Sized Telescope (LST). Because the SSTs improve the CTAO’s sensitivity at the highest energies, they are more ideal for the southern site’s detection of higher-energy gamma rays, while the LSTs, optimized for lower-energy sensitivities, will be installed on the northern array. The MSTs, sensitive to the CTAO’s core energy range, will be installed on both sites. The graphics below illustrate the approved layouts of the telescope arrays in both the northern and southern hemispheres, the so-called Alpha Configuration. This configuration includes 13 telescopes, four LSTs and nine MSTs,  distributed over an area of about 0.5 km2 in the CTAO Northern Array and 51 telescopes over a ~3 km2 area, consisting of 14 MSTs and 37 SSTs, in the CTAO Southern Array.

Layout of the CTAO Northern Array on La Palma (Spain), including the elements defined in the Alpha Configuration.
Layout of the CTAO Southern Array in the Atacama Desert (Chile), according to the Alpha Configuration.
»The telescope structures will stand between about 8 and 45 metres tall and weigh between 17.5 and 100 tonnes.«

CTAO Telescopes and Technology

The CTAO is not the first ground-based gamma-ray detector, but it will be the first open ground-based gamma-ray observatory and the most advanced instrument of its kind. The current generation started yielding results in 2003 and increased the number of known gamma-ray-emitting objects from around 10 to around 200. The CTAO will build on the advances pioneered by its predecessors (H.E.S.S., MAGIC and VERITAS) in order to expand this catalogue fivefold, detecting more than 1,000 new objects.


Three classes of telescope types are required to cover the full CTAO energy range (20 GeV to 300 TeV). For its core energy range (100 GeV to 10 TeV), the CTAO is planning 23 Medium-Sized Telescopes (MSTs) distributed over both array sites. Furthermore, four Large-Sized Telescopes (LSTs) and 37 Small-Sized Telescopes (SSTs) are planned to extend the energy range below 100 GeV and above a few TeV.


While the individual telescopes may vary in size and design, the CTAO telescopes will be constructed and will perform similarly. Each telescope is composed of a segmented mirror that reflects the Cherenkov light to a high-speed camera that can digitize and record the image of the shower. 

»The CTAO will use both photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs) to provide around 125,000 ultra-fast light-sensitive pixels.«

CTAO Cameras

The CTAO telescopes will use almost 3,500 highly-reflective mirror facets (90 cm to 2 m in diameter) to focus light into the telescopes’ cameras. Once the mirrors reflect the light, the CTAO cameras capture and convert it into data. Each telescope has its own variation of camera (see example of one of the proposed camera prototypes below), but the designs are all driven by the brightness and short duration of the Cherenkov light flash.


A Cherenkov light flash lasts only a few billionths of a second and is extremely faint. The cameras are sensitive to these faint flashes and use extremely fast exposures to capture the light. The CTAO will use both photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs) to convert the light into an electrical signal that is then digitised and transmitted.


From Data to Discovery 


Once the telescopes record the Cherenkov images of a cascade, any undesirable “noise” in the image will be suppressed to reduce its size before it is analysed in real time. Processed images will then be transmitted to central computing facilities for further processing and to be archived. The calibrated image data will be used to reconstruct the properties of individual gamma rays. The energy and arrival direction of the gamma rays will be provided to science users of the Observatory. The CTAO Science Data Management Centre (SDMC), to be located on the DESY campus in Zeuthen, Germany, will coordinate the processing and long-term preservation of the data, in addition to providing the data, tools and support to the scientific users of the facility. 


Go to the Project page to learn more about the CTAO telescopes and technology.

CTAO: Cherenkov Telescope Array Observatory


Together, the northern and southern CTAO arrays will constitute the CTAO, which will be the first ground-based gamma-ray observatory open to the world-wide astronomical and particle physics communities as a resource for data from unique high-energy astronomical observations. The CTAO will be operated as an open, proposal-driven observatory for the first time in very high-energy astronomy. This is expected to significantly boost the scientific output of the CTAO by engaging a much wider research community.


Additionally, the CTAO will feed its data into a virtual observatory, which will allow scientists to probe multiple data centres seamlessly and transparently, provide analysis and visualization tools and give other observatories a standard framework for publishing and delivering services using their data.