How CTA Works

How CTA will detect Cherenkov light

Detecting Cherenkov Light

The gamma rays that CTA 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 CTA. CTA’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 CTA will be using more than 100 telescopes spread between two array sites in the northern and southern hemispheres to improve the chance of capturing them. The graphic below illustrates a potential layout of the telescope arrays in both the northern and southern hemispheres.


While the northern hemisphere array will be more limited in size and will focus on CTA’s low- and mid-energy ranges from 20 GeV to 20 TeV, the southern hemisphere array will span the entire energy range of CTA, covering gamma-ray energies from 20 GeV to 300 TeV. Three classes of telescope will be distributed in the northern and southern hemisphere based on their sensitivity: the Small-Size Telescope (SST), Medium-Size Telescope (MST), and Large-Size Telescope (LST). Because the SSTs are tuned to be the most sensitive to detect high-energy gamma rays, they are more ideal for the southern site’s detection of higher-energy gamma rays, while the MSTs and LSTs will be installed on both sites. The below schematics and illustrate the proposed layouts of the northern and southern hemisphere arrays.

Proposed Array Layouts
»The telescope structures will stand between about 8 and 45 metres tall and weigh between 8 and 100 tonnes.«

CTA Telescopes and Technology

CTA is not the first ground-based gamma-ray detector, but it will be the most advanced 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 more than 100. CTA will build on the advances pioneered by its predecessors (H.E.S.S., VERITAS and MAGIC) in order to expand this catalogue tenfold, detecting more than 1,000 new objects.


Three classes of telescope types are required to cover the full CTA energy range (20 GeV to 300 TeV). For its core energy range (100 GeV to 10 TeV), CTA is planning 40 Medium-Size Telescopes distributed over both array sites. Furthermore, eight Large-Size Telescopes and 70 Small-Size Telescopes 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, CTA telescopes will be constructed and will perform similarly. Each telescope will have a mount that allows it to rapidly point towards targets and will be comprised of a large segmented mirror to reflect the Cherenkov light to a high-speed camera that can digitize and record the image of the shower.

Large-Size Telescope (LST):

Because gamma rays with low energies produce only a small amount of Cherenkov light, telescopes with large mirrors are required to capture the images. The LST mirror will be 23 metres in diameter and parabolic in shape. Its camera will use photomultiplier tubes (PMTs) and will have a field of view of about 4.5 degrees. The entire structure will weigh 50 tonnes but will be extremely nimble, with the goal to be able to re-position within 20 seconds. More about the LST.


Medium-Size Telescope (MST):

This will be CTA’s “workhorse” due to its sensitivity to the faint energy flux of gamma rays. The MST mirror will be 12 metres in diameter and will have a camera that uses PMTs. Its large field of view of 7-8 degrees will enable the MST to take rapid surveys of the gamma-ray sky. More about the MST.


Small-Size Telescope (SST):

The SST is sensitive to the highest energy gamma rays, which come from our own galaxy. Since our galaxy is best observed from the southern hemisphere and the corresponding showers produce a large amount of Cherenkov light, the SSTs will outnumber all the other telescopes and will be spread out over several square kilometers in the southern hemisphere array only. The SST mirror will be about 4 metres in diameter and will have a large field of view of about 9 degrees. Three different SST implementations are being prototyped and tested. More about the SSTs.

»CTA will use both photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs) to provide more than 200,000 ultra-fast light-sensitive pixels.«

CTA Cameras

CTA will use more than 7,000 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 CTA 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. CTA will use both photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs) to convert the light into an electrical signal that is then digitised and transmitted.



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

CTA Observatory

Together, the northern and southern CTA arrays will constitute the CTA Observatory (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. CTA 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 CTA by engaging a much wider research community.


Additionally, CTA 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.


Visit the Users page to learn more about the Observatory operations and expected CTA performance metrics.