About CTA

An Introduction to CTA


CTA stands for Cherenkov Telescope Array. It is a new array of telescopes that will aim to study the high-energy Universe. Construction on CTA will start in 2015, and will hopefully be complete by 2020. CTA is a huge project, involving over 1000 scientists from 27 countries, and will cost about 185 million euros (about 150 million pounds).

CTA will be made up of around 120 telescopes. These will be split across two sites- one in the Northern hemisphere, and one in the Southern hemisphere. These telescopes aren’t just any old telescope though- they are Cherenkov telescopes.

Cherenkov Telescopes

CTA is going to be made up of Cherenkov telescopes. CTA will continue the work of successful Cherenkov telescopes such as MAGICH.E.S.S. and VERITAS. The most Cherenkov telescopes in an array at the moment (2012) is four- although H.E.S.S. is adding a fifth telescope later in 2012. CTA is going to have 120 telescopes!

The 120 telescopes of CTA will be split between two sites: one in the Northern hemisphere, and one in the Southern hemisphere. There are lots of factors to consider when deciding where to place these sites:

  • Weather: dry, clear skies.
  • Light: we are looking at very faint light sources, so need to be away from the light pollution from large towns, cities or roads:

Showing areas of high light pollution. Credit: Data courtesy Marc Imhoff of NASA GSFC and Christopher Elvidge of NOAA NGDC. Image by Craig Mayhew and Robert Simmon, NASA GSFC


  • Latitude: placing telescopes somewhere between the 30 degree lines of latitude means that they can easily observe the most interesting parts of the sky.
  • Altitude: the upper part of the Earth’s atmosphere constitutes a part of the detector, making the light we need to detect very high energy gamma-rays; the lower part of the atmosphere then re-absorbs that light. The higher the telescope, the less atmosphere for the telescope to look through. However, we don’t want to be too high – the astronomers have to be able to get to the telescopes without worrying about breathing equipment or altitude sickness. There is then a range of altitudes just right for placing a Cherenkov telescope – that is not too high and not too low.

  • Ground: we need a large, flat area on which to place the telescopes. We don’t want any leaning telescopes, so the ground should be nice and solid.
  • Infrastructure: we need basic things like power, water and a road to bring the telescopes (and astronomers) to the site. High-speed internet is important for transferring results and a nearby city (within a few hours drive) is useful for astronomers travelling to the telescopes.
  • Other: earthquakes would be bad news, and if the telescopes are in a desert, then we don’t want sharp, corrosive sand as this might damage the telescope mirrors.
  • The Scientific Community: it would be useful to have a local University or research centre nearby. It’s good to talk!

The site in the Southern hemisphere will be used for looking at Galactic sources – these are objects which emit gamma-rays in our Galaxy, the Milky Way. The site in the Northern hemisphere will be looking at sources from outside our Galaxy, which tend to be detected at lower energies.

To look at all these gamma-ray sources with different energies, we need telescopes which are sensitive to different energies. We do this by changing the size of the telescope – or more accurately, the size of the telescope mirror which collects the light. CTA will have a few large telescopes (23m diameter) to look at the low-energy sources. Some 12m diameter telescopes will look at the medium-energy sources, and there will be many 4m diameter telescopes to study the highest energies.

Next: How will CTA Work