CTA (Cherenkov Telescope Array)

Group members: Patrizia Caraveo, Andrew Chen (up to Oct 2013), Andrea De Luca, Mauro Fiorini, Salvatore Incorvaia, Nicola La Palombara, Nicola Sartore (up to Aug 2013), Luca Stringhetti

A formal international collaboration has been set up, which has now over 1000 members from 27 countries from Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which should lead to production readiness of CTA in 2014.

To reach its ambitious performance goals, and specifically the wide energy range to be covered, the instrumentation needs to be optimised for three adjacent energy ranges.

• The low-energy range <100 GeV: To detect showers down to a few tens of GeV, the Cherenkov light needs to be sampled efficiently with the fraction of area covered by light collectors being of the order of 10%. Since event rates are high and systematic uncertainties in the background limit the achievable sensitivity, the area of this part of the array can be relatively small (of the order of few 10⁴ m²). The CTA design assumes a small number (< 4) of closely placed large-size telescopes (LSTs), typically with a mirror diameter of about 24 m, to collect as many Cherenkov photons as possible from the low energy showers.

• The core energy range 0.1-10 TeV: The foreseen performances call for an array of mid-sized telescopes (MSTs) with about 12 m diameter mirrors and a spacing of about 100 m. Improved sensitivity compared to existing instruments will be obtained both by the increased area covered and by the higher quality of shower reconstruction, since individual showers will typically be imaged by a larger number of telescopes than for current few-telescope arrays. For the first time, array sizes will become much larger than the Cherenkov light pool of a shower, ensuring that images will be uniformly sampled across the light pool and that a number of images are recorded close to the optimum distance from the shower axis (about 70-150 m), where the light intensity is large and intensity fluctuations are small.

• The high-energy range >10 TeV: Here, the key limitation is the number of detected γ-ray showers. Consequently, for large improvement the array needs to cover an area of several square kilometres. At high energies the light yield of a shower is large, so that showers can be detected well beyond the 150 m radius of a typical Cherenkov light pool. Thus telescopes can be relatively small (4-6 m diameter, known as Small Size Telescopes SSTs) and their spacing should match the size of the light pool (100-200 m).

A large number of SSTs will be needed and INAF plans to play a major role in CTA by providing a significant fraction of the array of such small telescopes. Currently, the design and construction of an end-to-end prototype is being carried out within the ASTRI flagship project.