The ”Cherenkov Telescope Array” (CTA) will be an advanced facility for ground based very- high-energy gamma ray astronomy, based on the observation of Cerenkov radiation. It builds on the mastering of the Imaging Atmospheric Cherenkov Telescope technique developed by the H.E.S.S. and MAGIC installations. From the successes of H.E.S.S. it exploits the concept of telescope arrays and stereoscopic analysis for improving the current sensitivity by one order of magnitude. From the success of MAGIC it exploits the use of large telescopes to attain the lowest possible threshold. Both approaches have proven to be extremely successful for gamma rays of energies above few tens of GeV and have wide-open a new window in astronomy: the detailed study of the universe at the largest energies to study the most extreme astrophysical phenomena and fundamental physics.
The main wishes of the European VHE gamma ray community to be fulﬁlled by CTA are:
• A wide energy coverage: four decades, from some 10 GeV to beyond 100 TeV
• A sensitivity at least one order of magnitude better than any existing installation: better than 1 miliCrab at the intermediate energies.
• Two observatories, operated under a common framework, for all-sky monitoring capability: a southern one mainly for galactic studies.
Reaching such features will require building observatories consisting in an array of several tens of Cherenkov Telescopes probably of two or three different sizes: few large telescopes in a compact conﬁguration for the lowest threshold, few tens of mid-size telescopes for the high-sensitivity intermediate-energy region, several tens of small telescopes spread in a large area for the for the highest energies. Actually, the present results obtained from Monte Carlo simulations validated with H.E.S.S. and MAGIC data suggest that such a system will probably consist of about 50 to 100 telescopes with a total of about 100 000 to 200 000 electronics channels and O(10 000) m2 total mirror area with a total cost estimate of O(150) MegaEuros.
CTA may discover and study in detail the spatial structure, light curve and energy spectra of around thousand sources and, given the fast variability of many VHE gamma ray sources, shall be in operation while Fermi-GLAST is still active since both installations nicely complement each other. Operational overlap with the Fermi satellite mission will provide seamless coverage of 20 octaves of the spectrum.
In 2008 the project met two very important milestones:
• On the one hand, in January 2008 the CTA consortium decided to meet the challenge of designing an installation fulﬁlling the above goals in a Design Study that had its kick-off meeting in Barcelona, co-organized by IFAE.
• On the other hand, by mid 2008 the project was recognized as one of the most important future European Scientific Infrastructures by including it in the ESFRI Roadmap. This together with the inclusion as high priority project in the European Astroparticle Physics roadmap by ASPERA and in the European Astrophysics roadmap by ASTRONE, makes CTA the most important future project in our field. IFAE members have played a very important role in all this process.
During 2008 the Design Study proceeded organized in twelve Work Packages. IFAE members participated actively in many of them and coordinated the one on Advanced Techniques for Atmospheric-monitoring and Calibration (ATAC).