Cosmic gamma-rays arriving to the Earth are absorbed by the atmosphere and, thus, gamma-ray telescopes are usually installed aboard satellites. Satellite-based gamma-ray astronomy has progressed significantly in the last years since the launch of the CGRO in 1991. In particular the EGRET detector has observed more than 350 sources in the energy range 10 MeV - 20 GeV. Beyond this upper limit, gamma-ray fluxes are not enough to get sufficient statistics because of the limited collection area available on a satellite. In the future the GLAST mission will be able to reduce significantly this lower limit.

        Likely the interaction of a high energy gamma-ray with the atmosphere produces a particle shower which can be observed at ground by the appropriate detectors. In particular these Extensive Air Showers EAS contains many high-energy electrons and positrons which generate Cherenkov radiation. Cherenkov photons (wavelength range around blue and near UV) arrive to the ground as a particle front about 1 m width and more than 100 m radius. A small size detector at ground "sees" this front as a fast light pulse of around 3 ns duration. The so-called Cherenkov telescopes are able to detect the Cherenkov pulses coming from the direction of the cosmic source. At low primary energies the Cherenkov pulses are very week (similar to the Poissonian fluctuations of the light of the night sky) and, therefore, the sensitivity of these telescopes is severely limited by the mirror area. A natural procedure for decreasing the energy threshold of a Cherenkov telescope is to increase the mirror size. Unfortuntely the night sky background NSB also increases with the mirror size. Since the Poissonian fluctuations grows with the square root of the NSB intensity, the S/N ratio for the detection of the Cherenkov light increase with the square root of the mirror area. Consecuently, very big collection areas are needed for a significant decrease in the energy threshold.