Many years ago Danaher et al. (1982) proposed to use the mirrors of a heliostat field for the collection of the air-Cherenkov light, converting a solar power plant in a ground-based gamma-ray telescope with low energy threshold. Later on Tumer et al. (1990) envisaged an optics which images the Cherenkov light from every individual heliostat in a single PMT minimizing in this way the NSB contribution. The technical details of this device were finally solved and presently the following projects are using this technique:
 

    The main difference of GRAAL with respect to the above experiments is that the light reflected by groups of heliostats (ranging from 13 to 18 heliostats per group) is collected in the central tower by a non-imaging optical concentrator (Winston cone) into a single PMT. GRAAL is a much simpler and lower cost approach at the price of a higher energy threshold but still reaching the 200 GeV energy range.