  The gamma-ray energy region between 20 and 250 GeV is largely unexplored.
Ground-based atmospheric Cherenkov detectors offer a possible way to explore
this region, but large Cherenkov photon collection areas are needed to achieve
low energy thresholds. This paper discusses the development of a Cherenkov
detector using the heliostat mirrors of a solar power plant as the primary
collector. As part of this development, we built a prototype detector
consisting of four heliostat mirrors and used it to record atmospheric
Cherenkov radiation produced in extensive air showers created by cosmic ray
particles.
