The CELESTE experiment will be an Atmospheric Cherenkov detector designed to bridge the gap in energy sensitivity between current satellite and ground-based gamma-ray telescopes, 20 to 300 GeV. We present test results made at the former solar power plant, Themis, in the French Pyrenees. The tests confirm the viability of using a central tower heliostat array for Cherenkov wavefront sampling. 1 e-mail: giebels@cenbg.in2p3.fr 2 Visiting from McGill University, Montreal, Canada. 3 Deceased. Preprint submitted to Elsevier Preprint 27 October 1 INTRODUCTION The CELESTE experiment uses the Electricit'e de France central receiver solar power plant at Themis (N. 42.50 ffi , E. 1.97 ffi , 1650 m. a.s.l.) as a gamma-ray telescope [1]. The project is fully funded and will begin observations in early 1998 with 18 heliostats, growing to 40 in the following year. Figure 1 sketches the principal of the CELESTE approach. This paper describes test results accumulated during the design and co...