Gamma-rays from celestial objects cannot reach the ground since they collide with atmosphere. Detectors on man-made satellites have been the tool to study gamma-rays of low and medium energies. Very-high-energy gamma-rays, on the other hand, initiate cascading showers of particles in the atmosphere and Cherenkov photons emitted by these showers can reach to the ground, which provide a method to detect gamma-rays indirectly. However, cosmic-rays bombarding the earth also generate similar showers and become backgrounds of Cherenkov observation. At the end of 1980's a clever method, called an imaging method, was developed to discriminate gamma-rays showers from backgrounds. Since then ground-based Cherenkov telescopes had begun reliable detections of very-high-energy gamma-rays.