Gamma ray telescope. Cerenkov telescope at the High Energy Gamma Ray Astronomy (HEGRA) site at the Roque de los Muchachos Observatory on La Palma in the Canary Islands. Cerenkov radiation is the pulses of blue light given off by electrons when a high-energy gamma ray from space strikes the atmosphere. These gamma rays have up to 10 thousand billion electron volts of energy. This instrument was one of the world's most sensitive Cerenkov detectors, using the array of mirrors to focus the incoming light flashes onto a detector (upper right). This device, one of several at the HEGRA site, ceased operations in September 2002.