Gamma ray astronomy. 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 creat- ed 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 is one of the world's most sensitive Cerenkov detectors, using an array of small mirrors to focus the incoming light flashes onto a detector at upper left. HEGRA has many in- struments, of several types, spread out over hund- reds of square metres. Photographed in early 1997.