An instrument for recording gamma rays from celestial objects. Direct detection of gamma rays at low energies (100 keV to 10 MeV) makes use of coded masks or collimators coupled with solid-state or gas-filled detectors, such as NaI detectors or proportional counters. Between 20 MeV and 30 GeV, spark chambers and NaI detectors are used. At higher energies the gamma rays are detected indirectly, through the Cerenkov radiation emitted by secondary electrons produced after the gamma rays strike the atmosphere.