Experimental upper limits of the high-energy gamma-ray flux (above 100 MeV) from point sources are about 10−5 cm−2 s−1, and realistic theoretical predictions for the strongest sources are an order of magnitude lower than this, while the diffuse background flux is about 4 × 10−5 cm−2 s−1 sr−1 in space, and 100 times higher at balloon altitudes. To meet the need for instrumental sensitivity and angular resolution adequate to measure the small but important gamma-ray source strengths, a telescope of large area (5 m2) and fine angular discrimination (0.5 deg at 300 MeV, 0.3 deg at energies above 1 GeV) has been developed.