HEAO 1

HEAO 1 was the first High Energy Astronomy Observatory, a satellite launched in 1977 to survey the high-energy sky. Its instruments covered complementary X-ray and gamma-ray bands, allowing a catalog of discrete sources and measurements of diffuse backgrounds.

Its A2 experiment measured the cosmic X-ray background, while scanning observations distinguished sky-dependent signals from detector contributions. This illustrates a central survey problem: an instrument must characterize its own response and background before converting counts into an astronomical brightness distribution. HEAO 1 was one mission, distinct from the later observatories in the HEAO series.^[ref]