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The cosmic X-ray background is the aggregate X-ray radiation received from the sky after accounting for foreground and instrumental contributions. In an instrument with limited angular resolution, many distant sources contribute an apparently diffuse signal. Resolving more sources changes the resolved fraction without making their summed radiation cease to be a background.

A measurement therefore requires both an energy spectrum and control of detector backgrounds and sky coverage. The HEAO 1 A2 experiment measured the cosmic background in the 3–50 keV band, providing a benchmark for separating a broad background spectrum from individual sources.^[ref]

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]