The COMPTEL Experiment and Its In-Flight Performance

COMPTEL, the only orbiting Compton Telescope since the start of the space age, measured steady and transient γ-ray sources in the 0.75–30 MeV energy range. The objects studied include the galactic plane, pulsars, quasars, black hole systems, novae, supernovae, the cosmic diffuse γ glow, and the Sun. It also made groundbreaking measurements of solar neutrons from flares. Its design was state of the art, providing the best available signal-to-noise ratio and the necessary effective area to conduct these observations. We discuss the features, subtleties, and nuances of COMPTEL’s time in space that directly affected its ability to measure and deduce the nature of the wide variety of energetic objects in the sky. We conclude that even after more than nine years in orbit, it was still fully capable of conducting measurements of transient objects, as it did in the earliest phases of the mission. However, we also conclude that the effect of several forms of background was underestimated, limiting the sensitivity of deep searches and long exposures.