HEAO-1 was an X-ray telescope launched in 1977. HEAO-1 surveyed the sky in the X-ray portion of the electromagnetic spectrum (0.2 keV – 10 MeV), providing nearly constant monitoring of X-ray sources near the ecliptic poles and more detailed studies of a number of objects by observations lasting 3–6 hours. It was the first of NASA's three High Energy Astronomy Observatories, HEAO 1, launched August 12, 1977 aboard an Atlas rocket with a Centaur upper stage, operated until 9 January 1979. During that time, it scanned the X-ray sky almost three times HEAO included four X-ray and gamma-ray astronomy instruments, known as A1, A2, A3, and A4, respectively (before launch, HEAO 1 was known as HEAO A). The orbital inclination was about 22.7 degrees. HEAO 1 re-entered the Earth's atmosphere on 15 March 1979. A1: Large-Area Sky Survey instrument The A1, or Large-Area Sky Survey (LASS) instrument, covered the 0.25–25 keV energy range, using seven large proportional counters. It was designed, operated, and managed at the Naval Research Laboratory (NRL) under the direction of Principal Investigator Dr. Herbert D. Friedman, and the prime contractor was TRW. The HEAO A-1 X-Ray Source Catalog included 842 discrete X-ray sources. A2: Cosmic X-ray Experiment The A2, or Cosmic X-ray Experiment (CXE), from the Goddard Space Flight Center, covered the 2–60 keV energy range with high spatial and spectral resolution. The Principal Investigators were Dr. Elihu A. Boldt and Dr. Gordon P. Garmire. A3: Modulation Collimator instrument The A3, or Modulation Collimator (MC) instrument, provided high-precision positions of X-ray sources, accurate enough to permit follow-up observations to identify optical and radio counterparts. It was provided by the Center for Astrophysics (Smithsonian Astrophysical Observatory and the Harvard College Observatory, SAO/HCO). Principal Investigators were Dr. Daniel A. Schwartz of SAO and Dr. Hale V. Bradt of MIT. A4: Hard X-Ray / Low-Energy Gamma-ray experiment The A4, or Hard X-ray / Low Energy Gamma-ray Experiment, used sodium iodide (NaI) scintillation counters to cover the energy range from about 20 keV to 10 MeV. It consisted of seven clustered modules, of three distinct designs, in a roughly hexagonal array. Each detector was actively shielded by surrounding CsI scintillators, in active-anti-coincidence, so that an extraneous particle or gamma-ray event from the side or rear would be vetoed electronically, and rejected. (It was discovered in early balloon flight by experimenters in the 1960s that passive collimators or shields, made of