artist concept of HEAO-1

* Mission Overview

The 3 HEAO satellites were completely dedicated to astronomical studies. Two of these were dedicated to X-ray astronomy: HEAO-1 - a spinning survey mission, and HEAO-2 (Einstein) - a pioneering imaging mission. HEAO-3 was instead dedicated to cosmic- and gamma-ray astronomy. The HEAO missions were launched on Atlas Centaur rockets. The payloads were about 2.5 X 5.8 m in size and ~3000 kg in mass. The telemetry rate was large, ~6400 bits per second compared to the less than ~1 kb/s typical of earlier satellites.

The HEAO-1 satellite was launched on 12 August 1977 into a nearly circular orbit with apogee 445 km, inclination 22.75°, and orbital period approximately 93 minutes. HEAO-1 was primarily a scanning mission; it rotated once per 30 minutes about the Earth-Sun line. In this manner, the instruments scanned a great circle in the sky that lay 90° from the Sun. As the Sun moved though the sky, the scan circle moved around the sky at 1° per day. The instruments had fields of view of order 1-4° (except for the 1° x 20° slat collimators of the high energy experiment). Thus a given source near the ecliptic was viewed for only a few days while sources near the ecliptic pole were scanned nearly continuously during the entire mission. In this manner a deep survey of the sky was obtained by each instrument over a six-month period. The sky was scanned in this way almost three times during the mission. The satellite had a limited pointing capability which was used on occasion during its final year to obtain continuous coverage of selected sources. It also had a high-telemetry-rate mode (128 kb/s) that was invoked for brief periods. On 9 January 1979, the satellite's attitude gas ran out and in March 1979 it re-entered the atmosphere.

HEAO-1 carried four instruments: the A1 instrument, also known as the NRL Large Area Sky Survey Experiment (LASS) achieved full sky coverage after the first 6 months of operation. Uniform exposure was not achieved during this period due to the failure of some of the detector modules. Ideal coverage was eventually obtained since 4 of the seven modules far exceeded their design lifetime; the A2 instrument also known as the Cosmic X-ray Experiment (CXE) (a collaborative effort led by E. Boldt (GSFC) and G. Garmire (Cal Tech/PSU)) consisting of 6 detectors covering 3 different energy ranges. The collimators were oriented so that the 3 degree angular response was always perpendicular to the scan plane. Thus, each rotation of the satellite scanned a great circle 3 degrees wide on the sky passing through the ecliptic poles; the A3 instrument also known as the MIT/SAO scanning Modulation Collimator (MC) was designed to measure the positions of X-ray sources with sufficient precision to identify optical and/or radio counterparts; the A4 experiment, also known as the UCSD/MIT Hard X-ray/Low-Energy Gamma-Ray Experiment, fanned circular beams across the sky of varying FWHM (up to 37 degrees).

* Instrumentation

The Large Area Sky Survey Experiment (A1) covered the energy range 0.25 to 25.0 keV. The experiment consisted of seven thin-window large aperture proportional counter modules with collimators of varying fields of view. Six of these modules were mounted on the -Y side of the spacecraft, the seventh on the +Y side. The Z-axis of the spacecraft pointed toward the Sun so the viewing directions of the seven A-1 detectors were roughly perpendicular to the solar direction. The experiment had sufficient sensitivity to detect sources as faint as 0.25 µJy at 5 keV for sources with a Crab-like spectrum. Data was collected in either a 5 or a 320 millisecond timing resolution mode: Full sky coverage for both time resolutions was achieved before the mission's end.

The Cosmic X-ray Experiment (A2) was designed to primarily study the large scale structure of the galaxy and the universe, yielding high quality spatial and spectral data over the energy range 2-60 keV. The experiment consisted of 6 separate multi-anode, multi-layer, collimated gas proportional counters covering three energy ranges. Two of the detectors, designated LEDs (Low Energy Detectors), were thin-window propane filled proportional counters, sensitive to X-rays from 0.15 - 3.0 keV, each had an open area of about 400 cm2. There was one MED (Medium Energy Detector) which consisted of an argon filled counter covering the energy range 1.5-20 keV. Finally, there were 3 HEDs (High Energy Detectors), which were xenon filled counters covering the range 2.5 - 60 keV. The MED and the three HEDs had roughly 800 cm2 open area each. The HEDs and the MED had various field of view combinations, 1.5° x 3°, 3° x 3° and 3° x 6° (FWHM), the collimators were oriented so that the 3° angular response was always perpendicular to the scan plane. Thus, each rotation of the satellite scanned a great circle 3 degrees wide on the sky passing through the ecliptic poles.

The MIT/SAO scanning Modulation Collimator (A3) instrument consisted of two four-grid modulation collimators. The angular response of the collimators is a series of transmission bands separated by 8 times the FWHM of each band. In this case the bands are 30" and 2´ FWHM, separated by 4´ and 16´, respectively for the individual collimators. Each collimator measures position in one dimension, perpendicular to its transmission bands. The X-rays were detected by four sealed proportional counters placed behind each collimator. The 30" collimator (MC1) had a net effective area of 400 cm2 maximum. One of the MC2 counters failed 2 weeks after launch giving it a net area of 300 cm2. The instruments' energy range was 0.9 - 13.3 keV according to pre-launch calibration. Its expected sensitivity of ~ 1 Uhuru count/s after two all-sky scans was sufficient to detect any of the then-known 1-10 keV sources. For the faintest sources, positional accuracy was projected to be ~2´.

The UCSD/MIT Hard X-ray/Low-Energy Gamma-Ray Experiment (A4), consisted of seven inorganic phoswich scintillator detectors surrounded by massive scintillators which served as active anti-coincidence against ambient radiations. The two Low Energy Detectors, optimized for the energy range 15 - 200 keV, were collimated with slats to a fan beam of 1.7° x 20° FWHM. The slats were inclined at +/- 30° to the satellite scan direction. The four Medium Energy Detectors, with a nominal energy range of 80 keV to 2 MeV, had a circular beam of 17° FWHM. The High Energy Detector had a nominal range of 120 keV to 10 MeV, and circular beam 37° FWHM.

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