|
HEAO-1
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).
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.
[HEAO-1 Home]
[About HEAO-1]
[Archive]
[Software]
[Gallery]
[Publications]
Page authors: Lorella Angelini Jesse Allen
HEASARC Home |
Observatories |
Archive |
Calibration |
Software |
Tools |
Students/Teachers/Public
Last modified: Tuesday, 21-Apr-2020 13:01:07 EDT
|