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SAS-3
SAS-3 was launched May 7 1975 from the Italian San Marco launch
facility located off the coast of Kenya. Its initial orbit
was equatorial. The mission was designed as a spinning satellite,
but the spin rate was controlled by a gyroscope which could be
commanded to stop rotation. In this way, all instruments could be
pointed providing about 30 minutes of continuous data on a given
source such as a pulsar, burster, or transient.
The nominal spin period was 95
minutes, which was also the satellite orbital period.
The SAS-3 scientific payload was designed and built at MIT.
SAS-3 carried four X-ray experiments: modulation collimator,
slat collimators, tube collimators and soft X-ray concentrator.
The figure shows the location of each experiment
as mounted on the SAS-3 satellite.
The Z-axis is perpendicular to the solar panels.
Below follows a brief description of the onboard experiments:
- 2 rotating modulation collimator systems. These were composite of
a modulation collimator in front of a bank of proportional counters
that detect X-ray in the bands 2-6 and 6-11 keV.
The collimator has a transmission band with a FWHM of 4.5 arcmin
and an overall FOV of 12° X 12° centered on the
direction parallel to the spin axis (satellite Z-axis).
- 3 crossed slat collimators each with proportional counter.
They were designed to monitor a large portion of
the sky in a wide band of directions centered on the plane
perpendicular to the rotation axis of the satellite (+Z). Each
detector consisted of a proportional counter and collimator with
an on-axis effective area of 75 sq-cm. The collimators define 3
long, narrow fields of view which intersect on the +Y axis and are
inclined with respect to the YZ plane of the satellite at angles
of -30°, 0°, and +30°, respectively. During scanning
mode, an X-ray source would appear successively in the 3
detectors. Three lines of position could then be obtained, and
their intersection determined the true source position. The center
collimator had a field of view with FWHM 1° by 32° and a FW
of 2° by 120°. The left and right collimators had narrower,
but similar responses, i.e., 0.5° by 32° (FWHM) and 1.0°
by 100° (FW). The proportional counters were filled with argon
and were sensitive in the range 5-15 keV. In addition the center
detector had also a xenon counter, located behind the argon detector,
that extend the response to 60 keV
Over the energy range 1.5-6 keV, 1 count/s = 1.5x10-10
ergs/sq-cm/s for a Crab-like spectrum. In any given orbit, ~60% of
the sky was scanned by the center slat detector with an exposure
ranging from 300-1125 sq-cm sec.
- 3 tube collimators (which were located above, below, and to the
left of the slat collimators) that defined a 1.7 degree circular
FOV. The tube collimator located above the slat collimator was
inclined at an angle of 5 degrees above the Y-axis, and could
therefore be used as a background reference for the other tube
collimators that view along the Y-axis.
- 1 low-energy detector system to the right of the slat
collimators. It consisted of a set of 4 grazing incidence,
parabolic reflection concentrators with 2 independent gas-flow
counters sensitive to X-rays in the range 0.15-1.0 keV.
The main scientific hightlights of the SAS-3 mission
included: the discovery of a dozen X-ray burst sources
(including the Rapid Burster);
the discovery of X-ray emission from HZ 43 (isolated white dwarf),
Algol and from AM Her (the
first highly magnetic white dwarf binary
system seen in X-ray) precise location of about 60 X-ray sources.
This latter effort
established many new Be-star binaries as a class of X-ray emitters,
brought about the first identification of bursting sources with
X-ray binary systems, identified the first quasar located
through its X-ray emission, and demonstrated the central
location of X-ray sources in globular clusters.
[SAS-3 Home]
[About SAS-3]
[Archive]
[Gallery]
[Publications]
Page authors: Lorella Angelini Jesse Allen
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Last modified: Wednesday, 08-Oct-2003 19:02:43 EDT
HEASARC Staff Scientist Position - Applications are now being accepted for a Staff Scientist with significant experience and interest in the technical aspects of astrophysics research, to work in the High Energy Astrophysics Science Archive Research Center (HEASARC) at NASA Goddard Space Flight Center (GSFC) in Greenbelt, MD. Refer to the AAS Job register for full details.
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