artist concept of SAS-3


* Mission Overview

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. Diagram of SAS-3 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.

* Science

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.

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Last modified: Wednesday, 08-Oct-2003 19:02:43 EDT