Launched on 13 November 1978 from Cape Canaveral Florida on an
Atlas-Centaur SLV-3D, HEAO-B (later to become known as the Einstein
Observatory) carried the first imaging, extra-solar X-ray telescope
(PI, R. Giacconi). A set of 4 Wolter type 1 nested mirrors focused
X-rays up to 8 keV in energy. Angular resolution was 5 arcseconds on
axis, degrading to 1.5 arcminutes at the edge of the 1 degree field of
view (see details in Giacconi et al. 1979, ApJ, 230, 540). The
Einstein Observatory satellite re-entered the Earth's atmosphere and
burned up on March 25, 1982
Einstein employed a turntable by which different detectors could be
placed at the telescope focus. X-ray imaging was the principal aim of
the mission. Both an imaging proportional counter (IPC) and
micro-channel plate detector
(HRI) were included.
The HRI's angular resolution was that of the mirrors, but energy resolution was poor.
The IPC recorded the energy of each event but angular resolution was
approximately 40 arcseconds at best. A third detector, the solid
state spectrometer (SSS) provided a factor of approximately 3 better
energy resolution but no spatial information since it recorded all
events occurring in its 6 arcminute field of view. A monitoring
proportional counter (MPC) was coaligned with the telescope to monitor
variability of the brighter X-ray sources.
The observatory included two high-resolution spectrometers. The
objective grating spectrometer (OGS) was a set of gold diffraction
gratings which could be inserted into the telescope light path. The
dispersed spectrum was recorded by one of the imagers. The
focal-plane crystal spectrometer (FPCS) was a curved-crystal Bragg
spectrometer which operated at the telescope focus. Effective area of
these spectrometers was very small (about 1 sq. cm or less), so they
were used on only the brightest sources.
The SSS had a lifetime of 11 months due to a limited onboard cryogen
supply. The observatory itself lost its ability to point after only
2.5 years in orbit. Operations ceased in April 1981 after the
exhaustion of the attitude-control gas. Einstein accomplished 5600
observations. By virtue of its ability to focus X-rays from point
sources onto a small region of a detector and as a result of the much
lower background associated with the small region of interest, the
Einstein Observatory sensitivity was several hundred times greater
than any previous X-ray mission. It was the first X-ray NASA mission
to have a Guest Observer program.
With the dramatic improvement in sensitivity and the ability to image
with angular resolution of a few arcseconds, for the first time
astronomers studied the structure of supernova remnants, detected
unexpectedly strong coronal emission from normal stars, resolved the
distribution of compact sources in external galaxies (M31 and the
Magellanic Clouds), detected hot gaseous coronae in early type
galaxies, detected cooling gas in the cores of galaxy clusters,
discovered X-ray jets co-aligned with radio jets, and found extensive
structure in the hot gas of galaxy clusters. Deep observations of
blank fields resolved 25% of the diffuse background into discrete
active galactic nuclei. The Einstein Observatory made unique
contributions to the study of all classes of astronomical objects and
paved the way for future, more powerful missions.