HEAO 2 (High-Energy Astronomy Observatory 2), also known as Einstein, was the second of three missions in a program of research in high-energy
astronomical phenomena. The specific objectives of this mission were imaging and
spectrographic studies of specific X-ray sources and studies of the diffuse X-ray
background. The spacecraft bus was identical to the HEAO 1
vehicle, with the addition of reaction wheels and associated electronics to enable the
telescope to be pointed at sources to within 1 min of arc. The instrument payload weighed
1450 kg.
A large grazing-incidence X-ray telescope provided images of sources that were then
analyzed by four interchangeable instruments mounted on a carousel arrangement that could
be rotated into the focal plane of the telescope. The telescope collected X-rays over an
angular range of approximately 1 deg × 1 deg, with the focal plane instruments determining
the limiting resolution up to a few arc-s for each measurement. The four instruments were
a solid-state spectrometer (SSS), a focal plan crystal spectrometer (FPCS), an imaging
proportional counter (IPC), and a high-resolution imaging detector (HRI). Also included
were a monitor proportional counter (MPC), which viewed the sky along the telescope axis,
a broadband filter, and objective grating spectrometers that could be used in conjunction
with focal plane instruments and an aspect system.
The scientific objectives were
- to locate accurately and examine X-ray sources in the energy range 0.2 to 4.0 keV. with
high resolution;
- to perform high-spectral-sensitivity measurements with both high-and low-dispersion
spectrographs; and
- to perform high-sensitivity measurements of transient X-ray behavior.
The spacecraft was a hexagonal prism 5.68 m high and 2.67 m in diameter. Downlink
telemetry was at a data rate of 6.5 kb/s for real-time data and 128 kb/s for either of two
tape recorder systems. An attitude control and determination subsystem was used to point
and maneuver the spacecraft. Gyros, sun sensors, and star trackers were employed as
sensing devices.
HEAO (High Energy Astronomical Observatory) 2 carried a X-ray telescope sensitive over
the approximate energy range 0.2-3.5 keV. which focused energy on following
instruments:
- High Resolution Imaging (HRI) - a digital X-ray camera which provided high spatial and
temporal resolution in the 0.15-3.0 keV.energy range;
- Imaging Proportional Counter (IPC) - a position sensitive proportional counter sensitive
between 0.4-4.0 keV.
- Solid State Spectrometer (SSS) - a cryogenically cooled lithium-drifted Si(Li) detector
with a range between 0.5-4.5 keV.
- Bragg crystal spectrometer (FPCS).
- Monitor Proportional Counter (MPC), which monitored the 1-20 keV.X-ray flux of the
source(s) being observed by the telescope
- Broad Band Filter (BBFS) spectrometer
- Objective Grating Spectrometer (OGS).
