HEAO 2

Description

The Einstein Observatory (HEAO 2) was the second of three missions in a program of research in high-energy astronomical phenomena. The science objectives of this mission were imaging and spectrographic studies of specific X-ray sources and studies of the diffuse X-ray background. More specifically, scientific objectives were (1) to locate accurately and examine X-ray sources in the energy range 0.2 to 4.0 keV, with high resolution; (2) to perform high-spectral-sensitivity measurements with both high-and low-dispersion spectrographs; and (3) to perform high-sensitivity measurements of transient X-ray behavior.

The Einstein Observatory spacecraft 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 spacecraft was a hexagonal prism 5.68 m high and 2.67 m in diameter. 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 x 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.

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. For more details, see R. Giacconi et al., Astropy. J., v. 230, p. 540, 1979.

The Einstein Observatory was shut down on April 26, 1981. It remained in orbit for another year slowly spiraling in to the Earth, and reentered on March 25, 1982.