Mission Overview

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.

Launch Date: 1978-11-13 at 05:24:00 UTC
Launch Vehicle: Atlas-Centaur
Launch Site: Cape Canaveral, United States
Decay Date: 1982-03-25
Trajectory Details
Type: Orbiter
Central Body: Earth
Epoch start: 1978-11-13 05:24:00 UTC
Orbital Parameters
Periapsis 465.0 km
Apoapsis 476.0 km
Period 94.0 minutes
Inclination 23.5°
Eccentricity 8.030000026337802E

Instrumentation

Monitor Proportional Counter (MPC)

This experiment consisted of a proportional counter that viewed space through a collimator coaligned to the high-resolution telescope. The system had an X-ray collimator, a thermal impedance covering the spacecraft viewing aperture, and an inflight calibration system. The active area was 667 sq cm, the spatial resolution 1.5 x 1.5 deg FWHM, and the temporal resolution 256 s. For more details, see T. L. Cline et al., Astrophy. J., v. 255, pp. L45-L48, 1982. This experiment was equipped with three identical HRI detectors. The HRI was a digital X-ray camera that provided high spatial and temporal resolution over the central 25 arc-min of the telescope focal plane. It was composed of two microchannel plates operating in cascade, a cross-grid charge detector, and a set of electronics. It had a spatial resolution of 1 arc-s, a temporal resolution of 7.8125 microseconds, and an energy range of 0.15 to 3.0 keV. Spectral studies could be performed using the interchangeable broadband filter and the objective grating.

High-Resolution Imager (HRI)

This experiment was equipped with three identical HRI detectors. The HRI was a digital X-ray camera that provided high spatial and temporal resolution over the central 25 arc-min of the telescope focal plane. It was composed of two microchannel plates operating in cascade, a cross-grid charge detector, and a set of electronics. It had a spatial resolution of 1 arc-s, a temporal resolution of 7.8125 microseconds, and energy range of 0.15 to 3.0 KeV. Spectral studies could be performed using the interchangeable broadband filter and the objective grating.

Focal Plane Crystal Spectrometer (FPCS)

The FPCS was a curved crystal Bragg spectrometer with a thin-window, gas-filled proportional counter as a position-sensitive detector. There were two identical counters for redundancy, and sufficient gas was carried to compensate for differential leakage through the windows. Six different crystal diffractors were available. The spectrometer and detector had an imaging capability with available apertures of 3 x 30, 2 x 20, and 1 x 20 arc-min, and a 6-arc-min diameter. The instrument could be operated as a conventional curved-crystal spectrometer or used in a modified defocused mode to achieve higher resolution.

Imaging Proportional Counter (IPC)

The IPC was a position-sensitive proportional counter that provided good efficiency and full focal-plane coverage with a 75 x 75-arc-min FOV and an effective area of approximately 100 sq cm. It had a spatial resolution of 1 arc-min, a temporal resolution of 63 microseconds, and 32 energy channels in the range of 0.15 to 4.0 keV. Two identical counters were included for redundancy, plus a background counter for anticoincidence and an inflight calibration system.

Solid-State Spectrometer (SSS)

This instrument was a cooled solid-state spectrometer and was used to detect weak sources and weak spectral features over a broad band of energies by employing a nondispersive spectral technique. A lithium-drifted, solid-state detector was operated at a temperature of 120 K. The primary detector was 9 mm in diameter and was surrounded by two veto guard counters. A two-stage solid cryogenic refrigerator was used to cool the detector. Spectral measurements were made between 0.4 and 4 keV, with a resolution from 120 to 150 eV, FWHM. The effective area was 200 sq cm, the FOV 6 arc-min in diameter, and the time resolution 2 to 5 microseconds. Observations with the instrument were terminated when the supply of the solid ammonia-methane cryostat was expended and operating temperatures could no longer be maintained.

Science

Summary