Mission Overview

This mission was the third in the OAO program and its second successful spacecraft to observe the celestial sphere from above the earth's atmosphere. A UV telescope with a spectrometer measured high-resolution spectra of stars, galaxies, and planets with the main emphasis on the determination of interstellar absorption lines. Three X-ray telescopes and a collimated proportional counter provided measurements of celestial X-ray sources and interstellar absorption between 1 and 100 A.

The OAO 3 spacecraft was an octagonally shaped aluminum structure with a 1.21 m hollow central tubular area, which housed the experiment container. Solar panels were mounted on each side of the spacecraft at angles of 34 degrees and had an area of 38.2 square m. A sun baffle protected the experiments and increased the length of the spacecraft to 4.9 m. Two inertial balance booms, one forward and one aft, extended approximately 6.8 m.

The spacecraft was equipped with an inertial reference unit (a high-precision three-axis gyro inertial system), sun sensors, a magnetometer, and star trackers, which enabled spacecraft pointing to be determined in many different ways. A boresight star tracker, sensitive to sixth magnitude, controlled pitch and yaw to within 5 arc-s. In addition, the high-resolution telescope experiment had a fine pointing control, which could control the pitch and yaw to within 0.1 arc-s on bright stars. Spacecraft attitude was controlled by inertia wheels and thrusters.

Redundant tracking beacons facilitated ground tracking of the spacecraft. Two UHF (400.55 MHz) transmitters provided wideband telemetry for transmitting digital data to the ground stations. Two redundant VHF (136.26 MHz) transmitters were used in a narrow-band telemetry link primarily for transmitting spacecraft housekeeping data, although they served as backups for the wideband telemetry system. Two redundant pairs of VHF command receivers were carried as part of command system capable of storing 1280 commands. Data were stored on an onboard tape recorder and in core storage. An onboard processor monitored telemetry data, issued commands, and was programmed via the command receiver uplink.

The observational life of the mission was August 1972 to February 1981 (9.5 years).

Launch Information

Launch Date: 1972-08-21 at 10:28:00 UTC
Launch Vehicle: Atlas-Centaur
Launch Site: Cape Canaveral, United States

Trajectory Description
Type: Orbiter
Central Body: Earth
Epoch start: 1972-08-21 10:28:00 UTC

Orbital Parameters
Periapsis 739.0 km
Apoapsis 751.0 km
Period 99.69999694824219 minutes
Inclination 35.0째
Eccentricity 8.420000085607171E


High-Resolution Telescopes

The primary objective of this Princeton telescope-spectrometer was to make quantitative observations of interstellar absorption lines with a resolution of about 0.05 A between 950 and 1450 A, and twice in first order between 1650 and 3000 A. The secondary objective was to observe UV spectra of selected brighter stars. The prime optical system was an 80-cm diameter Cassegrain telescope with a 16-m focal length (f/20). This telescope was coupled to a Paschen-Runge spectrometer with a concave grating which focused the spectrum on a 1-m Rowland circle, with a dispersion of 4.2 A per mm in first order. The entrance slit was 3 mm long and 24.2 microns wide, corresponding to 390 arc-s by 0.314 arc-s on the sky. The Bausch and Lomb grating was ruled with 2400 lines per mm, blazed for 2200 A in first order. The photons were detected by four phototubes, each equipped with its own exit slit and movable in pairs along the Rowland circle. A guidance error sensor attached to the prime optics controlled the spacecraft attitude to within 0.1 arc-s. This guidance system locked onto a star as faint as 7th magnitude. The overall system could make useful measurements on O-and B-type stars to 7th magnitude.

Stellar X-Rays

The University College London/Mullard Space Science Laboratory experiment used three grazing-incidence paraboloid mirror systems and one collimated proportional counter to observe celestial X-ray sources between 1 and 70 Angstroms. Between 1 and 3 Angstroms, the collimated proportional counter was used in conjunction with pulse-shape discrimination to reject background counts. The window was 0.51-mm beryllium, and the whole area of 17.8 sq cm collected radiation from a source on the axis of its collimating tube. From 3 to 9 Angstroms and 6 to 18 Angstroms, proportional counters located at the focus of two grazing-incidence reflecting telescopes, whose photon collect areas were 5.5 sq cm and 12.5 sq cm, respectively, were used with an anticoincidence scintillator also employed to reject background cosmic-ray counts. Their stellar windows were 3 mm in diameter and were of 76-micron beryllium and 5-micron aluminium, respectively.

A channel photomultiplier, located at the focus of a grazing-incidence telescope with a photon collecting area of 23 sq cm, was used to observe between 20 and 70 Angstroms. The channel photomultiplier suffered from high UV background, and provided no useful data. A six-channel pulse-height analyzer could be switched to any of the three proportional counters to improve the energy resolution. An image scanning photomultiplier tube behind a three-element lens of 610-mm focal length was an auxiliary system for measuring misalignments of the Mullard experiment from the pointing direction of the spacecraft and the Princeton spectrometer. The 3 to 9 Angstroms and 6 to 18 Angstroms systems became inoperable in June 1973 when the background shutter stuck in the closed position. Most of the observations after this were made with the 1 to 3 Angstroms system.