OAO 3 (Orbiting Astronomical Observatory), also known as Copernicus, 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).
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