An Overview of the Orbiting Astronomical Observatory

The world's first Orbiting Astronomical Observatory, OAO-II, was launched by the U.S.A. on December 7, 1968. Over one year of operation has proven its design concepts and utility. Built in an octahedron shape, 10 feet by 7 feet, the satellite weighs 4400 pounds. Two experiments are located centrally within the space-craft, each viewing space from opposite ends. Both experiments contain multiple optical and detector systems for spectrophotometry and surveys in the ultraviolet spectrum. The design was dictated by the stringent requirements of the experiments for pointing accuracy (one minute of arc anywhere in the celestial sphere), pointing stability (five seconds of arc), command capability, data handling, thermal environment, and the constraints of the orbit for ground-station contacts. Six two-gimbal star trackers are programmed by the on-board memory to acquire and track appropriate guide stars. Error signals are generated which drive the reaction wheels to obtain stellar stabilization. Coarse momentum wheels are used for slewing the spacecraft. Memories permit storage of 200 000 bits of experimental data. All information is relayed to the central control station in Greenbelt, Md. OAO-II has shown that it is practical to maintain a continuous viewing capability in space for long periods of time so that fruitful astronomical experiments can be performed.