Orbiting Astronomical Observatory 2

The Orbiting Astronomical Observatory 2 (OAO-2, nicknamed Stargazer) was a space observatory launched on December 7, 1968. An Atlas-Centaur rocket launched it into a nearly circular 750-kilometre (470 mi) altitude Earth orbit. Data was collected in ultraviolet on many sources including comets, planets, and galaxies. It had two major instrument sets facing in opposite directions; the Smithsonian Astrophysical Observatory (SAO) and the Wisconsin Experiment Package (WEP). One discovery was large halos of hydrogen gas around comets, and it also observed Nova Serpentis.

The Smithsonian Astrophysical Observatory, also called Celescope, had four 12 inch Schwarzschild telescopes that fed into Uvicons. Various filters, photocathodes, and electronics aided in collecting data in several ultraviolet light passbands. The experiment was completed in April 1970. By the time it finished about 10 percent of the sky was observed.

The Wisconsin Experiment Package had eleven different telescopes for ultraviolet observations. For example, there was a photoelectric photometer fed by a 16 inch telescope with a six-position filter wheel. WEP observed over 1200 targets in ultraviolet light before the mission ended in early 1973.