TheOrbiting Astronomical Observatory 2 (OAO-2, nicknamedStargazer) was the first successfulspace telescope (first space telescope beingOAO-1, which failed to operate once in orbit), launched on December 7, 1968.^[3] AnAtlas-Centaur rocket launched it into a nearly circular 750-kilometre (470 mi) altitude Earth orbit.^[4] Data was collected in ultraviolet on many sources including comets, planets, and galaxies.^[3][5] It had two major instrument sets facing in opposite directions; the Smithsonian Astrophysical Observatory (SAO) and the Wisconsin Experiment Package (WEP).^[5] One discovery was large halos of hydrogen gas around comets,^[5] and it also observedNova Serpentis, which was a nova discovered in 1970.^[3]

Orbiting Astronomical Observatory 2

Technicians in a clean room at NASA’s Kennedy Space Center in Cape Canaveral, Florida, check out the Orbiting Astronomical Observatory 2 before the mission’s Dec. 7, 1968, launch.
Names	OAO-A2, OAO2
Mission type	Astronomy
Operator	NASA
COSPAR ID	1968-110A
SATCATno.	3597
Spacecraft properties
Manufacturer	Grumman
Dry mass	2,012 kilograms (4,436 lb)
Start of mission
Launch date	7 December 1968, 08:40:09 (1968-12-07UTC08:40:09) UTC
Rocket	Atlas SLV-3C Centaur-D
Launch site	Cape CanaveralLC-36B
End of mission
Disposal	Telescope issues
Deactivated	February 1973 (1973-03)[1]
Orbital parameters
Reference system	Geocentric
Regime	Low Earth
Perigee altitude	768 kilometres (477 mi)
Apogee altitude	777 kilometres (483 mi)
Inclination	35.0 degrees
Period	100.30 minutes
Epoch	6 January 1969[2]
← OAO-1 OAO-3 →

The Smithsonian Astrophysical Observatory, also called Celescope, had four 12 inch (30.5 cm) Schwarzschild telescopes that fed into Uvicons.^[6] The Uvicon was an ultra-violet light detector based on the WestinghouseVidicon.^[7] Ultraviolet light was converted intoelectrons which were in turn converted to a voltage as those electrons hit the detection area of the tube.^[8] There has been a Uvicon in the collection of theSmithsonian Institution since 1973.^[7]

Various filters,photocathodes, and electronics aided in collecting data in severalultraviolet lightpassbands.^[6] The detectors showed a gradual loss of sensitivity^[9] and the experiment was turned off in April 1970.^[6] By the time it finished about 10 percent of the sky was observed^[6] resulting in a catalog of 5,068 UV stars.^[9]

The Wisconsin Experiment Package had seven different telescopes for ultraviolet observations.^[10] For example, there was a nebular photoelectric photometer fed by a 16-inch (40.64 cm) telescope with a six-position filter wheel^[10] that unfortunately failed a few weeks after launch.^[9]

Construction was supervised byArthur Code of theUniversity of Wisconsin-Madison.^[9] WEP observed over 1200 targets in ultraviolet light before the mission ended in early 1973.^[5]

In addition to the Celescope's catalog of UV stars, the WEP observedcomet Tago-Sato-Kosaka and found it to be surrounded by a cloud of hydrogen, confirming that the comet was largely made up of water, and detectedthe 2175-angstrom bump, an increase in UV absorption at that wavelength that is still not fully explained.^[9]

The observatory was built in the shape of anoctagonal prism. It measured about 10 by 7 ft (3.0 by 2.1 m) and weighed 4,400 lb (2,000 kg).^[11]

• Orbiting Astronomical Observatory
• Orbiting Solar Observatory

• OAO 2 observations of the Alpha Persei cluster
• OAO-2 Info and pics
• 50th Anniversary Overview of OAO-2 including video