The Cosmic Background Explorer (COBE) mission was launched in November 1989. The spacecraft contained liquid helium that cooled the DIRBE instrument to below 2K to allow it to image in the infrared wavelengths. Primary observation started December 11, 1989 and ran until September 21, 1990, when the liquid helium ran out. After that date only observations in the 1.25 to 4.9 micrometer bands could be carried out, at about 20% of original sensitivity. The DIRBE instrument was an absolute radiometer with an off-axis folded-Gregorian reflecting telescope, with 19 cm diameter aperture.