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The Far Infrared Absolute Spectraphotometer (FIRAS) instrument, one of three on NASA's Cosmic Background Explorer ({\sl COBE}) satellite, was tasked with the detailed measurement of spectrum of the relict radiation of the big bang, the Cosmic Microwave Background (CMB). The instrument was a differential Michelson interferometer, covering a frequency range from 2 to 100 inverse centemeters (100 micrometers to 5000 micrometers in wavelength), with a spectral resolution of 0.4 inverse centimeters. Careful design and implementation resulted in a successful achievement of its mission: a detailed measurement of the CMB spectrum from 2 to 20 cm$^{-1}$ that exactly matched a blackbody form within 0.03\%\ of the peak of the spectrum. Limits on the amount of deviation from a blackbody have been used to strongly constrain possible sources of energy release in the very early universe. Improvements in the instrument model and data analysis suggest the possibility that further refinements in the measurement of the CMB spectrum may provide information about the ionization history of the intergalactic medium. In addition to the CMB, the FIRAS was able to map the emission of dust and atomic lines from our local galaxy. Models of this emission can be used to try to detect a possible isotropic far infrared background (FIRB). Conservative limits can be placed on the absolute amount of emission in such a background, particularly in the band from 20 to 40 cm$^{-1}$. A detailed understanding of the foregrounds is necessary in order to make an actual detection of a FIRB.
The National Aeronautics and Space Administration / Goddard Space Flight Center (NASA/GSFC) is reponsible for the design, development, and operation of the Cosmic Background Explorer. GSFC is also responsible for the development of the analysis software and for the production of the mission datasets. The COBE program is supported by the Astrophysics Division of NASA's Office of Space Science.
