Calibration of the Far Infrared Absolute Spectrophotometer ( FIRAS ) on the Cosmic Background Explorer ( COBE 1 )

The FIRAS instrument on the COBE satellite was designed to accurately measure the spectrum of the cosmic microwave background radiation (CMBR) in the frequency range 1 to 95 cm ?1 with an angular resolution of 7. We describe the calibration of this instrument, including the method of obtaining calibration data, reduction of data, the instrument model, tting the model to the calibration data, and application of the resulting model solution to sky observations. The instrument model ts well for calibration data which resemble sky conditions. The method of propagating detector noise through the calibration process to yield a covariance matrix of the calibrated sky data is described. The nal uncertainties are variable both in frequency and position, but for a typical calibrated sky 2:6 square pixel and 0.7 cm ?1 spectral element the random detector noise limit is of order of a few times 1 The National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC) is responsible for the design, development, and operation of the Cosmic Background Explorer (COBE). Scientiic guidance is provided by the COBE Science Working Group. GSFC is also responsible for the development of the analysis software and for the production of the mission data sets. { 2 { 10 ?7 erg cm ?2 s ?1 sr ?1 cm for 2 to 20 cm ?1 , and the diierence between the sky and the best t cosmic blackbody can be measured with a gain uncertainty of less than 3%.