CALIBRATION OF THE COBE FIRAS INSTRUMENT

The FIRAS instrument on the COBE satellite was designed to accurately measure the spectrum of the cosmic microwave background radiation (CMB R) in the frequency range 1-95 cm(-1) with an angular resolution of 7 degrees. We describe the calibration of this instrument, including the method of obtaining calibration data, reduction of data, the instrume nt model, fitting the model to the calibration data, and application o f the resulting model solution to sky observations. The instrument mod el fits well for calibration data that resemble sky conditions. The me thod of propagating detector noise through the calibration process to yield a covariance matrix of the calibrated sky data is described. The final uncertainties are variable both in frequency and position, but for a typical calibrated sky 2 degrees 6 square pixel and 0.7 cm(-1) s pectral element the random detector noise limit is of order of a few t imes 10(-7) ergs cm(-2) s(-1) sr(-1) cm for 2-20 cm(-1), and the diffe rence between the sky and the best-fit cosmic blackbody can be measure d with a gain uncertainty of less than 3%.