Calibrator design for the COBE far infrared absolute spectrophotometer (FIRAS)

The photometric errors of the external calibrator for the Far Infrared Abso lute Spectrophotometer (FIRAS) instrument on the COBE are smaller than the measurement-errors on the cosmic microwave background radiation (CMBR) spec trum (typically 0.02 MJy sr(-1), 1 sigma) and smaller than 0.01% of the pea k brightness of the CMB. The calibrator is a reentrant cone, shaped like a trumpet mute, made of Eccosorb iron-loaded epoxy. It fills the entire beam of the instrument and is the source of its accuracy. Its known errors are c aused by reflections, temperature gradients, and leakage through the materi al and around the edge. Estimates and limits are given for all known error sources. Improvements in understanding the temperature measurements of the calibrator allow an improved CMB temperature determination of 2.725 +/- 0.0 02 K.