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

The photometric errors of the external calibrator for the Far Infrared Absolute Spectrophotometer (FIRAS) instrument on the COBE are smaller than the measurement-errors on the cosmic microwave background radiation (CMBR) spectrum (typically 0.02 MJy sr(-1), 1 sigma) and smaller than 0.01% of the peak 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 caused by reflections, temperature gradients, and leakage through the material 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 +/- 0.002 K.