  The photometric errors of the external calibrator for the FIRAS instrument on
the COBE are smaller than the measurement errors on the cosmic microwave
background (CMBR) spectrum (typically 0.02 MJy/sr, 1 sigma), and smaller than
0.01% of the peak brightness of the CMBR. The calibrator is a re-entrant 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 CMBR temperature determination of 2.725 +/-
0.002 K.
