O ct 1 99 8 Calibrator Design for the COBE 1 Far Infrared Absolute Spectrophotometer ( FIRAS )

(NASA/GSFC) was responsible for the design, development, and operation of the Cosmic Background Explorer (COBE). GSFC was also responsible for the development of the analysis software and for the production of the mission data sets. The COBE program was supported by the Astrophysics Division of NASA's Office of Space Science and Applications. ABSTRACT 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 σ), 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.