Cosmic Background Explorer/FIRAS
FIRAS instantaneous Beam Profile
A representative instantaneous FIRAS beam radial profile was determined using multiple on-orbit observations of the Moon. In practice, the finite dwell time of each FIRAS interferogram results in an effective beam which is elongated in the scan direction. The in-scan elongation is not incorporated in the tabulated instantaneous beam.
For an example of calibration comparison between FIRAS and another instrument, including treatment of the beam, see Fixsen et. al (1997).
FIRAS Explanatory Supplement, Chapter 7 "Errors and Uncertainty Estimation"