 Far Infrared Absolute Spectrophotometer

The Far Infrared Absolute Spectrophotometer measured the spectrum of the cosmic microwave background by comparing the sky with a calibrated reference. Its differential Fourier transform spectroscopy and blackbody calibration tested whether the radiation followed one Planck function, rather than only measuring differences between sky directions.

The key observable is a spectral residual after fitting a temperature and accounting for instrumental and foreground contributions. A small residual constrains departures from a blackbody spectrum and thus energy exchange in the early universe. This absolute spectral test is distinct from the directional map supplied by the Differential Microwave Radiometer.^[ref]