Design for the COBE1 Far Infrared Absolute Spectrophotometer (FIRAS)

The Far InfraRed Absolute Spectrophotometer (FIRAS) was built to measure the spectrum of diiuse emission from 1 to 100 cm ?1 , with particular attention to possible diierences between the spectrum of the cosmic microwave background radiation (CMBR) and a blackbody spectrum as small as 0.1% of the peak of the CMBR spectrum. The FIRAS has diierential inputs and outputs, a full beam external calibrator, a controllable reference blackbody, and a polarizing Michelson interferometer with bolometer detectors. It operated at a temperature of 1.5 K inside a liquid helium cryostat to suppress instrument emission and improve detector sensitivities. It has an intrinsic frequency resolution of the order of 0.7%, maximum path lengths of 1.2 and 5.9 cm, and a beamwidth of 7 , and achieved its goals for accuracy and rms sensitivity for I , which are better than 10 ?9 W/cm 2 sr over the frequency range from 2 to 20 cm ?1 .