Design for the COBE far-infrared absolute spectrophotometer (FIRAS)

摘要

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