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Far Infrared Absolute Spectrophotometer (FIRAS)

NSSDCA ID: 1989-089A-01

Mission Name: COBE
Principal Investigator:Dr. John C. Mather


The far-IR absolute spectrophotometer (FIRAS) is a cryogenically cooled polarizing Michelson interferometer used as a Fourier transform spectrometer. The instrument points along the spin axis and has a 7-deg field of view. This device measures the spectrum to a precision of 1/1000 of the peak flux at 1.7 mm for each 7-deg field of view on the sky (over the range 0.1 to 10 mm). The FIRAS uses a special flared trumpet horn flux collector having very low sidelobe levels and an external calibrator covering the entire beam; precise temperature regulation and calibration are required. The instrument has a differential input to compare the sky with an internal reference at 3 deg K. This feature provides immunity from systematic errors in the spectrometer, and contributes significantly to the ability to detect small deviations from a blackbody spectrum. The instrument weighs 60 kg, uses 84 W and has a data rate of 1200 bps.

Alternate Names


Facts in Brief

Mass: 30 kg
Power (avg): 84 W
Bit rate (avg): 1.2 kbps

Funding Agency

  • NASA-Office of Space Science (United States)


  • Astronomy: Infrared

Additional Information

Questions and comments about this experiment can be directed to: Coordinated Request and User Support Office



NameRoleOriginal AffiliationE-mail
Dr. Samuel GulkisOther InvestigatorNASA Jet Propulsion
Dr. Rainer WeissOther InvestigatorMassachusetts Institute of
Dr. George F. Smoot, IIIOther InvestigatorLawrence Berkeley
Dr. David T. WilkinsonOther InvestigatorPrinceton University
Dr. Michael G. HauserOther InvestigatorNASA Goddard Space Flight
Dr. John C. MatherPrincipal InvestigatorNASA Goddard Space Flight

Selected References

  • Mather, J. C., et al., Early results from the far infrared absolute spectrophotometer (FIRAS), in "After The First Three Minutes" AIP Conference, Preprint No. 90-06, Proc. 222, pp. 161-178, doi:10.1063/1.40411, Oct. 15-17, 1990.
  • Wright, E. L., Preliminary results from the FIRAS and DIRBE experiments on COBE, Preprint No. 91-05, Proc. The Infrared and Submillimetre Sky After COBE, Les Houches, doi:10.1007/978-94-011-2448-5_13, Mar. 1991.
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