= COBE FIRAS blackbody spectrum experiment
{c}
<FIRAS> compared sky radiation with an adjustable internal <blackbody> using an <interferometer> and cold <bolometers>. An external <blackbody> could cover the sky entrance for <measurement calibration>. The team adjusted the internal <temperature> to minimize the difference signal.<Nobel Lecture: From the Big Bang to the Nobel Prize and beyond by John C. Mather>[§III.D, pp.1337–1338; §V.A, p.1341]{ref}
Comparisons across detectors and mirror strokes tested instrumental errors; sky mapping separated <Milky Way> emission. <John C. Mather>'s lecture reports a final root-mean-square spectral departure of 50 parts per million of peak brightness. This is neither the preliminary 1990 precision nor a relative error in every spectral bin. Calibration drift and nonideal reference emission required correction.<Nobel Lecture: From the Big Bang to the Nobel Prize and beyond by John C. Mather>[§V.A, p.1342: calibration checks and final spectrum]{ref}
\Image[/cobe-firas-comparator-and-temperature-residual.png]
{title=FIRAS comparison principle and a chosen blackbody temperature residual}
{description=Functional comparison, not an optical layout. Calculated curves use stipulated <temperatures>, not measured COBE data.}
The advantage of a <null measurement> is that a small difference can be resolved without assigning the same absolute precision to a large detector output. Nevertheless, matching an internal reference is meaningful only after its relation to an independently calibrated <blackbody> has been tested. A bound on departures from a <Planck spectrum> constrains energy redistribution after <thermal equilibrium>; it does not require the <cosmic microwave background> to have exactly the same <temperature> in every direction.
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