XCALeXternal Calibrator
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When FIRAS first begins to transfer data to the Earth, a calibration file using Xcal had not been collected in space [22, p.
4, FIRAS was unable to yield proper nulls, either with the sky and Ical, or with Xcal and Ical.
The problems with correctly establishing temperatures for Xcal and Ical were central to the mission, as these investigators recognized: "There were two important problems.
The temperature errors on Xcal are fitted with an "arbitrary offset in the Xcal thermometer and the result was -7.
As represented in Figure 1, FIRAS functions as a differential spectrometer, wherein the sky or the external reference, Xcal, are being constantly compared to an internal reference blackbody, Ical.
In analyzing the bottom trace in Figure 7, it is evident that a null cannot be achieved, when Xcal is set at nearly the same temperature as Ical (Xcal = 2.
It is vital to understand the exact temperatures for Xcal and Ical, when a null spectrum is achieved by the two calibrators.
The error bar in setting the absolute temperature, using Xcal, drops precipitously from 60 mK to 10 mK (see Table 1).
A critical aspect of the calibration procedure is that the external calibrator, Xcal, is treated as providing a perfect blackbody signal to the rest of the instrument.
This could become more apparent when Xcal and Ical are at very different temperatures.
Furthermore, the FIRAS team has not shown that it can minimize residuals, using their final calibrations across all ranges of temperatures for Xcal, Ical, the sky horn, and the reference horn.
001 K (see Table 1), which relies on Xcal (see page 581, section 4.