XCALeXternal Calibrator
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It is also interesting that Mather describes significant problems with Xcal prior to launch, as follows: "Now without gravity to help hold it in place, the calibrator popped out of the horn every time the test engineers inserted it by means of the same electronic commands they would use once COBE was in orbit.
On the ground, the Xcal could "pop out" of the horn if the satellite was turned on its side [22, p.
When FIRAS first begins to transfer data to the Earth, a calibration file using Xcal had not been collected in space [22, p.216].
Yet, as will be seen below, in sections 2.3.3 and 2.3.4, 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 -7.4[+ or -]0.2 mK for this offset" [38].
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 = 2.750 K, Ical = 2.759 K).
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.
Calibration involves: "comparison of the sky with an ideal movable external blackbody calibrator (Xcal) that can fill the aperture of the sky horn.