= COBE DMR sky anisotropy experiment
{c}
In 1992 the <COBE DMR> team, including <George F. Smoot> and <Charles L. Bennett>, used <radiometer> <antennas> to compare <antenna temperatures> between sky directions $60^\circ$ apart at $31.5$, $53$ and $90\,\mathrm{GHz}$, with two nearly independent channels per <frequency>. Repeated sky coverage supported <least-squares solution> map reconstruction.<Structure in the COBE Differential Microwave Radiometer First-Year Maps by George F. Smoot and others>[§§1–2, pp.L1–L2]{ref}
The analysis fitted magnetic response and removed the assumed kinematic <angular dipole> and its second-order <Doppler effect> contribution. Channel sums retained sky structure; channel differences estimated <noise>. Cross-channel correlations, different <frequency> maps and <Milky Way> latitude cuts tested the interpretation. Residual large-angle structure supported <Cosmic microwave background anisotropy>; Galactic contamination particularly limited the <cosmic microwave background quadrupole>. Brightness fluctuations alone were not sufficient evidence of primordial structure.<Structure in the COBE Differential Microwave Radiometer First-Year Maps by George F. Smoot and others>[§§2–3.1, p.L2: controls, motion subtraction, Table1]{ref}
\Image[/cobe-dmr-differencing-and-sky-modes.png]
{title=Directional differencing rejects a monopole and responds to a chosen quadrupole}
{description=Ideal point beams and a stipulated low-order sky, not COBE observations. The ring scan deliberately has less coverage than a full sky survey.}
The two differences serve different purposes. Comparing two sky directions measures angular variation. Comparing two independently calibrated receiver maps observing the same sky tests <noise> and consistency. A common sky signal cancels in the latter comparison if their <antenna beam patterns> and response scales match; a systematic error shared by both channels need not cancel. Separating observer motion, local emission and intrinsic <Cosmic microwave background anisotropy> requires these additional tests.
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