Spectral analysis of the Planck 70, 100, and 143 GHz maps. Columns show results computed using the three sky masks in Fig. 1, with, from left to right, f_sky = 69.4%, 59.6%, and 39.7%. Top row: CMB anisotropy spectra binned over a range of multipoles Δℓ = 40, for ℓ ≥ 30, with (2ℓ + 1)-weighting applied within the bin. Error bars are computed as a measure of the rms-power within each bin, and hence comprise both the measurement inaccuracy and cosmic variance. The grey curve is the best-fit Planck 6-parameter ΛCDM model from Planck Collaboration XVI (2014). Noise spectra computed from the half-ring-difference maps are shown: for the 70 GHz channel, the S/N ≈ 1 at ℓ ≈ 650. Middle row: residuals of the same power spectra with respect to the Planck best-fit model. Bottom row: power ratios for the 70 vs. 100 GHz and 143 vs. 100 GHz channels of Planck. The ratios are calculated ℓ by ℓ, then binned. The error bars show the standard error of the mean for the bin. The effect of diffuse foregrounds is clearly seen in the changes in the 143/100 ratio with sky fraction at ℓ ≈ 100. Bin-to-bin variations in the exact values of the ratios with sky fraction emphasize the importance of making such comparisons precisely.