Differences between power spectra evaluated from the union masks G22∪PSA82 and G35∪PSA82 (≡ mask0 and mask1 respectively in the legend), showing the presence of Galactic dust. For ℓ ≤ 1400 the spectra show the 217−143 “double-differenced” power spectrum, rescaled to correct for dust emission at 143 GHz. For ℓ > 1400 the 217 mask differenced power spectrum is plotted. The blue line shows the 857 GHz mask-differenced power spectrum scaled to 217 GHz as described in the text. The green dotted line shows the best-fit of Eq. (9) scaled to 217 GHz. This is the model that we use to subtract dust emission at high multipoles from the 143 × 143, 143 × 217 and 217 × 217 spectra with suitably adjusted scaling factors.