N₂D⁺ (J:1–0) data reduction in TP mode compared to the original frequency SM reduction. The residual in the central part of the spectrum is shown in blue with a negative offset of −0.5 K. The ends have been erased because the negative lines from the frequency SM folding have no counterpart in the TP mode data reduction. Red vertical lines mark the masks used for the TP mode data reduction to compute the baseline, while the black vertical lines mark those used for the frequency SM data reduction. Part of the negative lines are outside the window.