Wavelength-calibration process on a planetary nebula spectrum. Top: global function and its minimum (black circle) for the planetary nebula PNG321.0+3.9 observed at CTIO. The sharp steps at the top of the plot arise when certain lines are detected. Bottom: calibrated spectrum of PNG321.0+3.9. The vertical lines indicate emission or absorption lines that are detected, positioned at their tabulated values. The dashed blue lines show the fitted background (whose degree depends on its length), and the plain blue lines show the Gaussian profiles fitted on data.