Left: example of telluric line correction in the weak line regime by means of the rope-length method applied to the spectrum of HD 170740. Telluric lines of O₂ are corrected first, then H₂O lines. Right: example of telluric line correction in the strong line regime by means of a two-step method and a composite instrumental profile adjustment (see text). Residuals remain at the location of the deepest lines, especially when the model does not predict their shape and exact Doppler shift very accurately. The positions of the 9577 and 9632 Å DIBs attributed to C are indicated.