Spectrum of a rapidly rotating B star containing only telluric absorption lines. The positions of the telluric lines are marked by red and orange lines (red lines deeper than orange ones, see Sect. 3.4), and the rest wavelengths of the six C I lines are marked with dotted blue lines. Depending on the (geocentric) radial velocity of the target, the C I lines will be either red- or blue-shifted and may, or may not, be contaminated by the telluric lines.