Telluric contamination due to H₂O vapor in Earth’s lower atmosphere. Shown as solid lines are rest-frame spectra of HD 140283 taken at different epochs and line-of-sight radial velocities. The apparent motion with respect to the heliocentric velocity v _helio = −169.00 km s⁻¹ is indicated in the legend. All dashed lines represent the same topocentric telluric absorption model (gray, see text for details) but being shifted in wavelength space to match the radial velocity of their observed counterpart (light-colored). For better visibility the models are shifted in flux direction as well.