A telluric line is an absorption line due to Earth atmosphere in observation data from ground telescopes. In studying astronomical bodies, such lines constitute a distortion of the target body's spectrum (termed telluric contamination), and are a motivation for space telescopes. The contamination is not the same everywhere and all the time: it depends upon weather conditions, altitude, etc. Telluric correction and telluric line subtraction are terms for efforts to tease out the astronomical data from that contributed by Earth atmosphere. For stars, a well-established method is to include in the FOV, a star with known spectral energy distribution which would reveal the telluric contamination at the time and place, allowing it to be quantified (a telluric star). Compensating for telluric lines is vital for very precise/accurate radial velocity measurements using HRS. Advanced methods using post-processing are now used, such as methods using principle component analysis (PCA).