2006PASP..118..399G

Telluric absorption inside our coudé spectrograph is used as a reference spectrum for precise radial velocity measurement. The optical path for this telluric absorption is stable and is not compromised by atmospheric disturbances, such as the jet stream. As an added bonus, the telluric fiducial spectrum is free; i.e., one does not need to construct, regulate, maintain, or suffer throughput losses from a captive-gas absorption cell. The telluric and stellar spectra span the same portion of the detector and are observed sequentially, but in different diffraction orders. In this way the stellar line profiles are not contaminated by the telluric spectrum and can therefore be used for normal astrophysical line profile analyses, as well as radial velocity studies. However, precise measurements of the temperature and pressure are needed for differential corrections between orders, because the index of refraction of the air in the spectrograph is wavelength dependent. Precision in radial velocity of ∼25 m/s is demonstrated.