  Observations with ground-based telescopes are affected by differential
atmospheric dispersion when seen at a zenith angle different from zero, a
consequence of the wavelength-dependent index of refraction of the atmosphere.
One of the pioneering technology in detecting exoplanets is the technique of
radial velocity (RV), that can be affected by uncorrected atmospheric
dispersion. The current highest precision spectrographs are expected to deliver
a precision of 10 cm/s (e.g., ESPRESSO). To minimize the atmospheric dispersion
effect, an Atmospheric Dispersion Corrector (ADC) can be employed. ADC designs
are based on sky dispersion models that nonetheless give different results;
these can reach a few tens of milli-arcseconds (mas) in the sky (a difference
up to 40 mas); a value close to the current requirements (20 mas in the case of
ESPRESSO). In this paper we describe tests done with ESPRESSO and HARPS to
understand the influence of atmospheric dispersion and its correction on RV
precision. We also present a comparison of different sky models, using EFOSC2
data (between 600nm and 700nm), that will be used to improve on the design of
ADCs.
