  Differential atmospheric dispersion is a wavelength-dependent effect
introduced by the atmosphere. It is one of the instrumental errors that can
affect the position of the target as perceived on the sky and its flux
distribution. This effect will affect the results of astronomical observations
if not corrected by an atmospheric dispersion corrector (ADC). In
high-resolution spectrographs, in order to reach a radial velocity (RV)
precision of 10 cm/s, an ADC is expected to return residuals at only a few tens
of milli-arcseconds (mas). In fact, current state-of-the-art spectrographs
conservatively require this level of residuals, although no work has been done
to quantify the impact of atmospheric dispersion. In this work we test the
effect of atmospheric dispersion on astronomical observations in general, and
in particular on RV precision degradation and flux losses. Our scientific
objective was to quantify the amount of residuals needed to fulfill the
requirements set on an ADC during the design phase. We found that up to a
dispersion of 100 mas, the effect on the RV is negligible. However, on the flux
losses, such a dispersion can create a loss of ~2% at 380 nm, a significant
value when efficiency is critical. The requirements set on ADC residuals should
take into consideration the atmospheric conditions where the ADC will function,
and also all the aspects related with not only the RV precision requirements
but also the guiding camera used, the tolerances on the flux loss, and the
different melt data of the chosen glasses.
