which depends on the wavelength 
(see f.i., Simon, 1966). Because of the dependence in wavelength of this
refraction angle, polychromatic images of such an object are actually spread
into a small vertical spectrum. For instance, for a 250 nm bandwidth centered
at 500 nm, the typical atmospheric dispersion is 1 " for an elevation
of 
and 2 " for 
.
This effect can be neglected for most astronomical observations, because
it is smaller than the FWHM (Full Width Half-Maximum) seeing of the atmospheric
turbulence. In PISCO, the atmospheric dispersion is corrected with ``Risley
prisms,'' i.e., two identical sets of two prisms (cf. Fig. 4).
Each set consists of two prisms of different dispersion law and roof angles,
placed in an upside-down position. These prisms have been designed to have
a null mean deviation, and a dispersion allowing atmospheric correction
from the zenith down to an elevation of 30 
for blue wavelengths (450 nm) with a 50 nm bandwidth.
During the observations, the control program (cf. §2.1) computes the elevation of the star and the corresponding atmospheric dispersion from models by J.C. Owens, ( Owens, 1967, formulae 29-31). The Risley prisms are set so that their total dispersion has the same magnitude as the atmospheric dispersion, with an opposite direction (cf. Fig. 4).
Figure 4: Risley prisms to correct for the atmospheric dispersion.
For more details, see Risley prisms design: parameters computation