The goal of the design of the telescope/enclosure system is to keep the image quality of the telescope at a level where the limitation is given by the natural seeing rather than by the disturbing influences of the local environment. It is therefore convenient to evaluate theses influences with the CIR criterion which weighs them with respect to the natural seeing.
Equations (
) and () are used to pass from
angular image sized to the corresponding CIR terms.
For the computation of each particular case the natural seeing term,
which was measured close to the zenith, will be corrected for
the actual zenithal angle 
with the factor
.
The reference for this evaluation is given by the CIR budget of the VLT in which the terms relevant here are detailed here below.
Figure 6.2: CIR error budget for the terms caused by local turbulence