Introduction

A centered optical system is compsed of a succession of diopters or mirrors. It has a symetrical axis of revolution. We have seen that generally stigmatism does not exist. Paraxial approximation helps to disregard stigmatic imperfections and to suppose that these systems are perfect. This is essential in order to calculate the position of the images.

Moreover, optical systems are generally spared from stigmatisms leading to approximate stigmatism The position of the images are thus that of paraxial imagery, even if incidence angles on optical surfaces are well above paraxial approximation.

In fact stigmatism implies that all luminous rays converge to the same point.

Paraxial stigmatism presents a convergence position which then becomes that of the totality of luminous rays emerging from the system. Following figure 30, a marginal ray must converge on the paraxial image so that stigmatism is ensured.

Moreover, aberrations correction yields an absence of field curvature and a constant magnification, which is also one of the caracteristics of paraxial conjugations.

Any centered optical system free from aberrations has an imagery perfectly determined by paraxial imagery.