Glossary

Mirror: surface reflecting light rays symmetrically regarding the normal to the surface (specular reflexion) and therefore without scattering them
Clear aperture: aperture diameter of an optical system
Longitudinal spherical aberration: algebraic distance from the paraxial image to the marginal image obtained with rays propagating through an optical system from an on-axis point
Transversal spherical aberration: diameter of the spot formed in the paraxial image plane with rays propagating through an optical system and coming from an on-axis object