Coatings are used to alter the reflective properties of a surface. They can be used to increase transmission (AR Coatings), increase reflection (HR Coatings) or selectively transmit or reflect certain wavelengths.
The clear aperture is the pupil of the system that preserves all specifications defined by the manufacturer. It is always smaller than the diameter of the optic.
Quantifies the range of angles over which an optical system can accept or emit light. In a single lens, it is proportional to the optics diameter and inversely proportional to the focal length.
Thickness indicates the distance between two surfaces. Larger thickness allows for greater heat dissipation and higher resistance to stress in machining, blocking, and handling, however it increases weight and dispersion.