Definitions for "Optical axis"
Is an imaginary line passing horizontally through the center of a compound lens system.
A line passing through the centers of curvature of a lens or series of lenses in an optical system.
As applied to birefringent crystals in polarized light and DIC microscopy, the path followed by the ordinary wavefront in a birefringent material. Aligned optical systems possess an optical axis consisting of a straight line joining the centers of curvature of lens surfaces contained within the system.
Keywords:  concave, mirror, sharp, stable, able
Responsible for the sharpness of  a projection image. Only if the projection head is positioned vertically above the lamp house and the concave mirror the user is able to get a sharp projection image. It is important that there is a stable connection between projection head and projection arm