Properties:

A cylindrical, externally threaded item having a centrally located hole specifically designed to retain by means of either a raised box (shoulder), or counterbore, countersink, or a similar shaped recess, an optical element such as a lens, reticle, cell, and the like, in optical assemblies. It must have drilled holes or slots to facilitate assembly and disassembly. For items which house the optical element(s), and form the internal or central portions of an optical system, see cell, optical element. Excludes ring, externally threaded.