"Diffraction Limited" means that the limits of image detail are determined by the physical properties of light, and not by optical defects in the telescope.

Instead of a single thumb screw locking the eyepiece in place, a brass ring compressed by a thumb screw secures the eyepiece. This spreads clamping force over a larger area of the eyepiece's barrel, which provides a better connection. The compression ring also prevents the tip of the thumb screw from producing unsightly scratches and divots on the eyepiece barrel.