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The Baader FR-4 Focusing Ring Collar slips onto a 1-1/4" eyepiece barrel and enables to adjust and lock a reference stop for repeatable eyepiece focus. This allows you to set an eyepiece to be precisely parfocal with another eyepiece or piece of equipment. Multiple collars can also be useful in pre-setting. The Bino-Viewer eyepieces for repeatable focus. The focusing collar is 0.23" (5.84 mm) thick and incorporates two nickel plated brass lock screws, which are spherical-tipped to minimize eyepiece barrel damage.