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These tube rings have an inside diameter (ID) of 100mm (3.9"). Orion Tube Rings for telescope Optical Tube Assemblies come as a set of two with quick-release, hinged clamp knobs for easy installation on your favorite telescope. Each tube ring features a felt lining to keep your telescope optical tube free of scratches. Mounting bolts and washers are included with each set of tube rings. A threaded piggy-back camera adapter is integrated on one of the two tube rings in each set. Many different sizes are available for almost any telescope. Inside diameter (ID) is listed. Choose the size that matches the outside diameter of your telescope tube. These cast-aluminum rings feature quick-release, hinged clamp knobs and felt lining. They are designed for use with most Orion equatorial and altazimuth mounts, and may fit some other mount brands as well. One of the two rings in each set has a 1/4"-20 piggyback camera adapter on it, the other has a threaded 1/4"-20 hole. The outer diameter (OD) of your telescope should match inner diameter (ID) of rings. If OD of the telescope is a little smaller than ID of the rings, that is OK; you can "shim" inside of rings with extra felt. Inside diameter (i.d.) is 100mm/3.9". One-year limited warranty.