Orion's cast-aluminum tube ring set features quick-release, hinged clamp knobs and felt lining. Designed for use with Orion telescope mounts, they fit some other brands of mounts as well. One of the two rings in the set has a 1/4"-20 piggyback camera adapter on it, the other has a threaded 1/4"-20 hole. The piggyback camera adapter can be removed for mounting an adapter plate for a guide scope on top of the tube rings. For use with SkyView Pro or Atlas telescope mounts, you will also need a dovetail mounting plate.
Set of two rings includes mounting bolts and washers. Inside diameter (ID) is listed. Choose the size that matches the outside diameter of your telescope tube.
Outer diameter (OD) of the 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.
The column labeled "Rings Fit" can be used as a guide for selecting appropriate rings if you don't know the OD of the telescope.
Vixen dovetail plates are compatible with the SkyView Pro telescope mount.