The 8-inch Dovetail bar (CGE) from Celestron is for mounting a telescope optical tube to a mount or to attach accessories to the top of the optical tube. Having an extra dovetail bar means that a telescope can have its tube rings attached to its own dovetail bar. To use a different telescope for the same mount, all that would then have to be done would be to slide the bar into the mounts clamp and tighten it.