The Lumicon Cassegrain Tele-Extender threads on to the 1.25" eyepiece holder/visual back that was supplied for many years with many Schmidt-Cassegrain telescopes (and is currently supplied with Meade LX90 scopes).  Most of those 1.25" visual backs threaded on to the larger 2" rear cell threading of the catadioptric scopes and presented a rear which had an outer 1.375"-20 male thread and an inner 1.25" barrel with either a set screw or clamping ring to accept your 1.25" diagonal, eyepiece, or other accessory.

On the front of the Cassegrain Tele-Extender (where light would enter) is a female threading which mates to the 1.375"-20 thread of the 1.25" visual back, and on the rear of the Cassegrain Tele-Extender (where light would leave) there is a male T-mount threading.  You add your camera system's specific T-mount to the rear of this tube and then affix your SLR camera to the back of the T-mount.  This allows direct "prime focus" imaging without an eyepiece in the optical path, the main reason for this device, but because the Tele-Extender fits on to the outside of the 1.25" visual back you'll also be able to do eyepiece projection imaging if your eyepiece has a very small outer diameter.