The Celestron Visual Back for SCT and Refractors is also known as an "Eyepiece Holder", and screws onto the rear cell threads of most SCTs. It presents a 1.25-inch barrel on the other end so that accessories with like-sized barrels can slide into the visual back. The thumbscrews seen in the photo can then be tightened down so that the accessory will stay firmly in place.
The Visual Back can also screw onto the focusers of some refractors for the same purpose.
Celestron Product Number: 93653-A