An Off-Axis Guider (OAG) allows you to have an autoguide camera share the same optical path as the main camera. This solves the problem of flexure that can ruin your images. Because the telescope mount rotates your equipment, gravity acts in different directions. This means that a heavy guide can move fractionally in relation to the main scope. This problem is only made worse when using long focal length scopes, such as Schmidt-Cassegrains. An OAG therefore will give you more accurate guiding and better shaped stars.
The ToupTek Off-Axis guider comes with a choice of 48mm, 54mm or 68mm threaded ring on the telescope side. These match the thread on your coma corrector, field flattener, or nose piece. The body of the OAG and the mounting ring comes to 17.5mm. This body bolts to a 20mm spacer which bolts to the front of a deep-sky camera (with back-focus of 17.5mm) using six bolts. Overall, the train places the camera sensor 55mm from the corrector flange. If you are using a filter wheel, this replaces the 20mm spacer.
The ToupTek OAG includes a helical focuser with a thumb screw to lock the position. The focuser will not rotate the guide camera, meaning you won’t have to recalibrate the autoguiding program if you refocus the OAG.