An Off-Axis Guider is an astroimaging accessory considered by many to be the most accurate method of autoguiding.
The Celestron Off-Axis Guider uses a large 12.5mm prism to intercept a small portion of the telescope’s focal plane (outside the field of view of the main imaging camera) to locate a guide star. Any movement seen by the guide star will be the exact same movement seen by the imaging camera. Guiding this way will correct for tracking errors, as well as opto-mechanical errors or flexure, for pinpoint round stars.
NOTE: The Celestron Off-Axis Guider is not compatible for DSLR imaging with the 8" EDGE-HD Focal Reducer.