In astrophotography, Off-Axis Guiding (a.k.a. OAG) is an autoguiding technique used to improve the tracking accuracy of an equatorial mount. It increases the exposure time and improves the overall image quality.
Unlike the standard autoguiding technique, off-axis guiding doesn’t required an extra guiding scope. It consists of an adapter that mounts between the main scope and the imaging camera. This adapter has a tiny mirror, that redirects a portion of the light perpendicularly, towards the guiding camera.
One of the advantage of an OAG system is the weight: without a guiding scope, the overall weight is reduced. It is also said to be more adapted to long focal length.