The method of Off Axis Guiding is one way to overcome the problem that can occur when using a separate guide scope, such that the alignment of the imaging telescope and the guide scope becomes compromised. As the scope tracks across the sky, it can sometimes happen that the effects of gravity result in something known as differential flexure. This just means that the relationship between when the main scope and the guide scope are pointing changes. If this happens, then despite the guiding software keeping a guide star exactly positioned on the correct pixels of the guide camera you can see trailling in the main image due to the changing alignment.

When using an Off Axis Guider, the imaging camera and the guide camera are both connected to the main telescope so there is no loss of alignment that can take place. The Off Axis Guider uses a small prism to divert light from the edge of the field of view to the guide camera. The prism is positioned in a way to avoid creating any shadow on the main imaging camera's sensor whilst still collecting light from the main telescope. The use of a single telescope for both cameras means that one variable in the guiding performance is removed, and the final result is usually more reliable.

The use of this technique is not entirely without difficulies though. It can sometimes be challenging to ensure the imaging camera and guide camera are positioned correctly to ensure both are well focused at the same time. With long focal length systems it can sometimes be difficult to find a bright enough star for guiding to work well, because the field of view of the guide camera in this configuration can be very small.

Further details about the Off Axis Guider can be found on the Celestron website.