If you're serious about astrophotography, the Celestron Off-Axis Guider (OAG) is an essential astroimaging accessory. This OAG serves as the most accurate way to guide telescopes with larger focal lengths, compared to using a separate guide scope.
It works by using a prism to intercept a small area of the focal plane in order to locate the guide star. Note, this will not impact the field of view on the main imaging camera. The use of an OAG results in pinpoint round stars with the reduction of any tracking or mechanical errors and impressively clear imagery.