I want to autoguide with my Alt-Azimuth mount for long-exposure astrophotography. Will this work?

Technically, yes, it will work. Many modern autoguiders can keep up with the guide rates involved. In practice, no, guiding with an alt-azimuth mount will not work. Here’s why.

The autoguider will detect drift of the star you have selected to guide on and send commands to the mount to keep it at the same position relative to your scope’s axis. Meanwhile, as your imaging session goes on, all the other objects in the field of view will move relative to both the scope axis and the guide star. The guide star–whether it’s centered in the field or at its edge–will act as the axis of motion and all the stars will rotate about it.

The result: trailed stars everywhere in your picture except the guide star.

This field rotation is the same as for polar axis misalignment with an equatorial mount. Field rotation is discussed in more depth in another knowledgebase article under Astrophotography–Imaging FAQs.

Updated 12/21/13