In astrophotography, Autoguiding refers to a technique used to improve the tracking accuracy of an equatorial mount. It increases the exposure time and improves the overall image quality.
When using an equatorial mount, the motors within the mount are never perfect, and they tend to cause small tracking inaccuracies, called Periodic Error (similarly to a clock that advances one second every 5 minutes). To correct PE, which occur periodically, one can use a guiding system.
An autoguiding system consists of several elements, that all communicate together via cables:
- A guiding scope, generally small and lightweight, piggybacked with the imaging scope;
- A guiding camera, attached to the guiding scope. It is usually dedicated for astrophotography and hosts a very sensitive sensor.
- A computer equipped with a guiding software, that controls the camera and the mount.
Here is how it works:
- Using the software, the guiding camera is locked on a guide star near the object being photographed. The guide star will serve as a reference, a point in the sky that must remain centred.
- The guiding software will first study the mount and log the periodic errors (based on the distance between the live position of the guide star and where it is supposed to be).
- Since the tracking errors are periodical, the software will learn and remember when they occur.
- Finally, during the imaging, when the guide star starts shifting due to the PE, the computer will automatically command the mount to correct the movement, eliminating any tracking error.
A similar way of tracking a guide star, is a technique called off-axis guiding (OAG). It follows the same general principle, without the use of a guide scope.