Guide star

In astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a heavenly body, whose motion across the sky is primarily due to the rotation of the Earth.

Accurate telescope pointing and tracking is critical for obtaining good astronomical images and astrophotographs. However, because the Earth rotates, the sky appears to be in a constant state of motion relative to the Earth. Although this movement appears to be relatively slow when viewed with the naked eye, with the high magnification and consequently smaller field of view provided by even a small telescope this motion becomes apparent on timescales of the order of seconds.