Guide star

Accurate telescope pointing and tracking is critical for obtaining good astronomical images and astrophotographs.

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.

Computer-controlled electric motors are commonly employed to allow the telescope to move in sync with the apparent motion of the sky, according to a pre-computed pointing model.

By measuring the observed motion of the guide star, and making minute distortions to the primary mirror, the telescope can produce images with much greater sharpness than is possible without adaptive optics.

However, only about 1 percent of the night sky is close enough to a natural guide star to use adaptive optics, so various methods to create artificial laser guide stars have been developed, including the sodium laser system developed by the Lawrence Livermore National Laboratory and used by the University of California's Lick and Keck observatories.