Guide star

In astronomy, a guide star is a reference star used to keep atelescope pointing in the right direction while moving to follow the sky rotation.

Telescope pointing is critical for obtaining good images and photographs. Unfortunately, the sky is in costantly moving, while the Earth is rotating around its axis. If the movement is visible to the naked eye only on timescales of minutes or our, with the high magnification provided by a telescope the movement becames significant in seconds or even less.

Computer-controlled electric motors are commonly used to have the telescope move in synch with the sky. But every mechanical piece is bound to introduce some error in the movement, along with structural bendings.

Most modern telescopes can use a guide star: a sensor is pointed to a sufficently luminous star near the object being observed and, if the pointing begins to drift, the error can be detected and the movement corrected. This is best done, again, by a computer-controlled system, but it is common for amateur telescope to have manual correction (requiring the observer to continuously look at the star for the exposure period, which may be well over 30 minutes at times.)