Guide star

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See GuideStar.

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 photographs. 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.

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. However there is usually a significant non-zero error associated with the model, which is an approximation to the true motion of the sky.

Most modern professional telescopes use a guide star. A sensor is pointed to a sufficiently luminous star that lies near the object being observed and, if the pointing begins to drift, the error can be detected and the movement corrected. This is most accurate when the corrections are applied by a computer, but it is common for amateur telescopes to have manual correction (requiring the observer to continuously follow the star by eye for the exposure period, which may be a significant length of time).