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A guide star (GS) assists in guiding something: the term is used for identifiable stars that indicate a position in the celestial sphere, and for other assistance in observation. Guide stars have been used to assist in navigation (e.g., on the sea) throughout history, to determine the traveler's position and direction of travel. Within astronomy itself, determination of an observed astronomical object's position involves comparisons of angular distances to objects for which positions have been established, termed guide stars. For telescopes with very small fields of view, guide stars may be necessary simply to determine whether the telescope is anywhere near pointing in the right direction. Catalogs of guide stars have been compiled, such as the Guide Star Catalog for the Hubble Space Telescope
The term guide star is also used regarding modern adaptive optics systems, which use the apparent position of some known star (termed its guide star) within the field, to adjust the optics to compensate for varying seeing issues that would distort the field. For fields without (enough) guide stars suitable for this, a laser is used to produce a spot high in the sky, which is termed a laser guide star (LGS). The term natural guide star (NGS) is used to specify use of an actual star for this function.