ledestjerne

Ledestjerne, or guide star, is a reference star used by a telescope’s guiding system to maintain precise tracking during long-exposure astronomy. The guiding process compensates for small pointing errors in the mount, atmospheric distortion, wind, and mechanical flexure, enabling sharp images and accurate astrometry over time.

Typically, a guiding camera continuously images a small region of the sky near the science target. The software measures the drift of the designated guide star relative to its initial position and sends real-time corrections to the telescope’s mount in right ascension and declination. Corrections are applied at a cadence of roughly 1 to several times per second, producing steadier tracking during long exposures.

Choosing an appropriate ledestjerne depends on several factors. The star should be bright enough to yield a reliable signal with the guiding camera, yet not so bright that it saturates the detector. It should be relatively isolated to avoid blending with nearby stars and ideally lie within the field of view close to the science target. In many setups, astronomers select one or more guide stars within the same frame or nearby fields to optimize accuracy.

In professional astronomy, guiding may involve multiple guide stars or advanced techniques, and in some systems, a laser guide star can be used to aid adaptive optics. However, a ledestjerne commonly refers to a natural star used for autoguiding rather than the artificial laser guide star used for atmospheric correction.

Historically, autoguiding transformed long-exposure imaging, moving from manual adjustments to automated systems in the late 20th century. Modern software and cameras have made ledestjerner a routine element of both amateur and professional observing.