astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...

, a guide star is a reference

star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...

used to accurately maintain the tracking by a

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...

of a

celestial body An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...

, whose apparent motion through the sky is primarily due to Earth's rotation. Accurate telescope pointing and tracking is critical for obtaining good astronomical images and astrophotographs. However, because Earth rotates, the sky appears to be in a constant state of motion relative to 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 A small telescope is generally considered by professional astronomers to be any reflecting telescope with a primary mirror that is less than in diameter. By amateur standards, a small telescope can have a primary mirror/aperture less than in dia ...

, this motion becomes apparent on timescales of the order of seconds. Though

space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO-2 launch ...

s are not mounted on a spinning planet, they still use guide stars including those listed in the HST Guide Star Catalog. Computer-controlled

electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate forc ...

s 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. An autoguider 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 amateur telescopes often 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). Guide stars are also employed in

adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tel ...

. In this application, the star is not used to correct for the rotation of the Earth, but to correct for

turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...

in the Earth's

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. ...

. 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 Lawrence Livermore National Laboratory (LLNL) is a federal research facility in Livermore, California, United States. The lab was originally established as the University of California Radiation Laboratory, Livermore Branch in 1952 in response ...

and used by the

University of California The University of California (UC) is a public land-grant research university system in the U.S. state of California. The system is composed of the campuses at Berkeley, Davis, Irvine, Los Angeles, Merced, Riverside, San Diego, San Fran ...

Lick Lick may refer to: * Licking, the action of passing the tongue over a surface Places * Lick (crater), a crater on the Moon named after James Lick * 1951 Lick, an asteroid named after James Lick * Lick Township, Jackson County, Ohio, United State ...

and Keck observatories.{{cite web , url=https://www.llnl.gov/str/June02/Pennington.html , title=Science and Technology Review: Adaptive Optics Sharpen the View from Earth , first=Arnie, last=Heller , publisher=

, date=2002-06-12 , access-date=2008-06-21

References

Observational astronomy

See also

References