Guide star
Encyclopedia
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 motor
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
. In this application, the star is not used to correct for the rotation of the Earth, but to correct for turbulence
in the Earth's atmosphere
. 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 star
s have been developed, including the sodium laser system developed by the Lawrence Livermore National Laboratory
and used by the University of California
's Lick
and Keck observatories.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, a guide star is a reference star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
used to accurately maintain the tracking by a telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
of a heavenly body, whose motion across the sky is primarily due to the rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...
of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
.
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
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
-controlled electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
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
Autoguider
An autoguider is an electronic tool used in astrophotography to keep the celestial object being photographed from drifting across the field of view.-Background:When imaging dim targets, usually deep sky objects, hours-long exposure times are often necessary...
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
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
. In this application, the star is not used to correct for the rotation of the Earth, but to correct for turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
in the Earth's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature 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 star
Laser guide star
Laser guide stars are a form of artificial star created for use in astronomical adaptive optics imaging.Adaptive optics systems require a wavefront reference source in order to correct atmospheric distortion of light...
s have been developed, including the sodium laser system developed by the Lawrence Livermore National Laboratory
Lawrence Livermore National Laboratory
The Lawrence Livermore National Laboratory , just outside Livermore, California, is a Federally Funded Research and Development Center founded by the University of California in 1952...
and used by the University of California
University of California
The University of California is a public university system in the U.S. state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University...
's Lick
Lick Observatory
The Lick Observatory is an astronomical observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton, in the Diablo Range just east of San Jose, California, USA...
and Keck observatories.