Editing Guide star (section)

{{other uses|Guiding Star (disambiguation)}}
{{More citations needed|date=December 2006}}
[[File:Green light for GALACSI's blue glow.jpg|thumb|300px|GALACSI<ref>{{cite web |title=GALACSI |url=https://www.eso.org/sci/facilities/develop/ao/sys/galacsi.html |archive-url=https://web.archive.org/web/20240213223017/https://www.eso.org/public/announcements/ann16025/ |archive-date=2024-02-13 |website=eso.org |publisher=[[ESO]] |access-date=2024-08-03 |quote=}}</ref> adaptive optics system for the [[Very Large Telescope]] uses artificial [[laser guide star]]s<ref>{{cite web |title=Announcement. GALACSI Adaptive Optics System Ready to be Mounted on the VLT. The VLT’s MUSE will soon be able to see even more clearly |url=http://www.eso.org/public/announcements/ann16025/ |archive-url=https://web.archive.org/web/20240720124401/https://www.eso.org/sci/facilities/develop/ao/sys/galacsi.html |archive-date=2024-07-20 |website=eso.org |publisher=[[ESO]] |access-date=2024-08-03 |quote=GALACSI will rely on 4 sodium lasers launched from the centre piece of one of the Unit Telescopes of the VLT to produce “artificial stars”, known as guide stars. Sensors then follow the motion of these guide stars as the light from them flickers in the turbulent atmosphere. That allows a computer to calculate the correction that must be applied to the telescope’s deformable secondary mirror (itself a new addition to the VLT) to compensate for the atmospheric disturbance. In this way, extremely sharp images of the real celestial objects can be obtained.}}</ref>]]

In [[astronomy]], a '''guide star''' is a reference [[star]] used to accurately maintain the tracking by a [[telescope]] of a [[astronomical object|celestial body]], whose [[diurnal motion|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 [[astrophotograph]]s. 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]], this motion becomes apparent on timescales of the order of seconds.

Though [[space telescope]]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]]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 Observatory|Lick]] and [[Keck Observatory|Keck]] observatories.<ref>{{cite web |archive-url=https://web.archive.org/web/20170209132107/https://str.llnl.gov/str//June02/Pennington.html |archive-date=2017-02-09 |url=https://www.llnl.gov/str/June02/Pennington.html |title=Science and Technology Review: Adaptive Optics Sharpen the View from Earth. Blur free imagereveal a wealth of astronomical detail |first=Arnie |last=Heller |publisher=[[Lawrence Livermore National Laboratory]] |date=2002-06-12 |access-date=2024-08-03 |quote=The dye laser, similar to that pioneered at Livermore for its Atomic Vapor Laser Isotope Separation program, creates a glowing star of sodium atoms measuring less than 1 meter in diameter at an altitude of about 100 kilometers above Earth’s surface. This artificial reference can be created as close to the astronomical target as desired so that the light from the laser star and the observed object pass through the same small part of the atmosphere.}}</ref>