Editing Heliostat (section)

{{Short description|Solar tracking device}}
{{About|solar tracking devices|optical communication equipment|Heliograph}}
[[File:Ekling Heliostat.JPG|thumb|right|Heliostat by the Viennese instrument maker [[Johann Michael Ekling|Ekling]] (c. 1850)]]
[[Image:heliostat.jpg|right|260px|thumb|A heliostat at the [[Themis (solar power plant)|THÉMIS]] experimental station in France. The mirror rotates on an [[altazimuth mount]].]]
[[Image:Solar two.jpg|thumb|right|260px|The [[Solar Energy Generating Systems|Solar Two]] [[Solar thermal energy|solar-thermal]] power project near [[Daggett, California]]. Every mirror in the field of heliostats reflects sunlight continuously onto the receiver on the tower.]]
[[Image:PS10 solar power tower 2.jpg|thumb|right|260px|The 11MW [[PS10]] near Seville in Spain. When this picture was taken, dust in the air made the converging light visible.]]
[[Image:Font Romeu France.jpg|thumb|right|260px|The [[Odeillo solar furnace|solar furnace]] at [[Font-Romeu-Odeillo-Via|Odeillo]] in the [[Pyrenees-Orientales]] in France can reach temperatures up to {{convert|3500|C|F}}]]

A '''heliostat''' 
({{etymology|grc|''ἥλιος'' (hḗlĭos)|sun
            ||''στατός'' (stătós)|standing}})
is a device that reflects sunlight toward a target, turning to compensate for the Sun's [[Diurnal motion|apparent motion]].
The reflector is usually a [[plane mirror]].

The target may be a physical object, distant from the heliostat, or a direction in space. To do this, the reflective surface of the mirror is kept [[perpendicular]] to the [[bisection|bisector]] of the [[angle]] between the directions of the Sun and the target as seen from the mirror. In almost every case, the target is stationary relative to the heliostat, so the light is reflected in a fixed direction. According to contemporary sources the heliostata, as it was called at first, was invented by [[Willem 's Gravesande]] (1688–1742).<ref>''A New and Complete Dictionary of Arts and Sciences,'' vol 2, London, 1763, p. 1600</ref> Other contenders are [[Giovanni Alfonso Borelli]] (1608–1679) and [[Daniel Gabriel Fahrenheit]] (1686–1736).<ref>Pieter van der Star, ''Daniel Gabriel Fahrenheit's Letters to Leibniz and Boerhaave'', Leiden, 1983, p. 7.</ref> A heliostat designed by [[George Johnstone Stoney|George Johnstone Storey]] is in the [[Science Museum, London|Science Museum Group]] collection.<ref>{{cite web |title=Heliostat, contrived by the late G. Johnstone Stoney, D.Sc., F.R.S., c. 1875. |url=https://collection.sciencemuseumgroup.org.uk/objects/co56796/heliostat-contrived-by-the-late-g-johnstone-ston-ancillary-instruments-astronomical-instruments-heliostats-astronomical |publisher=[[Science Museum Group]] |access-date=20 June 2022}}</ref>

Currently, most heliostats are used for [[Daylighting (architecture)|daylighting]] or for the production of [[concentrated solar power]], usually to generate electricity. They are also sometimes used in [[solar cooking]]. A few are used experimentally to reflect motionless beams of sunlight into [[solar telescope]]s. Before the availability of [[laser]]s and other electric lights, heliostats were widely used to produce intense, stationary beams of light for scientific and other purposes.

Most modern heliostats are controlled by computers. The computer is given the [[latitude]] and [[longitude]] of the heliostat's position on the Earth and the time and date. From these, using [[astronomical]] theory, it calculates the direction of the Sun as seen from the mirror, e.g. its compass bearing and angle of elevation. Then, given the direction of the target, the computer calculates the direction of the required angle-bisector, and sends control signals to [[Electric motor|motors]], often [[stepper motor]]s, so they turn the mirror to the correct alignment. This sequence of operations is repeated frequently to keep the mirror properly oriented.

Large installations such as solar-thermal power stations include '''fields of heliostats''' comprising many mirrors. Usually, all the mirrors in such a field are controlled by a single computer.

There are older types of heliostat which do not use computers, including ones that are partly or wholly operated by hand or by [[clockwork]], or are controlled by light-[[sensors]]. These are now quite rare.

Heliostats should be distinguished from [[solar tracker]]s or sun-trackers that point directly at the sun in the sky. However, some older types of heliostat incorporate solar trackers, together with additional components to bisect the sun-mirror-target angle.

A '''siderostat''' is a similar device which is designed to follow a fainter [[star]], rather than the Sun.