Editing Planetarium (section)

==Technology ==

=== Domes ===
<gallery mode= "packed" class="center" caption="Examples of planetarium domes">
Bundesarchiv Bild 183-1987-1008-020, Berlin, Zeiss-Großplanetarium.jpg|The Large Zeiss Planetarium in Berlin, 1987.
EugenFound-023.jpg|The dome of the [[Athens]] Planetarium.
Hamburg Planetarium.jpg|The [[Hamburg Planetarium]]
VU planetariumas by Augustas Didzgalvis.jpg|The dome of the [[Vilnius University]] Planetarium.
Planetariet.jpg|Inside of the Planetarium located in the [[Science Factory (Vitenfabrikken)]] in [[Sandnes]], [[Norway]].
Aleksandria raamatukogu IMG 0119.JPG|Dome of the [[Planetarium Science Center]] of the [[Bibliotheca Alexandrina]]
Planetobus - kopuła mobilnego planetarium Centrum Nauki Kopernik w Warszawie.jpg|A small inflatable portable planetarium dome.
Planetarium21-alex.jpg|GM-II starfield projector at [[Kerala Science and Technology Museum|Priyadarshini Planetarium]], [[Trivandrum]], [[India]]
Planetarium-alex.jpg|[[Priyadarshini Planetarium]], [[Trivandrum]], [[India]]
Søerne 5.jpg|[[Tycho Brahe Planetarium|Planetarium]], [[Copenhagen]], [[Denmark]]
</gallery>

Planetarium domes range in size from 3 to 35 m in [[diameter]], accommodating from 1 to 500 people. They can be permanent or portable, depending on the application. 
* Portable [[inflatable]] domes can be inflated in minutes. Such domes are often used for touring planetariums visiting, for example, schools and community centres.
* Temporary structures using [[glass-reinforced plastic]] (GRP) segments bolted together and mounted on a frame are possible. As they may take some hours to construct, they are more suitable for applications such as exhibition stands, where a dome will stay up for a period of at least several days.
* Negative-pressure inflated domes are suitable in some semi-permanent situations. They use a fan to extract air from behind the dome surface, allowing [[atmospheric pressure]] to push it into the correct shape.
* Smaller permanent domes are frequently constructed from glass reinforced plastic. This is inexpensive but, as the projection surface reflects sound as well as light, the [[acoustics]] inside this type of dome can detract from its utility. Such a solid dome also presents issues connected with heating and ventilation in a large-audience planetarium, as air cannot pass through it.
* Older planetarium domes were built using traditional construction materials and surfaced with [[plaster]]. This method is relatively expensive and suffers the same [[Acoustics|acoustic]] and [[Ventilation (architecture)|ventilation]] issues as GRP.
* Most modern domes are built from thin [[aluminium]] sections with ribs providing a supporting structure behind.<ref>{{cite web|title=ESOblog: How to Install a Planetarium A conversation with engineer Max Rößner about his work on the ESO Supernova|url=https://www.eso.org/public/blog/how-to-install-a-planetarium/|website=www.eso.org|access-date=21 February 2018|url-status=dead|archive-url=https://web.archive.org/web/20180507190501/https://www.eso.org/public/blog/how-to-install-a-planetarium/|archive-date=7 May 2018}}</ref> The use of aluminium makes it easy to perforate the dome with thousands of tiny holes. This reduces the reflectivity of sound back to the audience (providing better acoustic characteristics), lets a sound system project through the dome from behind (offering sound that seems to come from appropriate directions related to a show), and allows air circulation through the projection surface for climate control.

The realism of the viewing experience in a planetarium depends significantly on the [[dynamic range]] of the image, i.e., the contrast between dark and light. This can be a challenge in any domed projection environment, because a bright image projected on one side of the dome will tend to reflect light across to the opposite side, "lifting" the [[black level]] there and so making the whole image look less realistic. Since traditional planetarium shows consisted mainly of small points of light (i.e., stars) on a black background, this was not a significant issue, but it became an issue as digital projection systems started to fill large portions of the dome with bright objects (e.g., large images of the sun in context). For this reason, modern planetarium domes are often not painted white but rather a mid grey colour, reducing reflection to perhaps 35-50%. This increases the perceived level of contrast.

A major challenge in dome construction is to make seams as invisible as possible. Painting a dome after installation is a major task, and if done properly, the seams can be made almost to disappear.

Traditionally, planetarium domes were mounted horizontally, matching the natural horizon of the real night sky. However, because that configuration requires highly inclined chairs for comfortable viewing "straight up", increasingly domes are being built tilted from the horizontal by between 5 and 30 degrees to provide greater comfort. Tilted domes tend to create a favoured "sweet spot" for optimum viewing, centrally about a third of the way up the dome from the lowest point. Tilted domes generally have seating arranged stadium-style in straight, tiered rows; horizontal domes usually have seats in circular rows, arranged in concentric (facing center) or epicentric (facing front) arrays.

Planetaria occasionally include controls such as buttons or [[joystick]]s in the arm rests of seats to allow audience feedback that influences the show in [[Real-time computing|real time]].

Often around the edge of the dome (the "cove") are:
* [[Silhouette]] models of geography or buildings like those in the area round the planetarium building.
* Lighting to simulate the effect of twilight or urban [[light pollution]].

Traditionally, planetariums needed many [[Incandescent light bulb|incandescent lamps]] around the cove of the dome to help audience entry and exit, to simulate [[sunrise]] and [[sunset]], and to provide working light for dome cleaning. More recently, solid-state [[LED]] lighting has become available that significantly decreases power consumption and reduces the maintenance requirement as lamps no longer have to be changed on a regular basis.

The world's largest mechanical planetarium is located in Monico, Wisconsin. The ''Kovac Planetarium''. It is 22 feet in diameter and weighs two tons. The globe is made of wood and is driven with a variable speed motor controller. This is the largest mechanical planetarium in the world, larger than the ''Atwood Globe'' in Chicago (15 feet in diameter) and one third the size of the Hayden.

Some new planetariums now feature a [[glass floor]], which allows spectators to stand near the center of a [[sphere]] surrounded by projected images in all directions, giving the impression of floating in [[outer space]].  For example, a small planetarium at [[AHHAA]] in [[Tartu]], [[Estonia]] features such an installation, with special projectors for images below the feet of the audience, as well as above their heads.<ref name=Aru>{{cite journal|last1=Aru|first1=Margus|title=Under One Dome: AHHAA Science Centre Planetarium|journal=Planetarian: Journal of the International Planetarium Society|date=March–June 2012|volume=41|issue=2|page=37|url=http://c.ymcdn.com/sites/www.ips-planetarium.org/resource/resmgr/planetarian/201206planetarian.pdf|access-date=2017-06-02|url-status=live|archive-url=https://web.archive.org/web/20151002175556/http://c.ymcdn.com/sites/www.ips-planetarium.org/resource/resmgr/planetarian/201206planetarian.pdf|archive-date=2015-10-02}}</ref>

===Traditional electromechanical/optical projectors===
{{Main|Planetarium projector}}
{{Multiple image|perrow=2|total_width=300
|image1=Bundesarchiv B 145 Bild-P018935, Berlin, Planetarium.jpg|caption1=A [[Zeiss projector]] in a Berlin planetarium during a show in 1939.
|image2=ZeissPlanetariumProjector MontrealPlanetarium.jpg|caption2=Zeiss projector at [[Dow Planetarium]] in Montreal
|image3=Universarium in Planetarium Hamburg.jpg|caption3=A modern, egg-shaped Zeiss projector (UNIVERSARIUM Mark IX) at the Hamburg planetarium
|image4=Киевский планетарий. Аппарат "Большой цейс - 4".JPG|caption4=Zeiss projector at [[Kyiv Planetarium]]
}}
Traditional [[planetarium projector|planetarium projection apparatus]] use a hollow ball with a light inside, and a pinhole for each star, hence the name "star ball". With some of the brightest stars (e.g. [[Sirius]], [[Canopus]], [[Vega]]), the hole must be so big to let enough light through that there must be a small lens in the hole to focus the light to a sharp point on the dome. In later and modern planetarium star balls, the individual bright stars often have individual projectors, shaped like small hand-held torches, with focusing lenses for individual bright stars. Contact breakers prevent the projectors from projecting below the "horizon".{{Citation needed|date=October 2008}}

The star ball is usually mounted so it can rotate as a whole to simulate the Earth's daily rotation, and to change the simulated latitude on Earth. There is also usually a means of rotating to produce the effect of [[precession of the equinoxes]]. Often, one such ball is attached at its south [[ecliptic]] pole. In that case, the view cannot go so far south that any of the resulting blank area at the south is projected on the dome. Some star projectors have two balls at opposite ends of the projector like a [[dumbbell]]. In that case all stars can be shown and the view can go to either pole or anywhere between. But care must be taken that the projection fields of the two balls match where they meet or overlap.

Smaller planetarium projectors include a set of fixed stars, Sun, Moon, and planets, and various [[nebula]]e. Larger projectors also include [[comet]]s and a far greater selection of stars. Additional projectors can be added to show twilight around the outside of the screen (complete with city or country scenes) as well as the [[Milky Way]]. Others add coordinate lines and [[constellation]]s, photographic slides, [[laser]] displays, and other images.

Each planet is projected by a sharply focused [[Stage lighting instrument#Spotlights|spotlight]] that makes a spot of light on the dome. Planet projectors must have gearing to move their positioning and thereby simulate the planets' movements. These can be of these types:-
*[[Heliocentrism|Copernican]]. The axis represents the Sun. The rotating piece that represents each planet carries a light that must be arranged and guided to swivel so it always faces towards the rotating piece that represents the Earth. This presents mechanical problems including:
*:The planet lights must be powered by wires, which have to bend about as the planets rotate, and repeatedly bending copper wire tends to cause wire breakage through [[metal fatigue]].
*:When a planet is at [[Opposition (astronomy)|opposition]] to the Earth, its light is liable to be blocked by the mechanism's central axle. (If the planet mechanism is set 180° rotated from reality, the lights are carried by the Earth and shine towards each planet, and the blocking risk happens at [[conjunction (astronomy)|conjunction]] with Earth.)
*[[Geocentric model|Ptolemaic]]. Here the central axis represents the Earth. Each planet light is on a mount which rotates only about the central axis, and is aimed by a guide which is steered by a deferent and an epicycle (or whatever the planetarium maker calls them). Here Ptolemy's number values must be revised to remove the daily rotation, which in a planetarium is catered for otherwise. (In one planetarium, this needed Ptolemaic-type orbital constants for [[Uranus]], which was unknown to Ptolemy.)
*Computer-controlled. Here all the planet lights are on mounts which rotate only about the central axis, and are aimed by a [[computer]].

Despite offering a good viewer experience, traditional star ball projectors suffer several inherent limitations. From a practical point of view, the low light levels require several minutes for the audience to [[Adaptation (eye)|"dark adapt"]] its eyesight. "Star ball" projection is limited in education terms by its inability to move beyond an Earth-bound view of the night sky. Finally, in most traditional projectors the various overlaid projection systems are incapable of proper [[occultation]]. This means that a planet image projected on top of a star field (for example) will still show the stars shining through the planet image, degrading the quality of the viewing experience. For related reasons, some planetariums show stars below the horizon projecting on the walls below the dome or on the floor, or (with a bright star or a planet) shining in the eyes of someone in the audience.

However, the new breed of Optical-Mechanical projectors using fiber-optic technology to display the stars show a much more realistic view of the sky.

===Digital projectors===
[[File:ADLIP Jena.JPG|thumb|right|A [[fulldome]] laser projection.]]
An increasing number of planetariums are using [[Digital data|digital]] technology to replace the entire system of interlinked projectors traditionally employed around a star ball to address some of their limitations. Digital planetarium manufacturers claim reduced maintenance costs and increased reliability from such systems compared with traditional "star balls" on the grounds that they employ few moving parts and do not generally require synchronisation of movement across the dome between several separate systems. Some planetariums mix both traditional opto-mechanical projection and digital technologies on the same dome.
[[File:E&S Digital Laser Projector.jpg|thumb|An example of a digital laser projector installed in the Lohman Planetarium at the [[Museum of Arts and Sciences (Daytona Beach)|Museum of Arts and Sciences]], [[Daytona Beach, Florida|Daytona]]. This Projector employs a [[Fisheye lens|fisheye]] lens to project an image across the entire dome.]]
In a fully digital planetarium, the dome image is generated by a [[computer]] and then projected onto the dome using a variety of technologies including [[cathode-ray tube]], [[LCD]], [[Digital Light Processing|DLP]], or [[laser]] projectors. Sometimes a single projector mounted near the centre of the dome is employed with a [[fisheye lens]] to spread the light over the whole dome surface, while in other configurations several projectors around the horizon of the dome are arranged to blend together seamlessly.

Digital projection systems all work by creating the image of the night sky as a large array of [[pixels]]. Generally speaking, the more pixels a system can display, the better the viewing experience. While the first generation of digital projectors were unable to generate enough pixels to match the image quality of the best traditional "star ball" projectors, high-end systems now offer a resolution that approaches the limit of human [[visual acuity]].

LCD projectors have fundamental limits on their ability to project true black as well as light, which has tended to limit their use in planetaria. [[LCOS]] and modified LCOS projectors have improved on LCD [[contrast ratio]]s while also eliminating the "screen door" effect of small gaps between LCD pixels. "Dark chip" DLP projectors improve on the standard DLP design and can offer relatively inexpensive solution with bright images, but the black level requires physical baffling of the projectors.  As the technology matures and reduces in price, laser projection looks promising for dome projection as it offers bright images, large dynamic range and a very wide [[color space]].