Editing Altazimuth mount

{{Short description|Support mechanism with rotation about the horizontal and vertical axes}}
[[File:Dobson class.jpg|thumb|right|250px|A [[Newtonian telescope]] on a simple [[Dobsonian mount]]]]

An '''altazimuth mount''' or '''alt-azimuth mount''' is a simple two-[[coordinate axis|axis]] mount for supporting and [[Rotation around a fixed axis|rotating]] an instrument about two [[perpendicular]] axes – one vertical and the other horizontal. Rotation about the vertical axis varies the [[azimuth]] (compass bearing) of the pointing direction of the instrument. Rotation about the horizontal axis varies the [[altitude angle]] (angle of elevation) of the pointing direction.

These mounts are used, for example, with [[Telescope mount|telescope]]s, [[camera]]s, [[antenna (radio)|radio antenna]]s, [[heliostat]] mirrors, [[solar tracker|solar panel]]s, and [[weapon mount|gun]]s and similar weapons.

Several names are given to this kind of mount, including '''altitude-azimuth''', '''azimuth-elevation''' and various abbreviations thereof. A [[gun turret]] is essentially an alt-azimuth mount for a gun, and a standard [[Tripod (photography)|camera tripod]] is an alt-azimuth mount as well.

==Astronomical telescope altazimuth mounts==
When used as an astronomical [[telescope mount]], the biggest advantage of an alt-azimuth mount is the simplicity of its mechanical design. The primary disadvantage is its inability to follow astronomical objects in the [[night sky]] as the [[Earth]] spins on its axis. On the other hand, an [[equatorial mount]] only needs to be rotated about a single axis, at a constant rate, to follow the rotation of the night sky ([[diurnal motion]]). Altazimuth mounts need to be rotated about both axes at variable rates, achieved via [[microprocessor]] based two-axis drive systems, to track equatorial motion. This imparts an uneven rotation to the field of view that also has to be corrected via a [[microprocessor]] based counter rotation system.<ref>{{cite journal
  | last1 = Mahra
  | first1 = H. S.
  | last2 = Karkera 
  | first2 = B. N.
  | title = Field rotation with altazimuth mounting telescope
  | journal = [[Bulletin of the Astronomical Society of India]] 
  | volume = 13
  | pages = 88
  | date = 1985| issue = 1
 |bibcode = 1985BASI...13...88M }}</ref> On smaller telescopes an [[equatorial platform]]<ref>{{cite web |last=Vogel |first=Reiner |title=Equatorial Platform |date=2007 |url=http://www.reinervogel.net/Plattform/Plattform_e.html |access-date=16 January 2014}}</ref> is sometimes used to add a third "polar axis" to overcome these problems, providing an hour or more of motion in the direction of [[right ascension]] to allow for astronomical tracking. The design also does not allow for the use of mechanical [[setting circles]] to locate astronomical objects although modern [[Setting circles#Digital setting circles|digital setting circles]] have removed this shortcoming.

Another limitation is the problem of [[gimbal lock]] at [[zenith]] pointing. When tracking at elevations close to 90°, the azimuth axis must rotate very quickly; if the altitude is exactly 90°, the speed is infinite. Thus, altazimuth telescopes, although they can point in any direction, cannot track smoothly within a "zenith blind spot", commonly 0.5<ref>{{cite web |url=http://www.gemini.edu/media/factsheets/telescopestructure.html |title=Telescope Structure |publisher=Gemini Observatory |access-date=2015-08-08}}</ref> or 0.75 degrees<ref>{{Citation |url=http://www.oir.caltech.edu/twiki_oir/pub/Keck/NGAO/NewKAONs/KAON708.pdf |title=Limits to AO Observations from Altitude-Azimuth Telescope Mounts |first=Christopher |last=Neyman |date=February 10, 2010 |page=5 |publisher=W.M. Keck Observatory |id=Keck Adaptive Optics Note 708 |access-date=2015-08-08}}</ref> from the zenith. (i.e. at elevations greater than 89.5° or 89.25° respectively.)

===Current applications===
Typical current applications of altazimuth mounts include the following.

====Research telescopes====
In the [[list of largest optical reflecting telescopes|largest telescopes]], the mass and cost of an equatorial mount is prohibitive and they have been superseded by computer-controlled altazimuth mounts.<ref>{{cite book |url=https://books.google.com/books?id=wXCMYy0yF5cC&q=altazimuth+mounts+have+been&pg=PA147 |first1=Pierre |last1=Léna |first2=François |last2=Lebrun |first3=François |last3=Mignard |title=Observational astrophysics |page=147 |isbn=9783540634829 |date=1998-01-01}}</ref> The simple structure of an altazimuth mount allows significant cost reductions, in spite of the additional cost associated with the more complex tracking and image-orienting mechanisms.<ref>{{cite journal |last1=Dawe |first1=J. A. |last2=Watson |first2=F. G. |author-link2=Fred Watson |title=Design for a large altazimuth Schmidt telescope |journal=Occasional Reports of the Royal Observatory, Edinburgh |volume=16 |pages=15–18 |date=1985 |bibcode=1985ORROE..16...15D}}</ref> An altazimuth mount also reduces the cost in the dome structure covering the telescope since the simplified motion of the telescope means the structure can be more compact.<ref>{{cite book |url=https://books.google.com/books?id=4x5Fw0AWfZsC&q=altazimuth+mounts+have+been&pg=PA18 |first1=Siegfried | last1=Marx |first2=Werner |last2=Pfau |title=Astrophotography with the Schmidt telescope |page=18 |isbn=9780521395496 |date=1992-06-11}}</ref>

====Amateur telescopes====
* Beginner telescopes: Altazimuth mounts are cheap and simple to use.
* [[Dobsonian telescope]]s: [[John Dobson (astronomer)|John Dobson]] popularized a simplified altazimuth mount design for [[Newtonian reflectors]] because of its ease of construction; Dobson's innovation was to use non-machined parts for the mount that could be found in any hardware store such as [[plywood]], [[formica (plastic)|formica]], and plastic plumbing parts combined with modern materials like [[nylon]] or [[teflon]].
* "[[GoTo (telescopes)|GoTo]]" telescopes: It has often proved more convenient to build a mechanically simpler altazimuth mount and use a [[motion controller]] to manipulate both axes simultaneously to track an object, when compared with a more mechanically complex [[equatorial mount]] that requires minimally complex control of a single motor.

==Gallery==
<gallery mode=packed heights=175px>
File:heliostat.jpg|A [[heliostat]] at the THÉMIS experimental station in France. The mirror rotates on an alt-azimuth mount. The pointing direction of the mirror is perpendicular to its surface.
File:Paranal opendome.jpg|One of the {{convert |8.2 |m |inch |abbr=on}} telescopes at [[Paranal Observatory]]. The entire building constitutes the altazimuth mount, saving on mass and cost.
File:SmallAltAzMount.jpg|A [[refracting telescope]] (with [[finderscope]] and accessories) on a small alt-azimuth mount.
</gallery>

==See also==
{{Portal|Astronomy}}
* [[Dobsonian mount]]
* [[Equatorial mount]]
* [[Heliostat]]
* [[Horizontal coordinate system]] – a system to locate objects on the celestial sphere via Alt-azimuth coordinates
* [[Parallactic angle]]
* [[Solar tracker]]
* [[Tripod]]

==References==
{{Reflist|30em}}

==External links==
* [http://www.photoinfos.com/astronomie/Azimutale-Montierung/Unitron-Polarex-Azimutal.htm Images of the Unitron altazimuth mount]

[[Category:Telescopes]]