View source for Astronomy:Altazimuth mount

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

An '''altazimuth mount''' or '''alt-azimuth mount''' is a simple two-axis mount for supporting and [[Rotation around a fixed axis|rotating]] an instrument about two [[Perpendicular|perpendicular]] axes – one vertical and the other horizontal. Rotation about the vertical axis varies the [[Azimuth|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 [[Astronomy:Telescope mount|telescope]]s, [[Physics:Camera|camera]]s, [[Engineering:Antenna (radio)|radio antenna]]s, heliostat mirrors, [[Physics:Solar tracker|solar panel]]s, and [[Engineering: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 [[Engineering:Gun turret|gun turret]] is essentially an alt-azimuth mount for a gun, and a standard [[Engineering:Tripod (photography)|camera tripod]] is an alt-azimuth mount as well.

==Astronomical telescope altazimuth mounts==
When used as an astronomical [[Astronomy:Telescope mount|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 [[Astronomy:Night sky|night sky]] as the [[Astronomy:Earth|Earth]] spins on its axis. On the other hand, an [[Astronomy:Equatorial mount|equatorial mount]] only needs to be rotated about a single axis, at a constant rate, to follow the rotation of the night sky ([[Astronomy:Diurnal motion|diurnal motion]]). Altazimuth mounts need to be rotated about both axes at variable rates, achieved via [[Engineering:Microprocessor|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 [[Engineering:Microprocessor|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 = [[Organization:Bulletin of the Astronomical Society of India|Bulletin of the Astronomical Society of India]] 
  | volume = 13
  | pages = 88
  | date = 1985| issue = 1
 |bibcode = 1985BASI...13...88M }}</ref> On smaller telescopes an [[Astronomy:Equatorial platform|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|right ascension]] to allow for astronomical tracking. The design also does not allow for the use of mechanical [[Astronomy:Setting circles|setting circles]] to locate astronomical objects although modern [[Astronomy:Setting circles#Digital setting circles|digital setting circles]] have removed this shortcoming.

Another limitation is the problem of [[Gimbal lock|gimbal lock]] at [[Zenith|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 [[Astronomy: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. |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.
* [[Astronomy:Dobsonian telescope|Dobsonian telescope]]s: 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 [[Chemistry:Plywood|plywood]], formica, and plastic plumbing parts combined with modern materials like [[Chemistry:Nylon|nylon]] or [[Engineering:Teflon|teflon]].
* "[[Astronomy:GoTo (telescopes)|GoTo]]" telescopes: It has often proved more convenient to build a mechanically simpler altazimuth mount and use a [[Engineering:Motion controller|motion controller]] to manipulate both axes simultaneously to track an object, when compared with a more mechanically complex [[Astronomy:Equatorial mount|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 [[Astronomy:Refracting telescope|refracting telescope]] (with [[Astronomy:Finderscope|finderscope]] and accessories) on a small alt-azimuth mount.
</gallery>

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

{{Sourceattribution|Altazimuth mount}}