An altazimuth mount or alt-azimuth mount is a simple two- axis mount for supporting and rotating an instrument about two

perpendicular In geometry, two geometric objects are perpendicular if they intersect at right angles, i.e. at an angle of 90 degrees or π/2 radians. The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', � ...

axes – one vertical and the other horizontal. Rotation about the vertical axis varies the

azimuth An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...

(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 A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

camera A camera is an instrument used to capture and store images and videos, either digitally via an electronic image sensor, or chemically via a light-sensitive material such as photographic film. As a pivotal technology in the fields of photograp ...

s, radio antennas, heliostat mirrors,

solar panel A solar panel is a device that converts sunlight into electricity by using photovoltaic (PV) cells. PV cells are made of materials that produce excited electrons when exposed to light. These electrons flow through a circuit and produce direct ...

s, and guns 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 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 The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include moonlig ...

as the

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

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 A microprocessor is a computer processor (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...

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

based counter rotation system. On smaller telescopes an equatorial platform 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 (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...

to allow for astronomical tracking. The design also does not allow for the use of mechanical setting circles to locate astronomical objects although modern digital setting circles have removed this shortcoming. Another limitation is the problem of

gimbal lock Gimbal lock is the loss of one degree of freedom (mechanics), degree of freedom in a multi-dimensional mechanism at certain alignments of the axes. In a three-dimensional three-gimbal mechanism, gimbal lock occurs when the axes of two of the gi ...

zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...

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 or 0.75 degrees 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 largest telescopes, the mass and cost of an equatorial mount is prohibitive and they have been superseded by computer-controlled altazimuth mounts. 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. 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.

Amateur telescopes

* Beginner telescopes: Altazimuth mounts are cheap and simple to use. * Dobsonian telescopes: 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 ''Formica'' is a genus of ants of the subfamily Formicinae, including species commonly known as wood ants, mound ants, thatching ants, and field ants. ''Formica'' is the type genus of the Formicidae, and of the subfamily Formicinae. The type ...

, and plastic plumbing parts combined with modern materials like

nylon Nylon is a family of synthetic polymers characterised by amide linkages, typically connecting aliphatic or Polyamide#Classification, semi-aromatic groups. Nylons are generally brownish in color and can possess a soft texture, with some varieti ...

or teflon. * " 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

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 telescopes at Paranal Observatory. The entire building constitutes the altazimuth mount, saving on mass and cost. File:SmallAltAzMount.jpg, A

refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...

(with finderscope and accessories) on a small alt-azimuth mount.

References

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External links

Images of the Unitron altazimuth mount
Telescopes