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An instrument equipped with both horizontal and vertical graduated circles, for the simultaneous observation of horizontal and vertical directions or angles. Also known as astronomical theodolite; universal instrument.



(also astronomical theodolite, universal instrument), a portable instrument that is used to measure angles in the vertical and horizontal planes. Observations of stars and the sun by means of an altazimuth are used to determine clock corrections and the geographic coordinates of locations and to carry out azimuth determinations with a maximum precision of the order of 0.20”. Altazimuths may also be employed to solve many practical problems in geodesy. An altazimuth differs from a theodolite in the higher precision with which angles, especially vertical angles, are measured.

In a typical altazimuth (see Figure 1), a fixed graduated horizontal circle (1) for measuring horizontal angles is attached to the lower part of the instrument, which is mounted on a tripod with three leveling screws. The upper part of the instrument rotates about the vertical axis and carries an alidade (2) for the horizontal circle, a reading device, and an upright frame (3) that contains

Figure 1. The U-5″ five-second altazimuth

the bearings for the horizontal trunnion axis. An astronomical telescope (4), which is used for sighting, is mounted on and perpendicular to the horizontal axis. If the telescope has a broken optical axis (as in Figure 1), the line of sight is directed by means of a prism through a hollow horizontal axis; an eyepiece (5) with a reticle is at one end of the hollow axis, and a light source for illuminating the field of view during nocturnal observations is at the other end.

Altazimuths are provided with devices for the precision micrometer rotation of the telescope about both axes that is required when the telescope is pointed at an object to be observed. A graduated vertical circle (6), the alidade of which is equipped with a level, is attached to the horizontal axis; the level is used for measuring zenith distances, for taking into account any change in the tilt of the alidade when the vertical axis is not established with sufficient precision, and for taking into account any change in the position of the alidade during an observation. The level of the alidade is also employed to level the altazimuth, that is, to align the vertical axis of the instrument with a plumb line. The tilt of the horizontal axis is determined by means of a plate level (7).

The circles used in an altazimuth are divided into sections that cover 30′, 20′, 10′, or 5′; in some cases, the smallest division is 2′. The circles are read by means of micrometer microscopes, scale microscopes, or verniers. The accuracy of the readings ranges from 30″ to fractions of a second of arc. To check the stability with respect to azimuth, precision altazimuths are provided with a control telescope (8) and micrometer, which are attached to the lower part of the instrument. The control telescope is pointed at a distant fixed sighting mark or target during azimuth and triangulation measurements.

For the determination of latitudes and clock corrections without accurate reading of the circles, that is, by methods of equal zenith distances (seePRACTICAL ASTRONOMY), altazimuths are provided with Talcott levels, which are attached directly to the telescope. By means of such levels, the variation of the inclination of the line of sight with respect to the horizon is controlled during the observations.


Eliseev, S. V. Geodezicheskoe instrumentovedenie. Moscow, 1952.
Podobed, V. V., and V. V. Nesterov. Obshchaia astrometriia. Moscow, 1975.
References in periodicals archive ?
A popular altazimuth mount is the sturdy, boxy Dobsonian design.
In the 1917 May Journal Ainslie wrote a useful three page paper (37) entitled: Notes from the Instrument Committee--On the Choice of a Telescope in which he explained the relative advantages and disadvantages of reflectors, refractors, equatorial mounts and altazimuth mounts.
There is a plaque on the tube to testify to this, and that instrument was originally tripod mounted as an altazimuth system, only being converted onto a modern equatorial mount in the 1970s.
Many of today's telescopes have motor-driven altazimuth mounts that keep them pointed at celestial objects as they move from east to west across the sky.
An altazimuth variation that's popular on Newtonian reflectors of all sizes (from $200 starter scopes to 36-inch monsters) is the Dobsonian mount.
We were looking for complete telescopes of at least 70-millimeters aperture, mated to simple, sturdy, easy-to-use altazimuth mounts.
Meade's Infinity 90mm Altazimuth Refractor has enough aperture to produce satisfying views of the Sun, Moon, and planets, as well as bright deep-sky objects and double stars.
Superficially, the two have much in common--both are basic 5-inch Newtonian reflectors on altazimuth mounts.
If you want to take one of these telescopes along for casual observing on a weekend camping trip, you might want a mounting as simple (and portable) as a heavy-duty photographic tripod fitted with a lightweight altazimuth mount.
A 1998 follow-up study using the Russian 6-meter Large Altazimuth Telescope found magnitudes, diameters, separations, morphological types, configurations, and radial velocities.
Like Chuck, Art was looking for a way to improve on the conventional altazimuth mount.
The oldest and simplest kind of mount is the altazimuth, in which one axis turns to allow the telescope to change its azimuth or cardinal direction (north, south, east, and west), while the other allows the telescope to change its altitude (up and down).