Altazimuth(redirected from altazimuth mounts)
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(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
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.
REFERENCESEliseev, S. V. Geodezicheskoe instrumentovedenie. Moscow, 1952.
Podobed, V. V., and V. V. Nesterov. Obshchaia astrometriia. Moscow, 1975.