equatorial mounting


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equatorial mounting:

see telescopetelescope,
traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy; other types of
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equatorial mounting

equatorial mounting

A telescope mounting in which one axis (the polar axis) is parallel to the Earth's axis of rotation, while the second axis (the declination axis) is at right angles to it. Its great advantage is that when the telescope is clamped in declination, and the polar axis is driven to turn once in 24 hours in the opposite direction to the Earth's rotation, any star will remain stationary in the field of view. Until recently almost every large telescope was equatorially mounted, but the constantly changing stresses involved in swinging possibly hundreds of tonnes of asymmetrically shaped material about an inclined axis mean that the mounting must be extremely strong and hence extremely costly, so as a result many telescopes are now constructed with a computer-controlled altazimuth mounting.

There are several types of equatorial mountings, some being shown in the illustration. In the fork mounting the telescope swings in declination about an axis carried on two prongs of a fork; the fork itself rotates about a shaft that is the polar axis. In the yoke (or English) mounting the polar axis is in the form of a long frame with a bearing at each end. The telescope swings in declination about an axis between the sides of the frame. This mounting is simple, very rigid, and needs no counterpoise weights; the polar region is, however, inaccessible. The horseshoe mounting is a modification of the fork and yoke mountings: the upper end of the polar axis frame is made into a horseshoe shape to accommodate the telescope tube; the polar region may then be observed. Many of the giant reflectors use this exceptionally stable mounting. In the German mounting the declination axis is carried as a tee on the top end of the polar axis. The telescope is carried on one end of the declination axis and there is a counterpoise on the other end.

The drive mechanism for the telescope is usually called a drive clock; in modern telescopes it is often an electric motor controlled by a variable frequency generator. A telescope is usually driven, about its polar axis, at the sidereal rate so that one rotation is completed in 23h 56m 4s, the duration of one sidereal day. Small corrections are required in drive rates because of atmospheric refraction and when following the Moon or planets, which move relative to the stars. These corrections are achieved by varying the frequency supplied to the drive motor.

Equatorial Mounting

 

a telescope mounting that has two axes of rotation. One axis is directed toward the celestial pole and forms with the plane of the horizon an angle equal to the geographic latitude of the point of observation. The second axis is perpendicular to the first and lies in the plane of the celestial equator. The axes permit the telescope to be turned and directed at a point of the sky with specified coordinates (hour angle and declination). To compensate for the diurnal motion of the stars, the telescope is turned by a clock mechanism around the polar axis at a rate of one revolution per sidereal day.

equatorial mounting

[‚e·kwə′tȯr·ē·əl ′mau̇nt·iŋ]
(engineering)
The mounting of an equatorial telescope; it has two perpendicular axes, the polar axis (parallel to the earth's axis) that turns on fixed bearings, and the declination axis, supported by the polar axis.
References in periodicals archive ?
The 400mm and 600mm lenses are too heavy and cumbersome to mount alongside a standard small to medium refractor, unless, of course, one has a really substantial observatory-housed equatorial mounting.
However, the telescope has to be hung at an angle to the vertical, and in the case of an arrangement as bulky as the NNTT, an equatorial mounting would impose torques and shears that the system couldn't sustain.
Various options for replacing the instrument were considered, including purchasing a new 28cm fork-mounted Celestron 11 CPC with a far superior drive chain, or using the British company AWR to retrofit a stepper motor control into the LX200, or even purchasing a modern German equatorial mounting onto which somehow to remount the LX200 tube.