Equatorial mount
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An equatorial mount is a mount used for moving a telescope or camera along two perpendicular axes of motion. These two axes are known as right ascension and declination. The telescope mount's axis of right ascension should be pointed directly towards whichever celestial pole is above the horizon to work correctly. Setting the axis of the mount to the polar axis completes alignment. This polar alignment is dependent on the location of the observer and corresponds with the latitude of the observer's location. Equatorial mounts are often equipped with a motor drive for automatic tracking of objects across the night sky. They may also be equipped with setting circles to allow for the location of objects by their celestial coordinates.
The advantage of an equatorial mount lies in its ability to track an object moving across the sky using constant speed movement around only one axis, unlike an altazimuth mount, which requires variable speed motion around both axes to track the diurnal motion. Also, for astrophotography, the image does not rotate in the focal plane, as occurs with altazimuth mounts when they are guided to track the target's motion, unless a rotating erector prism is installed (or other field-derotator).
English mount
In the English mount the system is like big +-sign. The right ascension axis is supported at both ends, and the declination axis is attached to it at approximately mid point.The telescope is placed on other end of the declination axis, and suitable counterweight on other end of it.
German mount
In the German mount the primary structure is a big T-shape, where "vertical" bar is the right ascension axis, and "horizontal" is the declination axis. The right ascension axis has bearings below the T-joint, that is, it is not supported above the declination axis.The telescope is placed on other end of the declination axis, and suitable counterweight on other end of it.
This is most common type of telescope mounts, and many amateur telescopes from 60 mm (2.4") refractors to 14" Schmidt-Cassegrain telescopes are mounted on this mount.
English fork
In English Fork there is a frame that has right ascension axis bearings at the top and the bottom ends, and the declination axis is at its approximate midpoint. The telescope is fitted entirely inside the fork (or not, as is the case with Mt. Wilson 2.5 m reflector) and there are no counterweights like german mount has.Original english fork has the disadvantage of being blocked by the frame from pointing to near north (or south) pole of the sky. For example the Hale telescope is an english fork even though the north end has been changed into huge horse shoe so that it can point to north pole.
Open fork
Most modern mass-produced catadioptic (200 mm or larger diameter) reflecting telescopes tend to be of this type. The mount resembles an Altazimuth mount, but with the azimuth axis is tilted and lined up to match earth rotation axis with a piece of hardware usually called a "wedge."Many mid-size professional telescopes also have equatorial forks, these are usually in range of 0.5-2.0 meter diameter.
See also
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