Any
instrument with a magnification of more than about 7 times must have some
form of mounting if it is to be used to best effect. For small telescopes
and more powerful types of binoculars, the
altazimuth mount is commonly used. This permits movement of the telescope
both vertically (altitude) and horizontally (azimuth).
An altazimuth mounted telescope (left) moves in both horizontal and vertical planes. But because objects in the sky appear to move in a curved line, both axes have to be continually adjusted.
One of the most popular and widely used altazimuth mounts is the Dobsonian. This is an ideal instrument for a beginner. It is freely portable and allows even relatively large telescopes to be handled easily.
The
Dobsonian ounting, left, is a do-it-yourself housing for a Newtonian telescope.
The tube fits into a plywood box, which slots to a cradle. Two side bearings
allow the box to pivot, and the cradle can turn freely on the baseboard.
The altazimuth mount is fine for low-power, wide-field work, but it becomes increasingly difficult to keep objects inside the narrow field of view of a large telescope, and both axes of the mount require constant adjustment.
To overcome this problem, a more advanced design is needed, and many astronomers use an equatorial mount, which permits an object to be followed by the adjustment of one axis only. The equatorial mount has two axes polar and declination - placed at right angles to each other. The polar axis is permanently fixed, parallel to the Earth's axis and pointing to the celestial pole. Once the mount is set up, both axes are used to bring an object into the field of view. The declination axis is then clamped in position and the object followed by slowly turning the telescope tube around the polar axis.
Instead of turning mounts manually, they can be driven automatically. And although normally electrically driven, more sophisticated computer drives are now available.
Any type of mounting and drive system requires some routine maintenance. This includes lightly oiling any moving parts, lubricating bearings and checking components for wear and tear.
In some amateur telescopes the angle of view is so narrow that locating an object in the night sky can be a problem. To overcome this, viewfinders are available, 1 and 2, which fit on the top of the telescope tube and present a wider field of vision. Right-angle finders, 3 and 4, can be helpful when viewing at awkward angles and erecting prisms, 5 and 6, are made for terrestrial observation.
