The common alt-azimuth mount on most cheaper telescopes is typically prone to a certain wobbliness, vibration, and “backlash” that tends to make them hard to point accurately. Most noticeable when viewing at medium to high powers, backlash occurs when you turn the telescope directly with your hand. This condition causes the telescope to spring back in the opposite direction from which you moved it once your hand lets go. Some manufacturers get around these problems by introducing worm gears turned by cables to move the telescope. Still, even such improved mounts often vibrate in even a relatively small wind, thereby making focusing of the telescope difficult. Also, they tend to jiggle for at least a couple of seconds after the user turns loose of the focusing knob of the telescope.
Most of these mounts rest on adjustable tripods that may be raised or lowered by extending or collapsing the legs of the tripod. It is possible to lessen the vibration at least somewhat by keeping the legs of the tripod as short as is practical. In other words, the lower the mount the less vibration there is.
The Fork Equatorial Mount
Of the two primary types of equatorial mount, the fork equatorial mount is without a doubt the easiest to learn to use. This mount is essentially the alt-azimuth mount except for the difference that it is tilted in such a manner that one axis is parallel to the Earth’s axis of rotation. This particular axis is called either the polar axis or right ascension axis. The telescope turns about the declination axis to pivot up or down. Look at the example in Figure 7.
With the alt-azimuth mount, when the Earth’s rotation causes an object to leave the field of view, you need to turn the telescope in both the altitude and azimuth directions to reacquire the object. With the fork mounted equatorial, you need only turn the telescope toward the right (about the polar axis) to reacquire the object! This one direction tracking is possible because the polar axis is parallel to the Earth’s axis of rotation. The fact that the telescope needs to be turned at a steady speed in only one direction makes motorized tracking of an object easy. Thus, this mount lends itself well to astrophotography. For over a hundred years, major observatory telescopes have traditionally either been on fork equatorial mounts or German equatorial mounts.
copyright 2004 Singularity Scientific