Telescope Support, Mounts, and Drives Help
Telescope Support, Mounts, and Drives
Telescopes can be supported, mounted, and driven in various ways. Some systems are designed for simplicity and convenience; others are intended for ease of tracking once the telescope has been aimed. Any system can be driven by a motor that keeps it aimed at an object as the Earth rotates. The most sophisticated systems can locate objects using computer programs and can follow them so precisely that they remain in the field of view for an hour or so.
Tripod Versus Pedestal
With the exception of the Dobsonian mount (described below), the most common amateur telescope support is the tripod . A telescope tripod resembles the tripods used for photography or video recording, but the telescope design is sturdier and more resistant to vibration. A typical telescope tripod can be adjusted in height from approximately 1 m (40 in) to 1.5 m (60 in). As its name implies, the tripod has three legs, the lengths of which are independently adjustable. This allows you to level the instrument even if the surface is somewhat irregular.
The pedestal support is preferred by some people for use with large telescopes, especially those using the German equatorial mount (described below). A pedestal consists of a single, massive, thick vertical post. The post can be supported on a flat base, or it can be driven and cemented into the ground. Pedestal supports are less portable than tripods (and of course, not portable at all if permanently secured to the surface). This type of support is sturdier than most tripods. Care must be exercised to ensure that the pedestal is perfectly plumb (vertical).
The simplest, and generally the cheapest, set of bearings you can get for a telescope is the azimuth-elevation (az-el) mount . It goes by other names too, such as altitude-azimuth, altazimuth , or alt-az . Figure 20-10 is a simplified drawing of a refractor employing this system. The azimuth bearing turns 360 degrees in the horizontal plane. The elevation bearing rotates in the vertical plane for as far as the telescope will allow. Theoretically, only 90 degrees of elevation range is necessary, from the horizon to the zenith. Using the bearings in combination, the telescope can be pointed to any object in the sky. Because of the construction of the particular az-el system shown in Fig. 20-10, it is sometimes called a fork mount .
The az-el mount, while convenient for casual telescope users, has limitations. As Earth rotates, objects move across the sky in paths parallel to the celestial equator. Near the celestial equator, this motion is from east to west; near the celestial poles it is in circles, counterclockwise in the northern hemisphere and clockwise in the southern. In order to follow an object across the sky over a period of time, you’ll have to adjust both the azimuth and the elevation settings in an az-el mount (unless you happen to be at either the north or the south geographic pole). It would be much easier if you only had to move one of the bearings. This is possible with a simple modification of the az-el system.
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