Telescope Support, Mounts, and Drives Help

Fork Mount/wedge

If an az-el mount is tilted so that the plane of the “horizon” corresponds with the celestial equator rather than with the actual horizon, a telescope can track objects in the sky by continuous adjustment of only one bearing. The 360 degree azimuth bearing from the az-el system becomes a right-ascension (RA) bearing . When it is rotated, the telescope moves east and west in celestial longitude. The range of the elevation bearing from the az-el system must be extended to cover 180 degrees, and it becomes a declination bearing . When it is adjusted, the telescope moves north and south in celestial latitude.

The proper tilt for the converted az-el system is accomplished by means of a wedge, constructed or set at an angle that corresponds to the terrestrial latitude where the telescope is located. The fork mount , which gets its name from its shape, lends itself readily to this scheme (Fig. 20-11). This system is popular among SCT users.

Figure 20-11. A wedge can be added to a fork mount for easy tracking of celestial objects.

The fork mount/wedge requires alignment to work properly. The RA axis (the axis of the right-ascension bearing) must point precisely at the north celestial pole. A slight misalignment will result in improper tracking, especially over long periods of time. To ensure that the alignment is correct, the wedge is adjustable. You should determine your latitude down to the minute of arc. (There are several Web sites that can provide you with this information if you live in a town of at least medium size. Because the Web page locations change constantly, the best way to find them is to enter the phrase latitude and longitude into a well-known search engine such as google.com .)

Dobsonian Mount

Large Newtonian telescopes—those over 25 cm (10 in) in diameter—present a special challenge when it comes to mounting them and viewing through them. The Dobsonian mount , named after its inventor, is an az-el system that sits directly on the ground or pavement (Fig. 20-12).

Figure 20-12. A Dobsonian mount is convenient for use with large Newtonian reflectors.

Dobsonian mounts usually are constructed from plywood with Teflon bearings. The plywood helps to dampen vibrations transmitted through the ground, such as can be caused by heavy trucks on nearby streets. The Teflon bearings provide ease and smoothness of movement. Because the telescope sits lower to the surface than is the case with a tripod or pedestal mount, the eyepiece can be reached with less difficulty. However, with extremely large Newtonian reflectors (those over 40 cm across and/or with high f -ratios), a ladder is necessary for viewing objects near the zenith.

The limitations of the Dobsonian mount are similar to those of the az-el mount. Tracking can be inconvenient because both the azimuth and the elevation bearings must be moved. Special equatorial mounting tables are available for Dobsonian telescopes. The table is sloped, and the slope can be adjusted; it performs the same function as the wedge in the fork mount/wedge system. If you happen to live in the tropics, the slope of the table can interfere with the full range of movement of the Dobsonian mount. However, at higher latitudes, including most of Europe and North America, the equatorial table is a convenient option.

German Equatorial Mount

One of the best-known sets of telescope bearings is found in the German equatorial mount . It can be recognized by its unique configuration and counterweight (Fig. 20-13). It, like the fork mount/wedge, moves along RA and declination coordinates. The RA axis is adjustable and must be aimed at the north celestial pole. The most sophisticated German equatorial mounts are equipped with sighting scopes that make them fairly easy to align. Once the mount is adjusted properly and the telescope is aimed at an object in the sky, the object can be followed by moving only the RA bearing.

Some people find the German equatorial mount unnatural and awkward, especially when observing things in the circumpolar region. However, a well-made and well-adjusted system of this type is superior to any other for serious use with massive and bulky telescopes. Not surprisingly, the best German equatorial mounts are expensive. Some of the largest ones, including a heavy-duty pedestal base, cost more than $2,000.

Figure 20-13. Simplified drawing of a German equatorial mount.

Clock Drive

If you want to gaze at celestial objects through a telescope for a long time, a clock drive is a great convenience. This is especially true if you have a group of people, each of whom must look through the telescope in turn and not all of whom are experts at aiming it. The higher the magnification, the more quickly a celestial object will “drift” out of the field of view unless some compensation is made. Objects near the celestial equator drift faster than objects near the celestial pole.

The heart of the clock drive is a slow motor that makes one complete 360-degree revolution per sidereal day, that is, every 23 hours and 56 minutes. This is accomplished by gearing-down and precise regulation of the actual motor speed. Clock drives are typically designed to work with either a fork mount/wedge or a German equatorial mount because only the RA bearing needs to be connected to the motor. Clock drives can function with az-el drive systems, but they are more complicated and more expensive because both bearings must be adjusted by the device as time passes.

With any clock drive, the quality of the gears is particularly important. If there is significant play in the gears, the telescope will wobble with the slightest disturbance. This effect is particularly annoying at higher magnification and takes away much of the advantage of the clock drive. If you plan to do any time-exposure astrophotography with your telescope, you will need a clock drive, an equatorial mount, and a guiding device that makes minute corrections based on a guide star that you select within the field of view. Under these circumstances, telescope vibration and gear play cannot be tolerated at all.

Clock drives require a source of electrical power. This can be a battery, or it can be the household utility current. Batteries tend to wear out fast in most clock drives, and they never go dead when it’s convenient. (No time is convenient for a power failure!) Use of the utility power requires an extension cord.

Computerized Drives

In recent years, a number of amateur telescopes, particularly SCTs, have been made available equipped with microcomputers that automatically guide the instrument to any of several thousand objects in the sky. These telescopes are manufactured by Celestron, Inc., among others. The most sophisticated models incorporate Global Positioning System (GPS) receivers so that the alignment process is automatic. You don’t even have to know where on Earth you are to set up such a telescope and use it.

A computerized drive, like the venerable videocassette recorder (VCR), requires the user to climb a learning curve. The system must be programmed and objects selected according to a certain sequence of entries. The information is entered on a keypad, and the system status is displayed on a screen. You also can locate objects manually using a set of up/down and left/right (or north/south and east/west) buttons. The slew rate (the speed at which the telescope turns as you hold down one of the buttons) can be selected over a range from slowest to fastest, for example, from 1 (very slow) to 9 (quite fast).

Once an object has been located in the sky, the telescope can be programmed to follow it. This scheme can work with az-el or equatorial systems, but if you have astrophotography in mind, the equatorial system is a must. Celestial objects rotate in the field of view over time when an az-el clock drive is used because an az-el–mounted telescope doesn’t maintain a constant attitude (orientation) with respect to celestial coordinates as the heavens sweep around the celestial pole. This will blur a time-exposure photograph, even if the bearings and the clock drive itself are aligned perfectly.

For a computerized drive to work correctly, the telescope must be aligned with great accuracy. If it doesn’t have the GPS feature, this means that it has to be set by using two or three reference stars. These stars must be far apart from one another in the sky, and they must all be above the horizon at the time of alignment. You will have to know your terrestrial latitude and longitude down to the minute of arc or better. In addition, you’ll have to know the exact time. This can be found at the following Web site (as of the time of this writing):

It will take some practice to get good at aligning a computerized drive system unless it has GPS built in. You will have to learn to carry out the whole process within a few moments. With each tick of the clock, your reference stars move across the heavens by several seconds of arc. Once you have the telescope aligned accurately, you can select the object you want by navigating the computer menu, and the telescope will aim itself. It’s best to use the lowest available magnification initially because this gives your telescope the greatest margin for error. You can then fine-tune the position of the telescope if you want more magnification.

Practice problems of this concept can be found at: Your Home Observatory Practice Problems