.. By Chris Stellis
One of the things many people do not think about when buying a telescope is the mount. Telescope mounts can make a huge difference in the quality of the viewing experience. An unstable, shaky mount can make it difficult to see detail on close objects and nearly impossible to see faint sky objects.
There are two main types of mounts and a number of sub-types. The altazimuth mounts are configured much like a camera tripod. There are controls for moving the telescope up and down (altitude) and controls for moving left to right (azimuth). One popular type of altazimuth mount is known as the Dobsonian. Dobsonian mounts are usually found on medium or large size reflecting telescopes.
The other main type or mount is called an equatorial mount. Equatorial mounts are designed to follow the motion of the sky. This is especially useful since the rotation of the earth can cause objects to quickly move out of view. Equatorial mounts are often configured with motorised drives which match the rotation of the Earth. This makes it easier to observe sky objects for longer periods of time.
The following section discusses the most common types of telescope mounts, showing the advantages of each type for various purposes:
- German Equatorial Mount
- Fork Equatorial Mount
- Altitude-Azimuth Mount
- Dobsonian Mount
German Equatorial Mount
A German Equatorial mount uses a counterweight on a long shaft opposite the telescope to counterbalance the weight of the telescope. The telescope is able to track the sky about a polar axis to compensate for Earth's rotation.
A German Equatorial mount has a Right Ascension (R.A.) axis which is aimed toward Polaris, the north star, to polar align the mount. Once aligned the telescope can track the sky using slow-motion controls or a clock drive to rotate the Right Ascension axis. This axis allows motion from east to west. The telescope rotates around the mount's declination (dec) axis in order to allow movement north and south.
- Allows automatic tracking with clock drive
- Very stable
- Easy to point to most areas of the sky
- Good for photography or CCD imaging
- Heaviest type of mount
- Longer set-up time for large scopes
Fork Equatorial Mount
A Fork Equatorial mount holds the telescope on the end of one or two arms. The term comes from the original two-arm design's resemblance to a tuning fork. Some smaller, lighter model telescopes incorporate a single arm to reduce weight when two arms are not necessary. The fork arms of this type of mount are pointed north to allow the mount to track the sky as Earth rotates.
A Fork Equatorial mount has a Right Ascension (R.A.) axis (the fork arms) which is aimed toward Polaris, the north star, to polar align the mount. Once aligned the telescope can track the sky using slow-motion controls or a clock drive to rotate the Right Ascension axis. This axis allows motion from east to west. The telescope rotates around the mount's declination (dec) axis in order to allow movement north and south.
- More compact than German equatorial mount Lighter weight and quicker set-up then German equatorial mount
- Allows tracking using a clock drive
- Good for photography or CCD imaging
- Eyepiece ends up in awkward position when telescope points north near Polaris
An Altitude-Azimuth (or Alt-Az) mount moves parallel and perpendicular to the horizon. This motion is very intuitive and is especially easy to use for terrestrial viewing. Most Goto telescopes are Alt-Az mounted, and the Dobsonian is a type of Alt-Az mount.
Altitude refers to height above the horizon, and azimuth is the angle along the horizon from north, like a compass bearing (0 is north, 90 is east, 180 is south, and 270 is west). Alt-Az mounts are easy to use for terrestrial viewing, and for astronomical viewing they have a great advantage over equatorial mounts in that they keep the eyepiece in a convenient position at all times. However, they cannot automatically track unless the telescope is computer- controlled. Therefore, most Goto telescopes are Alt-Az mounted.
- Most compact type of mount
- Eyepiece is always in convenient position
- Easy to use for terrestrial viewing
- Will only track if computerised
Must be mounted equatorially (using a wedge) in order to be used photographically or for CCD imaging
The Dobsonian mount is a type of Alt-Az mount. It is a simple yet ingenious design: it works entirely by friction. The balanced telescope is held in place only by the friction between the telescope's bearings and Teflon pads on the mount itself. As long as the amount of friction is perfect the telescope can easily be moved just a tiny amount so that an object may be centred in the field of view or tracked as Earth turns. But when the observer lets go of the telescope it stays exactly in place. In keeping with the simple design of the mount, the telescope itself is the simplest and least expensive design, the Newtonian.
The Dobsonian is very easy to use and so is very popular with beginners. It is also the least expensive type of telescope for a given aperture and so it is also popular with advanced observers who wish to own a very large telescope. Dobsonians with apertures of 36" or more are becoming familiar sights at many star parties.
- Easy to use
- Easy to set-up
- Does not track (unless a special device is purchased which may not be available for many models)
- Cannot do photography or CCD imaging without being able to track
- Wide range of eyepiece positions depending on where telescope is pointing
There are now sophisticated drives and computer software that can allow automated viewing with either type of mount. Simply align the scope with a couple of known stars and the computer can guide your scope to any visible entity in the night's sky. These systems can add quite a bit of cost to your telescope. For beginners, however, finding sky objects manually can be one of the real joys of astronomy.
No matter what type of telescope you choose, having a quality mount will vastly improve your viewing experience.