The Dobsonian mount has the highest strength per unit weight of any modern mount

The Dobsonian mount has the highest strength per unit weight of any modern mount. If you turn a telescope on such a mount directly with your hand, you will not have to wait for any vibration to die down before you can see clearly. Dobsonians normally turn so smoothly that there is little backlash once you let go of the telescope’s tube after turning it. Having a much lower center of gravity than any other type of mounting system, the Dobsonian based telescope is the most stable of all. It is also the cheapest and easiest to produce of all modern telescope mounts, making Dobsonian based Newtonians with large objectives much cheaper than other telescopes with the same size aperture. Such large aperture low priced telescopes are now so common that they have been given the special name, light bucket.

Though you can buy a 16 inch light bucket for about the same price as an eight inch Schmidt-Cassegrainian, there is a down side to the Dobsonian mount. You must move the telescope in both the altitude and azimuth directions to keep an object in your field of view. Though this is not as hard an accomplishment as it is on a common alt-azimuth mount, it has kept Dobsonian telescopes from being used for serious astrophotography since polar mounts require only a simple one direction movement. This situation is now changing due to the availability of cheap computers.

Remember that with an equatorial mount, the motor drive system is simplified by the need to use only a constant turning speed in one direction. Driving a Dobsonian telescope is much more complicated than this. In a Dobsonian drive, there is a motor to turn the telescope in the azimuth direction and another motor to drive it in the altitude direction. Further complicating things is the fact that neither of the telescope’s axes are parallel to the Earth’s axis, making it necessary to vary the speed of each motor depending on where the telescope is pointing! Thus, you can see why a computer is needed to accurately drive a Dobsonian telescope!

The first people to use this new type of drive were the professional astronomers. Practically all of the largest “state-of-the-art” observatory telescopes use this type of drive. Most new giant telescopes are on a metal strutted Dobsonian mount with an alt-azimuth drive. It would have been nearly impossible to build a traditional fork equatorial mount to support such a massive telescope and the cost would have been (grin) astronomical!

Computerized drives for Dobsonian mounts are now available to the public. I have one on my 17½ inch light bucket and it does a great job. Also, the drive cost me less than a typical drive for an equatorially mounted telescope of similar size, but requires some effort to install. However, there is one other thing you should know about a motor driven Dobsonian. As it tracks an object, the image you see will appear to rotate at a very slow rate. (NOTE: Schmidt-Cassegrainians with the new fork alt-azimuth mount will also suffer from rotational distortion unless an adapter is added to convert the mount to be equatorial.) This is not a problem if you are viewing directly or making a short photographic exposure; by short, I mean about 15 minutes maximum. There is an accessory available which will rotate your camera at the same rate that the image is rotating if you wish to do very long exposures. A large Dobsonian with all of these features would still be cheaper than an equatorially mounted telescope of the same size.