Many modern professional scopes use large circular rollers driven by machined shafts. These avoid the errors inherent in worm and gear drives. Worm and gear errors include periodic and erratic errors. Periodic errors are caused by the elliptical shape of the gear and by mis-centering of the worm on its shaft. Erratic errors are caused by tooth to tooth differences and by backlash when the drive changes direction.
By using a gear reducer in the preliminary stage and a roller drive for the final stage, the errors present in the gear reducer are divided by the ratio of the final roller. For instance, if the gear reducer has an error of one arc minute, and the final roller drive ratio is 30:1, then the actual error present at the eyepiece will be two arc seconds.
The bearing surfaces should be converted to ball-bearings riding underneath Formica in smaller scopes, and aluminum or galvanized metal sheet in larger sizes. Face the altitude bearing rims with thin strips of aluminum. Substitute a drive shaft or drive hub for one of the four altitude bearing points. Attach a gear reducer powered by a stepper motor to this drive shaft. The azimuth drive is a drive shaft with a conical machined end that rides underneath the rocker bottom, faced with a thin metal plate. The other two contact points are ball bearings. Since the rollers are very large, the scope has a very high inherent stiffness. One advantage of a dobsonian over an equatorial is that gravity naturally tensions the rollers and drive shafts.
Steppers motors can be very quiet and they can be very noisy. To run them quietly, isolate the stepper from wood and other resonant materials by a thin piece of rubber or styrofoam or something similar. Use nylon screws or bolts to attach the stepper to its mounting plate. Use a short piece of automobile vacuum hose to attach the stepper to the input drive shaft of the gear reducer.
Comments about gear reducers and drives by
Tom Cathey.
Comments about roller drives by Louis Boyd.
Make your own drive gear: http://easyweb.easynet.co.uk/~chrish/worms.htm
I use Byers 7 3/4 inch diameter gear reducers that are bolted to machined hubs that ride underneath the altitude rim and azimuth plate. These gears are reputed to have errors in the 5 to 10 arcsecond range. After going through the 7:1 reduction of the drive hub to altitude rim and azimuth plate, these errors are reduced by seven times to one arcsecond. There is no perceptable back and forth swaying of the guide star using a high power cross hair eyepiece. My total reduction is the 359:1 of the gear reducers times the 7:1 of the final roller which equals about 3600:1. This is a tad high, meaning that the microsteps are tinier than 1/4 arcsecond each, and that the overall maximum slewing speed is slightly reduced. But this is the smallest ratio I could obtain the Byers gears in.
the altitude drive...
the azimuth drive...
Dirk Dhoore's simple dob modification...
Jay LeBlanc's 17" altitude and azimuth drive system using small inexpensive gear reducers from Torque Transmission