In the last couple weeks I've been having some success with an inexpensive
drive idea...for those that want motorized tracking, but don't want to shell
out the $$$ for a Byers gear set, and don't mind tinkering/building.

- Make a sector of plywood or plastic (cut it with a router or jigsaw)
- Wrap some nylon threaded rod around the sector (can be pre-bent to the
sector radius in boiling water)
- Apply JB weld in a fillet on both sides of the nylon threaded rod, let dry
- Remove threaded rod (nylon won't stick to the JB Weld very strongly, even
without a release agent)
- Trim/clean up the rough edges

Presto...a large sector drive!  How big?  Mine is about 11 inch radius.  You
can make yours larger if you want.  (This may not work very well on smaller
radii unless you use finer/thinner threaded rod.)  I used 5/16 -18 threaded
rod.  Use the same diameter/pitch threaded rod (nylon, brass, steel?) for
the worm.  Attach to a stepper or synchronous motor.

Wanna make a continuous worm?  Do it in two steps.  First mold a partial
section on the disk.  Then, the second molding step is the rest of the
disk...with the ends of the threaded rod firmly meshed into the ends of the
first molded worm gear section.  Can this really work?  It did for me...on
the first try...a seamless/continuous 21 inch diameter worm gear with 1187

I've been able to find nylon threaded rod in six foot lengths.  That can
make a pretty big worm!  ;-)

With a big enough worm you can use a "direct drive" system....which means
low/zero backlash, and only one source of periodic error (one worm meshing
with one worm gear, no reduction gear train).  One motor, one worm, one big
worm gear.  Choose whatever worm gear size and threaded rod pitch will meet
your design needs.

If done carefully, this is easily good enough for wide angle imaging.  I
have not tested it for longer focal length photography possibilities, but
periodic error in my first version is smooth, not jerky.  I bet an
autoguider could handle it better than many mass produced, inexpensive drive

Currently I'm using these gears to drive my sixteen inch, f/6 dobsonian.
With surplus steppers rated at 6 volts, driven at 24 volts, using a small
flywheel to smooth rotor resonance problems (lower fundamental resonance
freq of the rotor), and some series resistors for current limiting at low
step rates...I can halfstep slew this large telescope at 2.75-3 degrees per
second.  Duck!

Tom Krajci

PS.  Is this an original idea?  Nope.  See Sky & Tel ATM articles from 1974
and 1979.  The difference is that nylon threaded rod is now easily