Hi Everyone -
I wanted to post some info that may prevent beginners (like me) from making the same mistakes....and needlessly wasting time and money.
I have been building a 22" scope (an Equatorial Newtonian/Fundy) for over four years now. It has gone through many evolutions. The last drive arrangement involved Hurst 600:1 steppers driving friction pinned belts. The slop was horrenedous due to the Hurst gear drives. So bad that Mel wrote a special routine into his scope.exe that would incorporate encoders to reset the sloppy steppers and help point better due to backlash. I even had his backlash routine maxed out so far that he had to make some software changes. (Mel...what a patient man!)
Needless to say, tracking sucked, pointing was bad, and I was not a happy camper using this scope. All just to save some money by not buying worm gears and using a cheap off-the-shelf Hurst stepper/gearbox.
Anyway, I recently picked up a pair of plastic (HDPE?-like Nylon) worm gears from Andy Saulitis. He also made up a pair of 6:1 reducers ahead of the spring loaded worms for a total gear reduction of about ~ 2000:1 per axis. I drive them with standard 35 in/oz Astro-Syn 200:1 steppers. Also, Andy made a slip clutch for each axis....I've been known to slew into the stops and strip a gearbox or two in the past.
I then tore off the encoders, the Hurst 2 degree! slop boxes, and disabled the backlash and encoder routines. After installing the new worms (it took some time to mount and correctly align for maximum performance) I measure virtually ZERO backlash, and pointing is now tremendous.....I point and click on an object in Guide, toggle to scope .exe...whir whir and the object is right dead center in the field! You should have seen it before - I spent more time searching than looking sometimes.
I did a 750X cross hairs tracking test last night and watched in amazement as the test star stayed right on the money for full minute. As I hit the stepper controls back and forth, the motion is now instantaneous - (that is what an autoguider needs to work with, when I finish it)
The scope now is now both mechanically and electrically simpler, but so much better. The only thing I would do differently would be to use 1/4" thick steel for the worm/stepper brackets instead of 3/16"...there is a very slight bit of flex if I pull on the brackets, but not enough backlash to even notice when looking through the eyepiece and rocking the steppers back and forth. (This was MY doing, not Andys')
Another advantage: Using friction drives in the past required spinning the scope or using a level and making a best estimate of the final drive ratios to enter into the software...I was always suspect of this. Now, I just count the teeth and it's dead accurate- done.
I always thought worm gears were only for the finicky astro photo Byers' types, but have found the plastic ones to be rugged, relatively inexpensive, and the answer to this astro-geeks' problems. They are estimated to be about 20 arcsecond max periodic error, but that's OK with me. Again, an auto guider is the way to go for long exposures...
In fact, I ordered a second worm gear set and just installed them on my homemade dedicated "zoom lens" CB245 CCD equatorial. Works tight and clean. (BTW, thanks again to Dave Groski for getting the 245 camera bugs squared away)
Good luck -