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 -
Tom Cathey