If you want a REALLY good worm gear reduction, go for Byers worm gears:
http://www.deep-sky.co.uk/telescopemaking/tm14.htm
Otherwise, I believe any pitch will do: What you need is a decent number of teeth (180 ~ 240, wheel diameter is derived from this and it should be as large as could possibly be found, to reduce torque.. but 100mm in diameter is still OK).
My dobson used 240 teeth worm gear and 1:12 friction wheel reductor... quite adequate for 32 microsteps.
Small parts has only one adequate option anyway:
http://www.smallparts.com.au/store/p...ection/wide/1/
Have a look at what Steve (Kinetic) is doing, here:
http://www.iceinspace.com.au/forum/s...t=43091&page=7
Standard EQ6 (Synscan) has 200s/rev stepper, and overall reduction (from motor shaft) is 705.
(180-teeth worm wheel, and 12/47 spur gear reduction. This is done by use of 12 teeth gear on motor axis, a transfer gear (30 teeth or so? not relevant) and 47-teeth gear on worm shaft).
With 32 microstepping, this gives you enough resolution (0.14 arcsec per microstep), while still allowing relatively high slew rate (at 400steps/sec you have 0.5° per second slew.. decent).
A friend of mine is using only 1:4 timing belt reduction (on his EQ-6), which is directly driving the worm from motor shaft .
I was a bit more conservative, using 1:4 spur gears and 1:5 timing belt reductor to drive worm (I will remove 1:4 spur gears when Bartelising my EQ, when I am ready for permanent setup under the Sirius dome, one fine day (night)...).
It is worth considering alternative approach - friction roller drive:
http://www.scopecraft.com/Technical%...r%20Drives.htm
And, finally, Bert (Avandonk) has EQ-6 with only one belt reduction, 1:6 or 1:8 (I think), directly from stepper to worm.
http://www.iceinspace.com.au/forum/s...ght=belt+drive