Computer Controlled Telescope - Gearboxes

There are many ingenious ways to achieve a suitable drive ratio from the motor to the telescope, some of these can be found as links on Mel's Website and other ATM sites on the Web. One of the easiest ways is to use a gearbox between the motor and worm/wormwheel or drive disk. The advantage is that they are easy to fit and provide an accurate ratio. The drawback can be cost and some reports of introducing further periodic error, though I have not found this.

Drive Ratio

Mel recommends a drive ratio of between 1300 and 2600:1 to achieve a microstep size of 1/2 to 1/4 arc sec. The calculation, for my RA drive goes something like this.

I wanted 1/4 arc sec per microstep = 1/4 * 10 * 200 = 500 arc sec per motor rev.
This assumes 10 microsteps per fullstep and 200 fullsteps per motor revolution
There are 1,296,000 arc sec in 360 degrees so:
Reduction = 1,296,000/500 = 2592 per motor rev.
I already had a 720 tooth worm wheel so the further reduction required would be:
2592/720 = 3.6:1

The same calculation for my DEC drive with a 213 tooth wormwheel gives a further reduction of about 10:1.

Gearboxes

Suitable quality gearboxes with a rugged construction are hard to come by, after a lot of searching I opted to spend the money to get my system up and running and doing Astronomy instead of looking for gearboxes. I bought a 5:1 ratio 4Nm gearbox from RS Components, part number 718-852. This easily fitted to my standard frame size stepper motor with a kit, part number 718-953, which contains the pinion (small cog) to go onto the motor shaft and engage with the gearbox. The 5:1 ratio is a little high (see above) so I end up with slower slew speeds and finer arcsec per microstep than planned, but that's not a problem for me.
the pinion is glued on to the motor shaft with acrylic "high strength retainer", this is just like super-glue and works very fast - make sure you position the pinion quickly in the right position - once the glue has set believe me, you can't move the damn thing!!

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