Re: Steppers and EQM-35

Khalid Baheyeldin

On Wed, Apr 8, 2020 at 06:54 PM, Kane Mcvicar wrote:
So I want to modify a eqm-35 non goto to be a goto mount using onstep,
The question I have is which type of drive would be most suitable, more for astro photography than high slew rates, the RA is 180 tooth and the Dec is 65 tooth.
I'm thinking a precision planetary drive on nema17 200 step motor with a ratio of 1:40 for the DEC, and a planetary with a ratio of 1:20 for the RA, both coupled to the moint with a 1:1 drive belt to reduce harmonics. Would these ratios be suitable? Or should I lower or increase them? 
This is a good strategy that is not in wide use yet.
The motors you quoted should do a decent job with very good resolution, using 1/16 microsteps.
And with that, you should be able to get 2.5 to 3 degree/second, so not slow at all (but not super fast either).

My concern would be the RPM of the motors. If you use 10:1 for RA, and 30:1 for DEC, the RPM is lower.
In that case 1/32 microsteps will be better.

The geared motors allow using the 1:1 pulley which need less space, and have less chance of obstruction.

Most imagers prefer not to have any backlash, and therefore opt for 400 steps motors and a 3:1 pulley reduction.
That leaves out only the backlash in the motor and in the worm/wheel.

But even with some backlash, things do work out. OnStep can compensate for backlash, and Ph2 too.
So worth exploring this approach.

The other option would be a 400 step motor with no gearbox and directly couple it with a suitable drive belt and ratio? Although I wouldn't get the high ratios as I would with the planetary gearboxes.
See above. That is what most OnStep users do.

In that case, RA should work with 3:1 with good resolution.
However, DEC would be a challenge since you need 6:1 for a decent resolution.
Finding a pulley that is 6:1 will be a problem.

In the Wiki Showcase page, check the EQ3 and CG-4 mounts, some have a 65:1 worm wheel on DEC.

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