What is the best way to drive a motor (DC/Brushless/Stepper) at a constant, yet low RPM? It must have no jitter and no drift over time. I'm aware that some gearing will be required. Basically what motor and controller should be used to achieve this accuracy?

Application is a telescope mount which has to track the sky at a final ratio of 1 rotation per 23 h 56 min 4 s (sidereal day).


closed as off-topic by gbulmer, Asmyldof, PeterJ, Daniel Grillo, Adam Haun Apr 18 '16 at 21:28

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    \$\begingroup\$ Probably something with a gearbox with a large ratio. Maybe a microstepping stepper (to reduce vibration) or a servo motor with encoder feedback. Note that if you are tracking planets the ratio is not exactly the same as when tracking stars. \$\endgroup\$ – Spehro Pefhany Apr 17 '16 at 15:16
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    \$\begingroup\$ Hello Tanenthor and welcome to EE.SX. What is your skill level? A highly accurate tracking rig is not going to be easy. What have you tried so far? EE.SX exists as a resource to electronics engineers or students thereof. Questions must show some research effort on your part and a stab at a solution, else it may be closed. As it stands, this question is too broad for this site. Please edit your question, narrow it, and include more information. Have you searched google yet? "Telescope tracking mount diy" turns up 130k hits. \$\endgroup\$ – rdtsc Apr 17 '16 at 15:36
  • \$\begingroup\$ As rdtsc wrote, the question is too broad, and requires rewriting to provide essential information. Please read the help center to learn how to ask questions, and understand the type of topics the community helps with. I read your question as either a shopping question (out of scope), or too broad to answer, with no explanation of what best in "best way to drive a motor" might mean, and no effort on your part to find an answer. An obvious answer is look at astronomy equipment review sites, and based on their review buy the best you can afford. \$\endgroup\$ – gbulmer Apr 17 '16 at 17:36

This could be relatively easy if you are willing to live with a few limitations. They are: AC operation only, single speed (no slewing) and an equatorial mount.

If those limitations are ok with you, then an AC synchronous motor (a clock motor) with a lot of gearing will do the trick. This is what mid-to-high end telescopes came with before microprocessor-based drives became popular.

You can run such a drive with batteries if you add an inverter that is frequency accurate. You can also provide some slight slewing (around half-speed to 2x speed) if the inverter frequency can be adjusted. I built such an inverter back in the 90s for under $30 (to drive an existing Meade clock drive).

If you don't have an equatorial mount (like a Dobsonian mount), then it is much more difficult. You would need two motors and a computer control. Brushed DC motors with encoder feedback would work best. That would also allow you to slew at high speed.


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