[Astrophotography] Equatorial Mount: DC or stepper motor

Hello,

I am in the process of thinkering about making an home made equatorial mount to take deep sky images.

An equatorial mount is basically a 4 axis motorized things in which only one axis moves to compensate the movement of the earth. To achieve it the motor should move smoothly and constantly at a very low speed (around 15 degrees per hour).

My camera + telescope weight approximately 2kg but I would like to make a solution that can handle 5kg without problems (in case of future upgrades). Based on my actual design the distance between the motor mount and telescope is ~8cm with a gear ratio between pulley #1 and #2 of around ~4. I am planning to operate it at 12/24V.

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I have a few experiences with steppers but unfortunately I have never used a DC motor (with Arduino).

As we know steppers motors moves with steps so from a logical point of view they have already lost at the beginning in this case but using reduction gears they can actually be used (and they are used by other DIY and some commercial equipment).

At the beginning I was going to use the 17HS4401 stepper, which has these characteristics:

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I would have used it with a worm gear without microstepping

Then I have been tempted by DC motor with/without encoder.

On aliexpress I found these: GW4058 A58SW31ZY

As driver a fast search brought me to the L298N which can control 2 motors and is very cheap, or the DRV8838 and TB6612.

I am opening this topic because I would like to receive a few suggestions and discuss which kind of motor to use: DC vs stepper motor.

Please feel free to share your thoughts

I assume that the principal advantage of a DC motor is that you want the motion to be as smooth as possible. That could work if you could contrive the gearing so that the motor is running all the time. If it has to stop, you're no better off than using a stepper I suspect. I'd want an encoder so as to be able to adjust the PWM to fine tune the speed.

Personally, I'd use a stepper. What do the majority of the commercial offerings use?

What do the majority of the commercial offerings use?

DC motor

Yes, if the DC motor is used it needs to run continuously without stopping. But, since I need very slow speed (1 turn every 24hours), is it really possible to find so slow DC motors? The best I have found is 5rpm, which are 7200 turns per day. I can play with the gear ratio but I can't achieve something like this

Cheap gear systems tend to have a lot of backlash. I think you need specifically anti-backlash gear system for a telescope?

yes, the backlash would be a problem

I think using a DC motor coupled to a guide star would be a better option; a PID controller. Like using a camera pointed at a guide star; guide star centered and locked on, guide star drifts, motors compensate for drift.

See optical flow sensor for the camera. https://smile.amazon.com/Hobbypower-PX4FLOW-Optical-PIXHAWK-Control/dp/B01FJVF1QM/ref=sr_1_3?crid=2QU6DEQ3X8NX4&dchild=1&keywords=optical+flow+sensor&qid=1588677210&sprefix=optical+flow+sensor%2Caps%2C209&sr=8-3, to make an electronic RigelSys Quickfinder.

thanks but the guidestar is not needed with the equatorial mount

I think that we all agree that the DC motor would be a better option, but it is not easy to find a motor that can handle so low speeds

With a servo loop a DC motor can be as slow as you want, but you are limited by the encoder resolution.

It you need quick rapids, a servomotor is the best approach. Otherwise you'll need a slipping clutch for rapid slewing by hand.

MarkT:
With a servo loop a DC motor can be as slow as you want, but you are limited by the encoder resolution.

I can slow it down, but won’t I loose also torque?
I also wasn’t able to find any information on these cheap
AliExpress encoders. I would need to buy one and run some tests

aster94: I can slow it down, but won't I loose also torque?

Not with a servomotor, they use control loops for position control or speed control or torque control or hybrids of the same. Industrial ones are expensive though(*). Adding control loop is how to turn a motor + encoder into a servomotor.

I also wasn't able to find any information on these cheap AliExpress encoders. I would need to buy one and run some tests

At the high-end you get things like this: https://www.youtube.com/watch?v=1F4-plhdnj0

I am going to buy both a stepper and a DC motor + encoder to check in the real world how they compare in this user-case

I will write here back my results but months will pass