I am in the process of adding a motor drive and control system to my 200 mm Dobsonian telescope.
The mechanical aspects of the new platform have been completed, and will give improved precision and rigidity to the assembly.
I am now ready to add the motor and control components, however my knowledge of this topic is limited.
I would appreciate help and advice from those who have experience of this stage.
I do know that a motor (stepper?), reduction gearbox unit, a micro controller, a motor driver is required
A 12V supply is preferred, but could go to 24V if required.
The telescope axis can be driven from either 1. North end via a 200 mm dia wheel (via a toothed timing belt, and small dia sprocket), or 2. South end via an 880 mm dia circle segment (via open timing belt and sprocket).
Images of telescope can be supplied.
Thanks in anticipation.
Peter
Hi @PETER298297
Welcome to the forum, great to see your project coming together! You’re definitely on the right track so far.
For precise tracking, a NEMA 17 stepper motor is a great choice. They’re accurate, reliable, and well-sized for telescope drive systems.
To control the motor, you’ll need a stepper driver. Here are two solid options:
- TMC2209 – ultra-quiet and supports microstepping up to 1/256
- DRV8825 – higher torque support, simpler to set up
A standard Arduino Uno is a great beginner-friendly controller and has lots of support in the DIY astronomy community.
For power, a 12V plugpack is ideal, it’s enough for the Uno and one NEMA 17 motor via the driver. You can power the Arduino via the barrel jack or the Vin pin.
If you’re planning to use timing belts for drive reduction (e.g. GT2 or HTD pulleys), you likely won’t need a gearbox, I think the mechanical advantage from large discs (like your 200 mm or 880 mm ones) plus microstepping gives you all the resolution you need.
Lastly, feel free to upload some photos of your setup, it’ll help the community give more specific guidance on mounting, layout, and belt routing!
Hi Ryan, thank you for quick reply. I have attached some photos. I did forget to mention that this is an equatorial platform, but I am sure it is obvious.
The virtual platform axis is tilted at 38 degrees (Melbourne latitude).
the telescope image is showing an earlier prototype platform, which has now been replaced (see images platform 1, 2).
re drive options - platform 1 image shows a 20 mm wide spacer between the platform support and the base plate bearing support. This can be removed and a 200 mm diameter toothed drive sprocket fitted.
The motor could be mounted to the side of the base bearing support housing.
Alternatively, the motor could directly drive via a toothed sprocket to an open timing belt fixed to the underside of the circle segment (880 mm dia).
I am not so sure of using a friction drive, but happy to consider other options.
Cheers
Peter
Hi @PETER298297,
Thanks for sharing the extra info and photos, that clears things up a lot. The equatorial tilt and those large arc segments definitely open up some good options for precise tracking!
Between the two drive options you mentioned, the 880 mm arc with the open timing belt is likely the better choice for smooth, consistent motion:
- The large radius gives you a natural mechanical advantage, meaning the motor turns more slowly and smoothly to track the sky, ideal when paired with microstepping.
- Open belts fixed to the arc segment (with adhesive or low-profile fasteners) can work really well, especially if you get the tensioning right.
I believe that the 200 mm sprocket option would still work fine if you want a more compact layout, but the smaller radius means you’ll need finer control from the motor to achieve the same tracking accuracy, something the TMC2209 can handle, but it may be a little less forgiving mechanically.
And yes, I’d generally avoid a friction drive in this case, even small slips can cause noticeable tracking errors over time.
Looking forward to seeing how it progresses, feel free to keep us updated with your build!
Hi Ryan, I have a few comments and a few more questions, hope you don’t mind.
I agree the 880 mm arc is probably the better set up re precision tracking. If a 2mm pitch, pulley and belt is used, (13 mm dia pulley will give 20 teeth), that will give a gear reduction of about 68:1 (880/13). There is a 12 tooth sprocket available (about 8 mm dia, probably too small).
Eliminating a gearbox from the stepper motor would be a real benefit as well (backlash).
How can I calculate the stepper motor output speed (in combination with micro stepping and the 68:1 gear reduction) ?
Is there an optimum balance of mechanical gear reduction and micro stepping to achieve the very low speed telescope platform rotation ?
The actual platform rotation speed requirement is 15 degrees per hour (1 hour viewing in a 24 hour day).
My calculations give 3 revolutions per hour for the stepper motor, that is 0.05 rpm ???
Is that possible ?
re NEMA 17 stepper motor.
There are a lot of stepper motors listed .ie. with a torque output upto 80 Ncm, 0.9 deg and 1.8 deg step angle, what specs do I require ?.
re Power supply.
Could a 12V battery pack (from a cordless power tool) be used instead of the plug pack, so as to give portability ?
Hope I am not too demanding, I am finding this project very interesting and challenging.
Cheers
Peter
Hi @PETER298297
So with a microcontroller it would be possible to define your target rotation and the time that it would take to move between its starting and finishing points. Something like the below would work for an Arduino Nano to use a TMC2209 Stepper Motor Driver. It will do the heavy lifting in terms of calculating the output speed.
const int stepPin = 2;
const int dirPin = 3;
// Stepper config
const int stepsPerRev = 200;
const int microstepping = 16;
const float gearRatio = 68.0;
void setup() {
pinMode(stepPin, OUTPUT);
pinMode(dirPin, OUTPUT);
digitalWrite(dirPin, HIGH); // Set fixed direction
}
// Calculate time between steps for ~2.83 RPM
void loop() {
float rpm = (1.0 / 24.0) * gearRatio; // ~2.83 RPM
float stepsPerMin = rpm * stepsPerRev * microstepping;
float delayMillis = 60000.0 / stepsPerMin; // Delay between steps in ms
digitalWrite(stepPin, HIGH);
delayMicroseconds(200); // STEP pulse
digitalWrite(stepPin, LOW);
delay(delayMillis); // Wait before next step
}
In terms of what you need for torque, it will depend on the force required to move the telescope, generally speaking though having more torque than required is always going to be better, for step angle 0.9° motors will be more expensive but will have a higher resolution and less vibrations.
You could definitely use a drill battery for this, you would likely need a DC-DC Adjustable Step-down Module 5A 75W | Buy in Australia | CE07271 | Core Electronics or two (power tool batteries are usually 18V+) to drop down to 12V for the motor and also 5V for the microcontroller.
