Astro Focuser with Stepper Motor

Update:

• 21.05.2026: Added a GT2 Focus Ring for infinte turning by using a GT2 350mm endless Belt, a M2 Hex screw and nut is used for tightening 

• 21.05.2026: Reviewed the description and recommended to use a Step-Down module for powering the Arduino to avoid some voltage peaks damaging the Arduino.

I wanted an "autofocus" system for my Canon lens that could move the focus ring without physically touch the lens, so I could focus on the stars without blurring the image in the preview.

The model was designed in three parts: two halves surrounding the lens and a separate part for the stepper motor. The halves surrounding the lens are slightly larger than the actual diameter of the lens, allowing them to flex slightly when tightened. All parts are designed for M3 threaded inserts.

I printed the parts in PETG with 4 perimeters and 40% honeycomb infill. Print the parts oriented like in the Picture.

For the stepper motor mount, I used M3x10mm Allen screws, and for the halves, I used M3x30mm Allen screws.

An Arduino Nano and a DRV8825 driver are used to control the stepper motor. A rotary encoder (KY-040) is used as a “remote control,” and a 3S LiPo battery serves as the power supply for the Arduino and the driver. For the Arduino I would recommend to use a separate Step-Down module to supply an isolated 5V voltage, since I had some issuses with a burned Arduino, wich I think had something to do with some voltage peaks.

The Arduino and the driver are simply attached to the printed mount with double-sided tape; maybe there will be another version someday and I’ll build a proper enclosure for the parts, but for my purposes, this is sufficient. And the battery is also just hanging loosely on the tracker.

The code for the Arduino is also attached. Thanks go out to Lumo (AI from Proton) for writing the code, since I have zero clue about coding :).

I have also attached an image from KiCad of the circuit diagram so you can see how everything is connected. The diagram is far from perfect but should be sufficient to connect everything.

Materials used.

• -Nema17 Pancake Stepper

• -DRV8825 Driver

• -Arduino Nano (Every)

• -Rotary Encoder (KY-040)

• -Cables

• -GT-2 Belt and Pulley

Also thanks to @DrGFreeman_723538for creating the Ring around the focus ring of the Lens. 

The Knob for the Rotary Encoder ist form @amanker_2918615

For now not really tested in night, since the weather is to bad at the moment.

Also I do not take any responsibility if my design damages your equiment! Please use my design with caution!