DIY Harmonic German Equatorial Mount (GEM) for OnStepX

A compact DIY harmonic German equatorial mount designed for lightweight astrophotography setups.

The mount is based on the Fysetc E4 v1.0 controller board running OnStepX firmware.

Update May 24th, 2026:

• Added a Fysetch holder - a dumb holder for the Fysetc plate I was actively using for the debug sessions. From the very beginning, I've decided not to make an additinal hole for the USB B port

• Fixed steps in Config.h

• Added a Station section for WiFi.

An archive with a modified version of OnStepX firmware is also attached. It includes several bug fixes and improvements and can be flashed directly to the controller and used as-is.

Designed for payloads up to approximately 3 kg. If you plan to use heavier equipment, do so at your own risk. The author assumes no responsibility for damage or failures caused by overloading.

Included Parts
Core
Case A
Case B
2 × NEMA motor mounts
Fysetc holder
Main Components
1 × FYSETC E4 v1.0 controller board https://www.aliexpress.com/item/1005001704413148.html
2 × NEMA 17 Stepper Motor stepper motors https://www.aliexpress.com/item/1005004586903773.html
2 × 1:100 harmonic drives for NEMA 17 https://www.aliexpress.com/item/1005008658589981.html
2 × GT2 60T pulleys, 6 mm belt width, 5 mm bore https://www.aliexpress.com/item/1005008672459615.html
2 × GT2 20T pulleys, 6 mm belt width, 5 mm bore https://www.aliexpress.com/item/1005008672459615.html
2 × GT2 belts, 158 mm, 6 mm width
2 × D-type shafts, 35 mm length, 5 mm diameter https://www.aliexpress.com/item/1005008459963157.html
Hardware Required
8 × M4x5x6 heat-set inserts
8 × M4x6 mm screws
8 × M3x6x5 heat-set inserts
4 × M3x10 mm screws
8 × M3x6 mm screws
4 × M3x12 mm screws
12 × M4x25 mm screws
12 × M5x20 mm screws
1 × DC power jack 5.5×2.1 mm (8 mm mounting diameter)
1 × 40×40×10 mm cooling fan
1 × 16 mm latching push button (power switch)
Print Settings

Recommended print orientation and support placement are shown in the provided images.

Core
8 walls
8 top/bottom layers
70% infill
NEMA Mounts
3 walls
3 top/bottom layers
30% infill
Case A / Case B
3 walls
3 top/bottom layers
15% infill
Fysetc holder
3 walls
3 top/bottom layers
15% infill
Material

Printed in PETG (333 Print) at 245°C.