SkyMover - Astrophotography Harmonic Mount

Make your own harmonic drive astrophotography mount. Achieve sub arc/second error rates for well under 500 USD. Unfortunately, projects like this need a large amount of bravery. I would rate this as 8 out of 10 difficulty. You have been warned. I spent over a hundred hours and several kgs of filament revising this design across different harmonic drives and planetary gearsets. Countless nights loosing sleep, lying in bed thinking about how to solve different problems. But hey, you can do this. The end result can be 5 minute long astrophotography exposures without spending thousands of dollars.

The prints themselves are not too bad, no overhangs, simple blocky design. The trick is, all the pieces need to fit together perfectly to achieve the desired end result. Sanding the flat interfaces between pieces is recommended and a tight fit and perfect alignment is the goal on everything. The fitment of every piece adds up to the final accuracy and precision of the mount. Consistent contact surfaces are critical to rigidity in the final performance. Tight interference fitment of all bearings, motors and moving pieces is the requirement.

I would use PET-CF for the material. I used ABS because I had a ton of it laying around, but this introduced way to many complications.

I will try to upload a diagram of the bolts and inserts for the final assembly.

No supports needed. Printed with 1.2mm walls and 35% infill. There are several tight fitting components, i.e. bearings, harmonic drive, planetary. You may want to check tolerances with a small cross section before printing the full model. Everyone's printer and filament are different.

There are several threaded inserts in parts, “Main_2_Bearing_Plate_F14” and "Main_4_NEMA_Mount_EG17_Precision".

Parts from Aliexpress change frequently, I have included the specifications for the Nema 17 F14 Harmonic drive.

I am running an unbalanced 10lbs astrophotography rig and able to achieve sub 1 arc/second error rates with active guiding. Some targets are better and some celestial targets are worse. The range is about 0.5-1.5 arc/second error across 5 minute exposures.

I have also been running a heavier Newtonian DBS150 rig, at approximately 15 pounds with excellent results. For some reason the heavier payload results in even better guiding rates at around 0.2 -0.5 total RMA arc second errors.

The motor drivers for this mount are using the OnStep platform. Specifically an OnstepX ESP32 and TMC2209 drivers.

The entire assembly is driven by Nina.

I also included the 40 degree wedge that I use for the mounting point. You will need to design your own wedge mount. The angle of your celestial pole is different depending on your location. Also your Tripod mounting holes are different. Use this only as a guide.

The only unsolved aspect of this model are the Arca Swiss mounting plates and clamp. These can be found for a good price but lately things are out of stock. The Arca Swiss plates can all be substituted for the Vixen mounts but this will add bulk and cost to the design. 

The other issue with the Arca Swiss plates is that I had to drill out new mounting holes for the M8 bolts. You will see both output plates in the files. One for Vixen and one for Arca. The Arca plates are used to join the Dec and RA assemblies together. The Vixen clamp is for the final output to the telescope.

In summary, the most difficult part of this build, is finding all the necessary parts, harmonic drive, planetary gearsets, mounting plates, etc.… It may be difficult to find the exact parts that I used, so inevitably, you will need to change the design to match those parts. The 3D printing is also difficult, getting all the surfaces to fit tightly, but this is at least within our control. I would consider this a guide and not an absolute roadmap for your end product. It is possible to build your own high precision tracking mount with an inexpensive harmonic drive and planetary gearset, but the end result may not follow the same path that I outlined in these documents. Have fun and don't loose too much sleep over this build.