CGEM Onstep Retrofit

In 2019, in an attempt to improve the autoguiding performance of my german equatorial mount, I went through the process of retrofitting my Celestron CGEM to OnStep Telescope Mount Goto Controller.

Components

• Hypertuned CGEM

• Oriental Stepper Motors (PKP244MD08A-L) – Nema 17 – 400 steps per turn

• Timing belt and pulleys (beltingonline.com)

  • Belts

    • 2 ea. MP (MXL) Synchroflex® Timing Belts

    • Width in mm – 6

    • Belt Length in mm – 182.88

    • Tension Cords – Steel

    • Material – Standard PU

  • Pulleys

    • 2 ea.15 Tooth MXL Pulley (AL15MXL025FB)

      • Bore – 5mm

      • Grubscrew Holes – 2 off M

    • 2 ea.60 Tooth MXL Pulley (AL60MXL025)

      • Bore – 12mm

      • Grubscrew Holes – 2 off M4

• Onstep MiniPCB 2

  • MCU: Teensy 3.2

  • Motor Controllers: TMC2130

Design

My goal was to validate if the stock CGEM could potentially have better tracking and guiding performance by replacing motors/gearbox, spur gears, electronics & firmware.

Using 400 steps stepper motors with the CGEM 1:180 worm gear and 1:4 pulleys, I calculated a 0.281 arcseconds tracking resolution using 16 micro-stepping.

No modifications were made to the CGEM assembly. I had to fabricate a plate for the motors to replace to stock plates

Fabrication & Assembly

The CGEM had to be disassembled in order to install the pulley to the worm gear. I had experience with the mount due to the fact that I had hypertuned it myself.

No modifications were made to the CGEM assembly. I had to fabricate a plate for the motors to replace to stock plates. Also bought a USB endoscope in order to be able to make the belts/pulleys alignment once the motors were in the mount.