OnStep EQ5 mount: summary

Few years ago, I bought a basic motorized EQ5 mount for my first telescope. It came with a simple RA/DEC pad that controls two tin-can motors. At first it was nice for pointing deep sky objects and track the sky (aka compensating for Earth rotation).

My first modifications to this mount was to computerize the pad. Using an Arduino, a USB-AScom driver and a bunch of wires, I was able to send Ascom commands to the Arduino which relayed them into “pushing” the RA/DEC buttons of the PAD.

Arduino Nano ST4 controller

I based my work on this project:

This setup allowed me to control the mount in RA/DEC but it has a few drawbacks: no slew (or reaaaaally slowly) and the polar alignment has to be perfect in order to get decent shots of deep sky objects.

I was still dreaming of a fully computerized mount able to slew to a specific position, correct for polar alignment errors etc.

I stumbled on what appears to me a very advanced project that was fitting my requirements: OnStep.


OnStep is a computerized goto system for stepper motor equipped mounts.

After thoroughly going trough the Wiki page and the forum, I decided to adapt the OnStep project to my mount.

The BOM was:

  • Arduino Mega 2560
  • DS3231 real time clock
  • Ramps 1.5 board
  • FYSETC stepper drivers
  • Wemos D1 Mini V3.0.0 Wifi Module
  • JK42HM48-1504 stepper motors

First I had to adapt the stepper motors on the mount. As speed is not my primary concern here, I chose a gear-pulley system which offers an additional ratio for further precision and limit the motor load. I designed and 3D printed adapters for RA and DEC motors.

Regarding the connectors and cables, I chose Cat 5 UTP with RJ45 connectors. Such cables have many advantages:

  • Low cost
  • I can crimp them at the desired length
  • They can sustain the motor power
  • Their locking mechanism is fast, simple and efficient
  • The plugs are easy to find

Now for the OnStep controller motherboard housing… I recycled an old 3D printer power/motherboard casing as it was designed to enclose a standard format PSU. To facilitate the integration of the components, I created a 3D model of the casing and designed the required additional components.

The back provides USB connection to the Arduino Mega and the Wemos wifi module. Not represented here is also the AC power socket.

The front gives access to RA, DEC, Focuser 1 and 2 RJ45 connectors. Below, the casing provides me with three 12V and 5V servo connectors. On the right, two LEDs give information on the system status (power, PPS clock locked, tracking, slewing…). The two rings on the bottom are designed to attach the casing on the mount tripod legs.

A bit of a design flaw here: the Arduino is attached to one half of the casing while the front panel is attached to the other half. But this is a permanent setup so opening the “messy cable box” won’t be a frequent operation.

This picture shows the Ramps Arduino shield, RA/DEC Stepper controllers (white) and the focuser Stepper controller (red). On the right are the real time clock module (required for the astronomical computations) and the Wemos Wifi module allowing me to remote control the mount.

Speaking of the focuser, I also had to adapt another stepper motor to the DPS focuser of the telescope. I started by modeling the focuser wheel itself and built the adapter around it.

Here again, gear-pulley and RJ45 are used.

To be continued