Overview

This blog is about an equatorial tracking platform I built for my son's 10" Dobsonian telescope. (A tracking capability for a telescope allows it to track the motion of the stars as the Earth rotates, avoiding the need to constantly reposition the telescope. Large reflectors like Dobsonians typically lack tracking.) When we got the telescope we quickly realized that tracking would be a great enhancement, so I got to work. I worked hard on it and learned a lot, and I'm very satisfied with the result. I thought I'd try to document my effort in case it's useful to someone else, hence this blog.

This post gives an overview of the platform. The next one is a User's Guide of sorts. The remaining posts detail the construction process, including the drive system and the electronics.

The platform is based on designs from this extremely helpful site: Telescopes and Deep Sky by Reiner Vogel. That site has a great overview of the basics of tracking platforms, so I will not repeat that material here. The design is similar to this one from André Heijkoop but with a conical (beveled) bearing surface proposed by Goerges d'Atume and employed by Ed Jones. The platform is constructed from Baltic birch plywood and solid maple.

The inclination of the platform's axis is 40°, corresponding to optimal use at a latitude of 40°N. (All my descriptions are in terms of the Northern Hemisphere. If you are south of the Equator, please forgive me; everything should translate easily.) It has been successfully tested here in Houston at 30°N by tilting it 10° northward; in theory it could extend as far as 50°N with 10° southward tilt.

The rotational extent of the platform is about 20°, giving just under 80 minutes of tracking time before needing to be reset.

Custom EQ platform designs endeavor to locate the telescope assembly's center of gravity on the platform's axis of rotation in order to minimize the torque required to rotate the platform. The center of gravity for this platform is calculated for the Celestron StarSense Explorer 10" Dobsonian OTA and its OEM rocker box base.

The drive uses a NEMA 11 stepper motor with a 100:1 planetary gearbox. The gearbox drives a urethane drive roller that engages the surface of the circular segment under the platform. The mechanical advantage results in more than adequate torque on the platform axis. The motor turns at just over 3 RPM to drive the platform at the sidereal rate.

Stepper motor (in rear), gearbox, and drive roller

There is a handheld speed controller with two controls. A rotary encoder knob adjusts the motor speed in increments of 0.1%, allowing the speed to be recalibrated to account for variations in the microcontroller's clock due to temperature or other factors, or to reduce the speed for lunar tracking. A joystick potentiometer allows momentary speed adjustments to increase or decrease speed by up to 4x. This is handy for centering objects in the eyepiece.

Hand controller; power supply at top

The drive is controlled by an Arduino Nano v3 microcontroller module, coupled to an Analog Devices Trinamic TMC2209 stepper motor driver module. The Nano sends pulses to the driver at the appropriate frequency, and the driver controls the current through the motor windings to step the motor smoothly and quietly at a precise speed. 

The Arduino also controls the hand controller and an OLED display. The display is a 1.3" 128x64 OLED display module. The display shows the current speed as both a percentage of the nominal (pre-calibrated) sidereal rate, and as the RPM of the motor shaft. The display also shows the rotational position of the platform since reset (from 0° to 20° over the approximately 80 minutes of tracking time).

OLED display

A box enclosure under the platform contains the Arduino and the stepper driver, mounted onto a "perfboard" type circuit board, with connections for the motor, display, power supply, and hand controller. A diagram of the circuit is here.

The circuit board and enclosure

The motor and control circuit are powered by a 5v DC supply, from a standard portable phone power bank via a USB-C connector. A 25800mAh power bank will run the drive for several hours. The hand controller connects to the enclosure using a standard ethernet cable (8-conductors with RJ45 connectors).

The Arduino's software is programmed into its non-volatile flash memory using the freely available Arduino IDE. The program (called a "sketch") was custom-developed for this platform and its control and display components. It uses a handful of open-source libraries from the Arduino ecosystem for the stepper control, encoder, and display. The sketch is published on GitHub and is freely available.

The platform includes an alignment tube that accepts a green laser pointer. When the laser is mounted in the tube, it precisely aligns with the platform's axis of rotation. Equatorial axis alignment is thus easily achieved by moving the platform and adjusting its feet to aim the laser directly at the North Celestial pole (very near Polaris).

Alignment tube with laser

The design is backed by a Google Sheets spreadsheet that computes all the parameters for the geometry and the motor drive. These specify the dimensions for the physical construction as well as speed and geometry constants used in the Arduino sketch. The geometry model used in the spreadsheet is based on this DrawIO diagram, which is adapted from the one from Reiner Vogel.

The remainder of this site provides more detail on all of the above. Please feel free to ask questions or send comments. This project has been a lot of fun for me, and I'm very interested in any feedback.