A DIY equatorial mount using harmonic drives

As an amateur astrophotographer will tell you, you just can't capture the really interesting objects without spending a ton of money on some decent kit pieces. Telescope aside, there really is a surprising amount of complexity, weight, and cost associated with the support telescope alone, let alone one that is capable of any type of programmable tracking. [Alan (Jialiang) Zhao] clearly wanted to up his game, and after experiencing some of the shortcomings of his Sky-Watcher HEQ-5 pro equatorial mount, he decided to go ahead and build an open-source mount, Alkaid, which hopefully works a bit better for them.

Put simply, the difficulty with photographing an extremely faint, distant object (or a larger but diffuse object) is that the camera sensor must spend a considerable amount of time averaging the signal, to collect enough for anything to be seen at all, through the noise. But, that ball of rock we're sitting on is constantly spinning, so the only solution is to track the object of interest, to compensate. C is called equatorial tracking, and helps compensate for the Earth's rotation during long exposure.

The design of each of the two axes revolves (sorry!) around the use of a NEMA-17 stepper motor with a 27:1 planetary gearhead, driving a harmonic reducer. Harmonic drives (aka stress wave drives) are pretty neat, working on the principle of a fixed, but circularly distorted ring gear, which transmits torque from the inside surface to the outside, with almost no backlash. This are expensive parts, but for super smooth motion, this is what you want. The huge torque output they allow allowed [Alan] to build a mount for a heavy telescope without any counterweights. Structurally, the set is constructed from 10mm thick aluminum plates which have been water jet cut and then milled to finish.

On the electronics side, a custom PCB was produced, with a pair of TMC2130 based stepper drivers, controlled by a Teensy 4.0. The simple design was built with Eagle PCB and can be found on Alkaid's GitHub project, along with frame details and a suitable copy of the OnStep telescope controller firmware. Some 3D printed side panels hold the electronics in place and enclose the internals, giving the stand tidy storage. kind of industrial look. An important metric for such a mount is the overall weight, which [Alan] says is about 5.5kg, less than half that of a HEQ-5 mount, without its counterweights. Hopefully it's heavy enough to dampen vibrations transmitted from the tripod, but testing will prove it one way or the other.

Too complex? Not enough time to build? How about a barn door tracking bracket? We also saw a star trail based on Raspberry Pi...