DIY Equatorial Mount

 A little bit of background: I study physical natural sciences at the university of Cambridge, where I became involved with the Cambridge University Astronomical Society, or CUAS. They are the largest body of students who share a large enough interest in astronomy, that they come together once every couple of weeks to use the Northumberland Telescope at the north west of Cambridge.

Images by me, 2021

I hope you can appreciate the sheer scale of this structure, and I was immensely impressed. The nights in which I had the time to look through its ocular were often overcast, and even when they weren't, the intense light pollution is at times difficult to ignore; the telescopes positioning puts the most interesting parts of the sky at roughly the same heading as the brightest parts of town.

As you can see, the telescope is mounted equatorially. That means it is mounted on an axle, which itself sits on a structure that can rotate about a different axle perpendicular to the first. The important thing to remember about this second axle is that it points directly at the north star, i.e.* is parallel to the earth's axis of rotation (*this is a two way implication only within our lifetimes, since the earth axis of rotation does precess). That way, in order to track a star, one only has to rotate about this second axis to cancel off the rotation of the earth, and fix the telescope's orientation with respect to the stars. Only ONE degree of freedom is required to follow stars, the initial starting point of both axles, and azimuthal adjustment of the first, is what determines specifically WHAT star is being looked at.

My telescope, April 2020

This means I should fairly easily be able to construct a similar mount for my small Dobson telescope (750mm primary mirror) which can rotate in much the same manner. No need to solve a bunch of linear equations in Euler angles or quaternions or God knows what to get component-wise solutions for the path of a star in the sky in the earth's frame. Effectively, whatever matrix I would have had to solve has been diagonalized by choosing a good axis to rotate about.

So- I should probably show you what I have come up with so far.

This is my design. I will mount a type of rebar or something onto a kind of easel, which is secured by a hinge/ball joint at the north end and two wound rods at the back. This makes the telescope portable, because I can change the direction of the rotation axis based on what latitude I'm using my telescope from (Northumberland and other such designs don't really need this because a significant alteration to the earths rotation, due to, say, a massive fucking asteroid, is very unlikely, and secondly we don't really have the technology to transport intact buildings across different latitudes instantaneously). The assembly with what looks to be two bike sprockets connected by a chain on it, is used to drive two bike sprockets connected by a chain. I ordered these really cheap stepper motors from omc-stepperonline.com, which, with some things I bought, can have a 200,000:1 reduction gear between the motor and the first sprocket, leaving an approximately 4:1 reduction to get the 1/1440 RPM required at the telescope axle. The whole thing is mounted to a good old europallet, so the thing could be transported by forklift if need be. The brains of the operation will be a raspberry pi zero 1.3 (which I couldn't find ANYWHERE due to the semiconductor crisis) with a bonnet board for stepper control. The whole axle is supported laterally by those two lunar lander looking things, and this thing will run on a car battery.

I have already ordered some stuff, so once exams are done, I'll get some friends to build this lil guy! I think I'll name it Ralph... (initialism suggestions wanted)