Building an equatorial mount for a Dobsonian-mounted telescope

Slow progress

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In this article series, I'm trying to build a working equatorial mount for a 10" Dobsonian telescope.

  1. Building an equatorial mount for a Dobsonian telescope - Getting started (Part 1)

I was hoping I would be much further with this project by now, but as usual, time and laziness got the best of me. I got a bit overwhelmed by the scale of this project, and kept thinking I would be work on it "next weekend". But now that spring has begun to push winter out of its way, I've got some newfound motivation towards finishing this project. Or rather to start it for real.

A sketch of the initial plan
My initial plan involved a layer to keep the telescope centered - I later realized centering would not be required as long as the telescope was always crossing the center axis of the mount.

Couple weeks ago I started to really think how I would go about building the mount. My first idea was to go all in on the automation. I thought of having the angle towards Polaris calculated with a GPS sensor and using a stepper motor to lift the base to the desired angle. I also thought I'd have to position the telescope to the center axis with an extra plywood layer and a set of wheels (seen in the sketch above), but luckily I realized I only would have to have the telescope cross the center axis (at all times) to avoid any image bluriness. In addition the telescope would of course spin on the mount to counteract the Earth's rotation.

But reality is a bitch, and I realized I don't have any experience in mechanical engineering or electronics. All of the automation would also cost much more, and be more likely to fail compared to a more simple solution.

I landed back to reality, and decided to keep it simple, at least for the start.

Final plan

The mount will have 3 round layers of plywood, each 60cm in diameter. The Dobsonian mount of the telescope is 56cm in diameter, so 60cm will leave a generous 2cm of padding on the sides of the telescope. One of the goals of this project is to build a plug-and-play mount, so that I don't have to modify the existing Dobsonian mount of the telescope. That way, I can still use the original mount for traditional viewing, while coupling it with the new-built equatorial mount for astrophotography.

Initially, the only motorized part will be the second layer, which will be responsible of spinning the topmost layer in sync with the Earth's rotation. Later on, I am hoping to add automatic tilting towards Polaris, and maybe even a keypad / locating software for locating objects in the sky automatically.

1st layer

The first layer is merely a base layer, responsible for keeping the mount nice and steady. It will have 4 padded feet to keep the mount slightly off the ground.

Between the 1st and 2nd layer, there will be an adjustable "metal rod" (for lack of a better term), which will allow setting the mount to the desired angle towards Polaris. The 1st and 2nd layers will be separated with a piano hinge to allow setting angle.

2nd layer

The second layer is basically where all the magic's at. It will house all electronics and the gearbox on its center, and it will have wheels on its edges to allow for rotation. In its dead center there will be a metal rod rotated by the stepper motor and gears, which is attached to the 3rd and final layer. The metal rod will cause the rotation of the mount alongside the wheels.

3rd layer

Like the 1st layer, the 3rd layer is mostly a support layer. It will have snug footholes for the telescope's Dobsonian mount's feet, so that the telescope wont slide off the equatorial mount when it is tilted towards the North star.

Going forward

Now that the plan has more or less solidified, I will start working on the motorized rotation, which I will discuss in the next post. Below are some early prototyping results.

Components and tools ordered from Sparkfun.com
Components and tools ordered from Sparkfun.com. Stepper motors, jumper wires, a breadboard, and motor drivers (EasyDriver). I also got soldering equipment from Verkkokauppa.com, and a claw for holding circuits while soldering from a local electronics shop (Bebek).
Early prototyping
I used one whole evening battling trying to get the motor spinning. I was sure I had accidentally fried the EasyDriver, but then by chance I tried to power the circuit through the Arduino's power jack instead of plugging power straight to the EasyDriver, and it started working. I'm sure the problem was with the current, as I had connected a 1000mA power source the EasyDriver, but had a 2250mA power source for the Arduino.
The schematic that got it working.
Schematic I used to finally get it working. The crucial thing was using a >2000mA power source. SparkFun source for schematic.

Short video of working prototype!

Building an equatorial mount for a Dobsonian telescope - Gear math (Part 3)

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