Ever since I tried to image the skies I have been plagued by the rotation of the earth. To counter this rotation you must have a mount for your camera or telescope that rotates in the opposite direction. Professionals use equatorial mounts, these precision instruments are quite pricey, in most cases more expensive than your telescope on its own. While looking for a cheaper way to build one myself I stumbled upon the designs for an equatorial platform. These platforms are an inexpensive way to emulate what a equatorial mount does.
This document will contain the basic design choices, sources, explanation of the containing files for my attempt at making an equatorial mount. This was also made for a school course “open design”
My goal was to make a mount that could track the sky for a few hours, it has to be able to support a camera and a small camera tripod. It only has to be accurate to about 2% over a range of up to 2 minutes.
Most important of all I wanted to build it for under 50 euros.
It only needs to work for my Latitude (I live in the Netherlands), very important! This design won’t work for anyone too far away from 51-52 degrees North. However some simple modifications can be done to change the inclination.
- 1m2 3.2mm (sold as 3mm) berch triplex plywood for laser cutting
- Glue
- Metal pins (any reasonable size you want)
- Washers and stops for your axles
- A method to rotate a gear (in my case I used bits of my Mechano set but you could also use Lego technic or even the mechanism from a clock or egg timer)
All files are uploaded to grabcad
https://grabcad.com/library/equatorial-platform-1
The following files are contained in the project:
- readme.odt, this file has a comprehensive list of all the things that were done designing and building this mount.
- build pictures/ this folder holds 10 picture showing various stages of the build in greater detail.
- final_model_acad_2004.dwg this file is a legacy AutoCAD drawing file, compatible with almost every version of AutoCAD and some other programs.
- final_model_acad_2014.dwg newer AutoCAD file
- cutting_template.ai this is an adobe illustrator file that contains the exported lines to use for this specific model.
If you are just going to make an exact copy of my mount you can skip step 1 and 2, just make sure to read chapter “What could have gone better”, for optimal performance you might want to alter a few things to my design.
First I designed my 3d model based on Reiner Vogel’s instructions (see sources). The inclination is 51.83 degrees (it will work fine between 50 and 55 though). Before you start making the model it is good to note what material you will be using. For the gearing I got 2 generated gears from woodengears.ca (see sources). The model was made with the material in mind, all surfaces can be cut from sheets of wood or acrylic.
2. Turn into 2d cutting image
Before you start making the model it is good to note what material you will be using and what cutting width your material has in a laser cutter, my 3.2mm thick birch plywood in that specific cutter lost 0.1mm on the cutting edge, to compensate for this I offset all my lines by 0.1mm outward, this made the interlocking pieces a snug fit. The lines themselves are polyline, these lines can be found under the layer “printlines” in the DWG files. The current print should fit on a 80cm by 40 cm plate.
3. Cutting, drilling and gluing.
How to cut these pieces depends on what laser cutter you have access to, the one I had required red lines and 0.01mm line widths. Once cutting was completed I had these pieces:
I drilled the holes after the cutting, this is because I have an exact drill and I was worried the laser had too much of a margin when cutting. So I made the laser cut 1mm holes then I used my 4mm drill with those guide holes.
Glueing is pretty straight forward, apply a little glue on the surfaces to bind and make sure there is enough pressure on the connection while it dries for 15 minutes. I did not have clamps so I used weights. I made sure to remove excess glue with a damp cloth as I connected the pieces.
When the pieces are done and glued you should be able to run axles though the holes and connect all the parts. Make sure to use washers for the rotating parts.
This step is most up to you, realize the big gear has to rotate once every 23 hours and 56 minutes (1 sidereal day). The small gear rotates 15 times for every rotation of the big gear so once every 95.733 minutes, you can use anything from an egg timer to a clock to power it, to make the gearing and reduction you may need some construction set parts, if you have Mechano you can replicate the following setup.
To test if the speed was right I used a rotary encode that gives 24 pulses per rotation, I input these pulses into my program, feed the gear ratio and see at what voltage I need to run my motor. In my setup a 1.95v input will drive the secondary shaft 10/24th of a rotation every minute, +-25 rotations per hour.
this is then reduced to 1.7857 rotations per hour at the (1:14) gears. This is the reduced to 0.627 rotations an hour at the (18:50 gears), this translates to 1/24th of a rotation for the main body of the mount at the (20:300,1:15) gear. The trick here to making the ratios work is to have a variable motor, I knew the motor could operate in a range between 1.5 and 5 volts, I mapped out the speeds for every step and I kept adding reductive gears until I got in that range, then I varied the voltage to match the speed required, finally I will make a 3 volt battery pack with a potentiometer to push the voltage down to 1.9 volts.
What went well
The laser cutting and gluing makes for a really ridged and light structure, I was worried that it would be too weak and inaccurate but it went well and is really cheap. The wood cost me under 2 euros.
The rotary encoder and motor made it really nice to test different speeds, it should be an easy mod to make a switch for tracking the moon (3% faster than the normal stars).
What could have gone better
The speed at which the main gear moves is so slow that the friction of the device makes it a little jagged, if I were to make this device again I would use bearings and smaller teeth on the wooden gears, maybe even try have the wood interlock with a metal gear.
The motor I have now has its own little gearbox, it is too loud, next time I will try to use a quieter one.
Original design by Adrien Poncet and later further pushed by Alan Gee and Georges d'Autume.
http://en.wikipedia.org/wiki/Poncet_Platform
http://en.wikipedia.org/wiki/Equatorial_platform
Gear generation:
https://woodgears.ca/gear_cutting/template.html
Project site that I based my designs on: http://www.reinervogel.net/index_e.html?/Plattform/Plattform_e.html
Personal open design statement.
It is a waste of a good idea to keep it to yourself. Like any science, designs can be subject to peer review, this process over time will make for better designs.
