Introduction: Equatorial Platform for the North
In this Instructable, I will show you how I designed an equatorial platform for my telescope. But first, what is an equatorial platform?
This device will let you observe an object for several minutes or take photos of the night sky without having to worry about star trails. Technically, it rotates at the same speed as the earth but in the opposite direction. You can now take long exposure images of your favourite constellations, nebulae and galaxies.
Most platform on the internet are designed for mid latitude observers. However, living above 60 degrees latitude, I needed to design my own platform. This is how I built it.
Step 1: Design
Since I am living in the north, Polaris sits quite high in the sky. This fact lead me to a triangular design with a polar axis in the south and 2 curved sections toward the north.
The drawings above shows 3 circles on the top of the platform. These are the position of my telescope feet. This is optional but it helps to secure your instrument in place.
The second drawing shows an upside down V shape. the top corner of this shape is where the polar axis is located. A threaded rod serving as the axis will be fixed at this location.
The base of the platform is similar to the top but extends further toward the north to accommodate bearings and a motor.
Please note that this design is one out of many. If you want to learn how to create your own or if you want to modify it, I invite you to take a look at Reiner Vogel's page and his detailed instructions.
Step 2: Parts
The parts are displayed on the pictures above. Here's a list of material:
- 1/2 inch birch plywood for the base, the top and the curved segments.
- 1/4 inch plywood for the circles (optional). They will hold your instrument in place (telescope, tripod).
- 5/16" threaded rod which will serve as the polar axis.
- 10 skate bearings (polar axis and circular segments rollers)
- 3/4" threaded rod to serve as a worm screw
- 72 teeth cog
- 4 rpm DC motor
- 3 hockey pucks
- Screws and small pieces of wood
Step 3: Cut the Top and Base
To begin with, we'll cut the top and base of the platform in a triangular shape. You can use the SVG file attached. The scale is 1:1. By rotating the part, it can be cut on a laser cutter with a 20x32 bed (2 layers of 1/4 in plywood). It can also be cut by hand but make sure to keep the original size (21" tall for the top board without the vertical segments attached).
The overall shape doesn't really matter. You just need to make sure you keep the position of the polar axis (see diagram) and the length, position and inclination of the 2 straight sides.
For the base, you can have the same shape but make the north side longer and extend the south side a little.
Step 4: The Polar Axis
We can start by assembling the polar axis. To do this, screw the threaded rod into a piece of wood and cut the other end of the wood at 30 degrees. You can then glue it under the top part of the platform, making sure that the axis of the rod goes through the polar axis (top corner of the triangle in the diagram).
Step 5: The Vertical Segments
We will now install the vertical segments onto the top part of the platform. Place the board upside down (polar axis up) and screw the segments into the sides of the board making sure the boards make a right angle. Pre-drilling the holes will help.
Step 6: Make the Bearings
These bearing will allow your platform to move smoothly. One of them is powered by a DC motor. The other one consists of 4 bearings mounted onto a 5/16" screw.
The powered one has the bearings mounted into its support (thick plywood). Around the threaded rod is a tube (plastic or metal) covered by a rubbery material. The final diameter should be about 1/2".
Step 7: Motorise It
In order to function properly, the motorised bearing rod should extend a few inches to the north side. On that rod, we'll put a system of worm and gear. This will create a strong but smooth circular motion.
I used a 3/4" threaded rod to make the screw. The cog has 72 teeth and the DC motor turns at 4 rpm.
You can use a stepper motor or a DC motor. I chose a DC motor because I already had one and I could adjust the speed easily by simply changing the voltage with a variable resistor.
Step 8: Assembly
You can now place the top part of the platform onto its base. The vertical segments should be on their bearings.
You will notice that the threaded rod for the polar axis has no support yet. We will add it now.
First, level the base of the platform and make the top and base parallel. They should both be horizontal. Now measure the distance needed for the support of the polar axis. You can then cut this support in a piece of wood and cut the bottom side at a 30 degrees angle. Drill a hole all the way through. You can now slide the polar axis inside.
To ease movements a little, the polar axis support can have one or more skate bearings inside. I have 2 on my platform.
Step 9: Add Feet
Your platform is almost complete. But there's one more thing we need to add. In order to cancel vibrations from the ground, and to make is more stable, we'll add 3 rubber feet. A good and cheap option is to screw 3 hockey pucks.
Step 10: How to Use It
It is fairly easy to use as long as you can see Polaris. Place the platform flat on the ground and turn the pointed side (polar axis) to the south. The side with the bearings should be facing north. If you are below latitude 60, you will have to raise the south side. If you are above it, you will need to raise the north side (2 rubber feet).
When you are fairly confident with your alignment, push the moving board (the top) to the west (left if you're looking at Polaris) and place your instrument on top of the platform.
You can now start the motor and look through the eyepiece or camera. The stars probably won't stay centred on your first try. You will need to adjust the speed of your motor while keeping your eye in the eyepiece.
Once the speed is set, your instrument should stay aligned on the objects you're aiming at.
The platform will run for about 60 to 90 minutes. Indeed, the vertical segments will reach their end after a while. You will have to push it back to its original position (west) and re-align your instrument.
Step 11: Results
You can now observe and take photos of galaxies, nebulae and planets at high magnification.
The more precise your alignment, the longer you can keep the objects centred in the eyepiece or on the sensor of your camera.
My platform lets me take up to 15 seconds exposure with a 1000 mm focal length. This is enough for common deep sky objects since you can stack multiple photos to get the same result as a single longer exposure.