Question: Why would you need a star tracker?
If you've ever photographed stars, you'll notice a pesky problem that appears after a minute or two. The earth will rotate and cause the stars to blur in your image. The longer the exposure, the more blurred the stars will become. To get really detailed photographs of the night sky, you need to counteract the rotation of the earth to keep the stars clear, and so your camera can absorb enough light from the faint stars to form a complex image of the sky.
Various trackers have been used for over 100 years to produce clear images of the sky, as well as for use with telescopes. This Instructable will show you how I built a mechanical star tracker which is accurate enough for wide angle astrophotography, but not accurate enough for use with a telescope.
Commerical motorized units exist, but they start at a minimum of $200, putting it out of the reach of someone like me who would only use it a few times a year.
The main mechanical issue of star trackers is that the earth spins once per day, but even the lowest electric motors run thousands of times faster than that. A synchronous AC motor, which should be reasonably accurate, runs at 1800rpm, 2.592 Million times too fast! Star tracker designs such as barn door trackers use a screw drive to reduce the rotational speed of the motor as simply as possible, but it is still difficult to do accurately. Often, a stepper motor and microcontroller is used to get an accurate but slow moving motor drive. The problem with motor-driven mounts is you need power, which you don't often have available wherever you are taking photographs of the night sky. My solution to this is to use a wind-up mechanical timer, like a kitchen timer. The timer I used is a 1 hour model that I tested to have better than 1% accuracy, so it will work for this purpose.
Step 1: Theory and Design
The idea is simply to rotate the camera at the same rate that the earth rotates, but in the opposite direction. However, this axis of rotation must be aligned with the same axis of rotation of the earth for the effect to take place.
The design is fairly simple. The timer has a gear connected to the shaft, and connects to a 24:1 gear train, with the shaft of the final gear being connected to a tripod ball-head. The camera then attaches and can be aimed in any direction (the direction you photograph is independent of the rotational axis). A red dot sight is used to align with the north star out in the field. To set the RDS, a laser is attached to the shaft of the final gear and the two are aimed at a wall, and the sight is adjusted until it overlaps the laser dot.
I designed and built this knowing what kind of tools I had access to, so it does rely on some fairly advanced machines, such as a laser cutter, mill and lathe. However, I don't think that not having these advanced tools will prevent you from accomplishing this job. With a little ingenuity, everything can be achieved with simple hand tools and common power tools.
The attached DWG can be used to laser cut the gears, if you wish.
The Nikon model I used is by Joel Anderson
The ball head is by Bob Jenney
Can't find the lower tripod head model anymore. The rest was done by me.
Step 2: The Build
I'm going to be brief about making this thing because you will have to do some problem solving to build this based on the tools and materials you have access to. For the most part, its not too difficult and the tools are fairly common. Some thinking may be required on your part.
The timer was removed from its plastic housing, and I found it had a convenient 1/4" shaft on it, which was easy to mount a gear on. The knob was kept to allow me to wind up the timer easily.
The gears are laser cut from acrylic, as is the top of the tracker. If you don't have access to a laser cutter or don't want to spend the money to have it cut by a laser cutting service, there are other Instructables on how to make wooden gears from plywood using a saw. You can definitely recreate these gears using this method. The gears were designed from the Solidworks Toolbox and altered slightly for my use.
The gears are held on shafts which have a flat side, and a point at one end used as a simple pin bearing. The idea is to minimize friction by having a very small surface area where the weight is. This proved to be insufficient for the large gear, so a spring and a bearing were added beneath it to hold it in place firmly, while still minimizing friction. The top of each gear shaft is held in a small nylon bushing which is machined to press into the acrylic top, keeping it in place.
The wooden portion of the housing is just two pieces of 1.5" thick poplar board from the local hardware store. I bolted the two pieces together while working on them, then glued it afterwards, because hole saws can't cut through that thickness so I needed to be able to separate the two pieces during the cutting.
The bottom is 18 gauge steel plate, cut to match the rest and align with the four bolts that hold the whole thing together. On the very bottom is a 0.5" thick aluminum block machined with a 1/4-20 hole where the tripod mounts to.
The alignment laser is mounted inside a little threaded holder that goes onto the end of the shaft where the ball head normally mounts. I use this to align the RDS correctly, then remove it and install the ball head. A simple version of this would be to cut some extruded aluminum to make an X-block that the laser could be strapped right to the side of the ball head, if the ball head has a good concentric face on it. Mine does so I will be using this method in the future to align, as it is easier and probably more accurate.
Step 3: Assembly
Since the timer is mechanically a very weak mechanism, designed only to have to rotate the knob back to the starting position, it doesn't have the strength to move the camera. Instead, a spring is hooked to the main gear and mounted to the top cover which pulls the gear in a clockwise direction. Now, the timer is effectively serving as an inhibitor to the spring's motion, and instead slowly allows the spring to rotate the camera in a clockwise direction. This eliminates the issue of torque in the drive system, and also keeps the gears tight to eliminate backlash. When the timer is wound up, the spring becomes increasingly stretched, and never fully slackens during use. You can see in the pictures that it is constantly stretched.
I finished the wood with some stain and spray lacquer, and let it cure for 2 weeks before using otherwise the bottom plate and/or top cover might stick to it, and I would have trouble opening it again.
Step 4: Finished
All assembled, ready to shoot. Using a ball head for both the tripod and the tracker lets you align the tracker with the tripod head, and then aim your camera wherever you want afterwards. I recommend buying an additional ball head on eBay for cheap.
Step 5: Results
The two images on this step are shot back-to-back. One was tracked, the other wasn't. The tracked one was 235 seconds long, and the untracked was 325 seconds long. A little bit longer, but the results are still dramatic. Polaris is not in this image so there are no stars which are not subject to warping, but polaris is up beyond the top-right corner of the image. Note how the tree is clear in the untracked image and blurry in the tracked image, indicating the camera was moving correctly.
It took a couple of test shots to get the tracker aligned right. I was having a hard time identifying the north star, so it's possible I wasn't very well aligned, resulting in the minor streaking in the tracked image. A larger spotting scope might make this easier to sight in, possibly something telescopic like an illuminated rifle scope, would be ideal.
I hope you enjoyed reading and I hope this Instructable helps to inspire you to make your own star tracker. They are a great way to take your astrophotography to the next level.