Star Tracker
June 2018 - Ongoing
Premise
Photography has been a hobby of mine for several years, and I always enjoy taking photos of the night sky. However, the camera needs a long time to capture enough light from the stars, usually at least 15 seconds. Over 15 seconds, the stars actually move quite a bit, and the stars are ever so slightly blurry in the final picture. This becomes even more apparent when the capture time is increased to 30 seconds. A star tracker allows the camera to follow the motion of the stars which means capture time can be increased to several minutes, and I can take clearer photos that expose more detail in the night sky.
First Design
When researching some other design concepts, I came across showed an example of one popular DIY design, the "barn door" tracker. In this style of tracker, the camera is mounted on a hinged platform, where the axis of the hinge is aligned with the North Celestial Pole, the center of rotation of the night sky. Here's a quick sketch I did to visualize this design:
As shown in the picture, a motor turns Gear 1, and in turn Gear 2. A threaded rod is attached to Gear 2, and a nut mounted on the threaded rod slides up and down, moving the upper plate. The upper plate is where the camera is attached, and the motion of the motor is calibrated so the upper plate moves about the hinge at a constant rate. Because the threaded rod is straight and I want the upper plate to move in a constant angular rate, the speed of the motor will change over time, and I calculated this equation using differential equations and trigonometry (I'll spare you the details).
Following that, I modeled the design using Fusion 360. I like Fusion 360 for personal projects due to it's top-down design approach which allows me to immediately design parts in the context of the full assembly, as opposed to Solidworks which requires each part to be modeled separately.
I fabricated the two gears using SLS printing for strength, although one downside was the support nodes which created some unwanted surface roughness. The two platforms were cut using a laser cutter, and the final prototype is shown here:
New Design
After playing around and tweaking this design for several months, I had gotten in a good place but there were a few things that bugged me
I was depending on the nut sliding against the upper plate smoothly without binding or friction, otherwise the tracker would not move properly
The upper plate would need to be zeroed to the same spot every time, since the motor speed is a function of time, or rather, what angle the upper plate is at
The overall device was elegant and decently compact, but I thought I could do better
Inspiration finally struck when I visited an old telescope shop, and got to see a commercial star tracker in person. After doing some thinking and asking the experts in the shop, I realized that the barn door design was good, but there was a much more simple and elegant design, which almost mimics the equatorial mount used for full size telescopes.
Demo
This is a demo of the current redesign. The basic mechanism is a double worm gear reduction (40:1 each) for a total of a 1600:1 gear ratio. A cheap 5V stepper motor (28byj-48) is used as the input. The worm gears, motor, and shaft are purchased off the shelf, and the housing was custom designed and 3D printed.