"Barn Door" Star Tracker


Be sure to click on these thumbnail photos for larger versions.

 1. This is my "Barn Door" star tracker.  Prior to buying a telescope, this is what I used to prevent stars from streaking in my pictures when I took long duration exposures with my SLR camera.
 2. It does this by rotating the camera around an axis that is parallel with Earth's rotation axis.  The rotation of the camera is in the opposite direction of Earth's rotation and at a speed equal to Earth's rotation.  This effectively prevents star streaks in long exposure pictures.
 3. I got the idea from an article in Scientific American on how to build this star tracker back when Comet Hale Bopp visited us.  Their plan didn't include a motor, instead you were instructed to turn the adjusting screw 1/4 turn every 15 seconds.  I decided to make two improvements to their plan.  1. Add legs to make it a complete self contained, full size tripod, and 2. Add a motor to turn the screw.
 4. The left two legs and the right leg are on opposite sides of a piano hinge that makes it easy to fold the thing up and transport it.
 5. This is what it looks like folded up.  Front side.
 6. Back side.
 7. The stationary side of the "Barn Door" hinge.  The barn with the door removed.
 8. Now the "door" is on, holding the camera mount and the drive motor.
 9. A little closer view.
 10. This is the motor assembly and the motor drive circuit.  The motor and gears were salvaged from an old computer printer.  I know a thing or two about electronics and built the motor drive circuit on the left to drive the motor at the proper speed.
 11. The largest gear to the right has a deep socket epoxied to it, that slides over and turns the head of the adjusting screw that rotates the camera.
 12. As I come around to the right, it gets easier to see what the adjusting screw does.
 13. As the deep socket turns, it screws the adjusting screw down through a threaded insert that is attached to the plywood "door".
 14. The adjusting screw pushes on the metal plate on the "barn" causing the "door" to open slowly, also causing the camera to rotate.
 15. This picture along with the next one shows a lot of "door" movement.
 16. It's also easier to see the threaded insert in this picture that the adjusting screw is threaded into.  There is enough thread on that screw for a 20 minute exposure.
 17. The wrench end of the deep socket was removed, so when the 20 minutes is used up, the deep socket just lifts off and the screw can be turned back out for the next 20 minutes of run time.  I have to admit that this method of turning the screw does cause trouble sometimes.  Every once in a while, that deep socket gear will jump a tooth.
 18. The three legs each have a lag bolt that can be used to adjust the height of each leg a little.
 19. I also made some 2x4 feet, but they seem to cause more trouble than help.
 20. Proper alignment is important to prevent star streaking.  The hinge pin needs to point to the North Star.  One way to do this is with a magnetic compass, a plumb bob and a protractor.  Use a compass to point the hinge pin to true north.  You need to adjust for magnetic declination.
 21. Then use a protractor and a plumb bob to adjust elevation.  The plumb bob string should pass through an angle that is equal to 90 degrees minus your latitude.
 22. The other option is to visually align the hinge pin with the North Star.
 23. I borrowed a rifle scope from my brother for this idea.  It worked great!