The Trott Double-Arm Type 4 Tracker 

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A barn door tracker is a simple device used to cancel out the diurnal motion of the stars. When taking photos in low light levels, a camera compensates by leaving the shutter open longer. When taking astrophotos, that length of time can be hours. During that time, the Earth is rotating. And if you're like me, your on the Earth, which means the camera is rotating too. This can cause the stars in your photographs to streak, and sometimes this can be desirable. Star trails are some of the easiest astrophotographs to take. But, if you want more color and detail, you need to cancel out that rotation. You can do this by purchasing an expensive motorized equatorial mount. Or, you can build one of these:

 

 A barn door tracker. 

I suppose it's called a barn door tracker because it functions a bit like a door. It's a piece of wood with another piece of wood attached to it via a hinge. When you "open" this "door" at the correct rate, with the hinges precisely aligned with one of the celestial poles, it will cancel the diurnal motion, and the stars will not trail. If you use the info on the pages in my links section, you will find that this rate equates to turning a 1/4-20 carriage bolt or threaded rod, placed the correct distance from the hinges, at 1 RPM.

It is quite simple to make, requiring only a few hand tools, some wood, hinges and screws, and a few other things. Pay attention to detail and dimensions, the more accurate the better. It is currently hand driven, but I'm in the process of motorizing it. I use the finder scope from my Simmons telescope for polar alignment.

I have it set up so I can adjust for latitude. It's set for about 40 degrees latitude and can be adjusted up to 50 degrees latitude. I live in Maine so this is plenty, and allows me to move around a little if I get the chance.

Here you can see the hinges I used, and how they're spaced. The side you put the hinges on will determine if you need to raise or lower the platform. In my case, I lower it.

Instead of buying a ball joint and having to mount it onto the platform, I decided to use my own method of adjustment. It allows the camera to be pointed in any direction, although aiming at things closer to zenith can be difficult. This could be fixed by simply adding a longer 1/4-20 bolt to the camera. None of these holes are threaded so all platforms can be rotated, the bolt and wing nut act as a clamp

Here is how it is currently driven, by hand. Each mark on the paper strip is 1 second. You only need to turn the knob 90 degrees every 15 seconds when using a 50mm lens, but this allows for greater accuracy if desired, or when using a longer lens. I use an analog watch I can listen to and time the clicks with rotation. And yes, that is a paper clip, I went all out on this project.