As I have the possibility of taking an after-school woodworking "class", I often go there and build neat little gadgets for astronomy and astrophotography.
Recently, I wanted a small, portable barn-door tracker that I could take on vacation with me, mount on a tripod, and snap some quick but well-tracked astrophotos. Having seen a similar design at a star party, I knew what I wanted and was able to complete the mount in approximately two hours. Instead of taking two big pieces of wood, my mount consists only of two very small and narrow stripes held together at one end by a hinge.
The critical dimensions are the screw thread and the distance from the hinge to the screw, which are proportional to another. You have to see this distance as the radius of a circle, that circle which the screw would describe if the wood was turned around the hinge once. The mount has to complete this circle every 24 hours (actually, it's 23 hours and 56 minutes, but it doesn't make much of a difference). That means the circle has a "circumference" of 1436 minutes. It is easiest to use a screw with a thread of 1mm/turn, since this requires a circumference of 1436mm. To calculate the radius from this circumference, divide by 2*Pi (3.1415). For the above screw, this results in a screw-to-hinge distance of 228.55mm. The distance does not have to be near that accurate; 22.8cm is fine.
With the theory explained, here is the actual construction of the tracker. First, I took two pieces of wood, about a third longer than the 23cm I needed and hinged them together at one end. Then, I added two pieces of wood to the sides to stabilize the construction, which, being held together by only one flimsy hinge, was rather shaky. Make sure that there is enough room for mouting a camera at the top. It is best to attach the stabilizers to point downwards, to where the tripod will be attached. However, don't make the stabilizers too long; exposures longer than 10 minutes cannot be realized with this kind of mount anyway because the angle at which the screw strikes the other strip of wood changes after a while.
Then I drilled a hole in the bottom piece of wood (the one without the stabilizers) and inserted a thread for my screw there. You can do without a thread and let the screw carve its own thread, but a ready-made thread (can be found in any hardware store) is much more reliable. Through this thread, I inserted the screw with the head in between the two pieces of wood. It is best to use a round screw head because the striking angle of the screw will not change by much this way. If you do not have a round-headed screw, glue a half sphere onto the flat head. I did this in my setup; I used a round head from another screw.
Especially if you are using a plain round screw head, you should glue a piece of metal onto the part of the opposite strip of wood (if you are using wood), so that the screw does not carve into the metal instead of pushing it up. This would be the case especially if you have a heavy camera.
I cut off the bottom of the screw after a length of about 2cm, then glued a wing nut to it in order to turn it from the bottom. Under this wing nut I marked a complete circle in 1/6 turns. I use these markings to turn the wing nut every 10 seconds (well within the limit for lenses up to about 135mm), instead of trying to keep a continuous motion. Turning the wing nut to the second hand of a watch works very well.
To keep the mount as close as possible together during exposures and storage, I added two screws connected by a spring to the side of the mount. This is not absolutely necessary, but I tried to think of everything that could eventually be useful for this mount.
I put a 1/4" (photo tripod) thread into the lower (larger) piece of wood. Now the mount can be screwed onto a small portable photo tripod.
Attaching the camera is the most difficult part. As the mount has to be tilted towards the north celestial pole, a camera that is mounted flat on the tracker would either point to Polaris or to the ground. To compensate, I took an old tripod head and screwed it onto the tracker. Had I bought this, it would have cost more than $10, but someone with more patience can surely build a suitable camera holder to stay within the $10 limit.
When you mount the tracker, make sure you know in which direction the stars are turning, so that you do not mount it the wrong way. Once you have explained to yourself that the way it is mounted is correct, mark "North" and "South" on the tracker so you'll know the next time.
The mount's hinge has to point directly towards the north (or south) celestial pole. This is the tracker's polar axis (above). How to turn the screw I described above, just make sure not to do it too violently, or your star images will again become streaks.
Some people have motorized their tracker, but I think that would be going too far. Instead, graduate from barn-door mounts and buy (or build) yourself an equatorial mount.
I have used my tracker a few times, but have not had very good results except with very wide-angle lenses. I assume this comes from my rather shaky tripod and the poor polar alignment. Since the hinge is very small, pointing it towards Polaris is not easy, and could be simplified by mounting a small finderscope parallel to the tracker's polar axis. However, I have not found the time to do this but will report if I modify my tracker in any way. Update (Oct. 8th, 2001): I do have a new tripod, and I am eager to test it, but the weather and my free time are both against me in this endeavour.
