A "Barn-Door" Tracker
Based on the reception received at a recent meeting, I've prepared this article to set down some of the basics of the tracker platform that I showed. We covered various methods of "wide" angle star photography. Earlier this year I knew that comet Hale-Bopp would soon be hanging in the twilight sky, and that co-workers would be inundating me with "How do I...?" type questions. One afternoon I disappeared into the basement and came out about an hour later with my version of a "barn-door" tracking platform. With this device I wanted to build a simple, compact, low cost camera platform, one that I could lend out to friends and co-workers so that they could try their hand at astrophotography. It's simple enough for just about anyone to make with a minimum of tools, experience and outlay of cash. I had seen articles on these things many times, but naturally I couldn't lay my hands on one at the time. Since the principle is simple enough I just re-engineered the idea into something that I knew would do the job, and be made from the parts I knew I'd be able to scrounge.
The theory of the device is that it's nothing more than an equatorial mounting for the camera. Rather than using the motorized drive usually associated with an equatorial mounting, this one is manually driven. Clock drives typically involve a large gear reduction to be able to rotate at the correct rate, about one revolution every 24 hours (1440 minutes). The principle at work here is that the large reduction of a clock drive can be approximated by the regular turning, by hand, of a screw. This forces open a pair of hinged boards. The hinge axis, which is the equatorial axis of the device, is aligned with the celestial pole (approximately the position of Polaris). One board is stationary, mounted to a tripod. The other board, onto which the camera is mounted, is forced to slowly rotate so that the camera remains stationary with respect to the stars.
Let's start by listing the required materials involved in building and using one of these devices. The first is that you have a camera suitable for the longer exposures required in taking wide field sky shots. Requirements would be a B(ulb) or T(ime) shutter speed, suitable cable release, and a shutter whose operation is preferably NOT dependent on the camera battery, etc. A normal 50mm lens for a 35mm camera will work just fine for starters. You'll also need a camera tripod, the sturdier, the better. The tracker will be adding a fair amount of weight, as well as cantilevering the camera's weight out from its normal position. This will put additional strain on the tripod head. You'll be turning the knob, by hand, in the dark, possibly with a stiff cold breeze, so rigidity of the tripod is very important . The tilt and pan motions must be lockable, and the tripod should be tall enough for you to turn the knob at a comfortable height.
Perhaps the most difficult thing to find in the whole project will be the means of mounting the camera to the upper board of the tracker. The mounting must allow the camera to be pointed in the desired direction in the sky, independent of the tripod. Remember, the tripod's motions will be dedicated to holding the tracker positioned on the celestial pole, so another means of pointing the camera will be required. I used an old flash extender that I had sitting around. This consists of a female 1/4-20 thread in a metal square the size of a camera flash "hot-shoe", and a lockable ball-and-socket with a male 1/4-20 thread on the end. Another alternative would be a ball and socket tripod head (just the head portion). Either of these should be obtainable from a camera shop for less than $30. A word of warning here, if you decide to make your own version, keep the length of the 1/4-20 screw the right length. If it's too short it may not grab the camera threads properly, and if it's too long it could go in far enough to damage the threaded insert. Worst of all, it could foul up the camera innards.
Other materials you'll need for this version of a tracker:
- --A plain door hinge that will define the polar axis of the device. Nothing fancy, I used an old one, discarded after redoing the kitchen pantry.
- --Two pieces of 1 by 4 wood (actual size 3/4 by 31/2) to form the stationary and rotating portions, each one just over 12 inches long.
- --Two 1/4-20 "tee" nuts, for mounting the stationary board to the tripod and for the tracking screw. Wood does not take well to "machine" threads, so these will keep the threads from stripping out the wood. Also suitable are threaded inserts, which are typically made of brass and have coarse external wood threads, and internal threads for engaging the machine screw thread, although they can be difficult to install.
- --A long (2 or 3 inch) 1/4-20 machine screw, or piece of threaded rod with locking nuts. I used a 3" long brass flat head screw. Power sand or file a blunt point on the end to define the pushing point of the screw.
- --A knob for turning the screw by hand. I used a 2" diameter by 1/2" long slug of black plastic left over from my first telescope mount. A jar lid with a centered hole would work just fine. I added a triangle of glow-in-the-dark tape for monitoring its position. Also required will be the use of a watch and dim red flashlight (helpful as well for recording exposure data), or some other means of timing the turning of the knob, in the dark, at the proper rate.
- --Miscellaneous hardware items:
- Flat head machine screws, washers and nuts for mounting the hinge, (the hinge is close to the end of the wood and wood screws will tend to split the wood).
- Spring and hooks to hold the open ends together ( or use the method that I prefer; the appropriate number of available rubber bands).
The only critical dimension in the device is the distance from the center of the hinge pin (the equatorial axis) to the center of the drive screw. The trick to getting the proper rotation of the upper board is to get the proper combination of screw pitch, knob rotation rate, and hinge / screw distance so that the rotation is equivalent to 1 whole turn of the hinge in 24 hours. Selecting the drive screw at a 1/4-20 means that for every turn of the knob the screw will advance by 1/20 of an inch (0.05 inches). To make things convenient the knob rotation rate is made one turn per minute. This would simulate the sweep second hand of an analog watch, but turning the knob 1/4 turn every 15 seconds will do just fine. These things set then, the screw pitch at 1/20 inch and the knob rotation rate at 1 turn per minute, the required hinge center / screw distance works out to be 11.43 inches or 11 7/16" as shown in the figure. There is a built in error in this type of "tangent" arm drive, in that as the screw advances its action is at a larger and larger distance. This changes the rotation rate of the upper board (about 1/2% in 15 minutes). For the short exposures that this device is intended for, this will not be a problem.
The location of the camera mounting adapter on the upper board can be anywhere, as convenient. The 1/4-20 insert in the lower, stationary board needs to be located close to the center of gravity of the tracker plus camera and lens. This point will change as the camera is pointed at different sky positions but the friction material on the tripod surface should be adequate to prevent slippage. For finishing touches I drew a small star map of Ursa Minor with the true pole position noted. It's not really necessary to correct for the small difference between Polaris and the true pole position, but it can't hurt either.
In use, the hinge is positioned on the west side when setting up the tripod and polar aligning. The tracker screw is unscrewed until the boards are close to parallel, and the camera is pointed at the desired sky location. The exposure is started and the knob is given a quarter turn every 15 seconds or so in the clockwise direction. (Assuming of course you're in the Northern hemisphere, and didn't special order a left-handed drive screw and insert) Every 60 seconds, then, the knob will be at its original rotational position. This will tend to keep things properly paced, and helps keep track of the elapsed time of the exposure in progress.
I would recommend resisting any temptation to motorize the rotation of the screw. Remember that this is intended for exposures of only a few minutes. Motorizing might be a temptation to try longer exposures, which will increase the tangent error mentioned above. What is more important, if you're going to the trouble of using a motor for rotation, you'd be better off building a device similar to the one which Rich showed at the meeting, a German type equatorial, or the one that I showed, a fork mounted equatorial. If I were to make any improvements I'd add a metal plate for the drive screw to push on. Alternatively imbed a pair of cylindrical steel pins into the wood, along the length of the boards, with the screw point pushing in the "vee" formed by them. The wood under the screw on mine has dented, and the force required to turn the screw goes up slightly as the angle between the plates increases. This is due to the increasing tangent arm length mentioned above. As the screw tries to move outwards, the tip tries to stay in the dent, tilting the screw and binding it slightly. Another improvement would be to use a much longer hinge, a "piano" type hinge, with deeper boards to accommodate the increased hinge length. Mounting the lower board on a couple of side boards, cut to hold the hinge axis at the proper angle, (the complement of the local latitude angle), would eliminate the need for a tripod. Setup could then be on any level surface, a picnic table, sidewalk, driveway..., and the only polar alignment required would be rotating east/west. The drawbacks with this would be a much less compact unit, additional wood working, and if you travel too far north or south you'll be shimming one end up to regain polar alignment in elevation.
One word of warning seems in order. Wide angle stellar photography, while it can be simple and fun, will reveal the limitations in that expen$ive len$ hanging out the front of your camera. Stars are point sources, and should focus to small dots, limited in size on the film only by the effects of diffraction and the scattering of light within the film's emulsion. A typical fast lens, wide open to minimize the exposure time, will most likely result in bloated images, probably fan shaped approaching the edge of the film frame. While these bloated images will mask some tracking and alignment errors, they may not be to your liking. Stopping the lens down 1 or 2 stops will lessen the effects, but this will require longer exposure times. As exposure times increase the chances of something going wrong also increase, like so many things it's a trade-off. YOUR optimum f-stop and exposure time will be determined by the film and lens used, local sky conditions and your preferences for star image size, sky background color, etc. Careful experimentation with film type, exposure bracketing, and writing down what you did, will help you decide the optimum settings for your setup.
Good luck and have fun.Published in the October 1997 issue of the NightTimes