"Affordable Astrophotography With a Motorized Barn Door. Astrophotography can be a fun and educational topic to cover with physics students.

Transcription

1 "Affordable Astrophotography With a Motorized Barn Door K. P. Trout Penn State University - York Campus (717) (work); (home); (fax); kxt7@psu.edu 1031 Edgecomb Avenue York, PA Abstract: Astrophotography can be a fun and educational topic to cover with physics students. Building the motorized barn door photographic device described here provides an affordable route to successful astrophotographs and provides unique out-of-class learning opportunities for physics and astronomy students. Key words: physics, astronomy, photography, education, teaching PACS 01.50P; 01.50M;

2 Introduction: The Barn Door, also known as the Haig mount or Scotch mount, is a common system of astrophotography allowing amateur astronomers to take timed exposures of the night sky on a hobbyist s budget. The topic is well published, including many ingenious variations. The variation described here is a motorized barn door device. The addition of a small D.C. motor makes this barn door device easier to use in the field and can improve performance. The first person to use a barn door for astrophotography was probably the Harvard astronomer, Donald Menzel. In the 1930 s Menzel devised the system, using an actual barn door, while setting up some solar eclipse experiments. 1 Later, in the mid-1970 s, Scottish astronomer George Haig redesigned the system into a smaller, more affordable backyard version. 2 This is the origin of the barn door s other names: Haig mount and Scotch mount. However, even this smaller backyard version that has become so popular can be tedious to use in the field. The basic structure of the barn door involves two wooden boards. The top board the drive board - is fastened to the lower board the base board - with a hinge. The basic operation in the Northern Hemisphere is to align the rotational axis of the barn door (i.e. its hinge) with Polaris, commonly known as the North Star. A camera, which is mounted on the device, is pointed at the region of sky that is of interest and the shutter of the camera is locked open. The photographer then continuously turns a screw to drive the two boards apart as the exposure progresses. If turned at the correct rate, the motion of 2

3 the camera on the barn door counteracts the apparent motion of the stars in the sky caused by the Earth s rotation, and one ends up with a clear image of the night sky. Without this sort of counteracting motion, the stars in the picture will appear smeared out in lines. This is called star trailing. With the barn door device, the star trailing can be virtually eliminated, and in addition, more stars can be seen in the photograph (see figure 1). The additional stars show up because the starlight from a dim star continues to be focused at the same location on the film throughout the exposure. Eventually the number of photons that hit the film is enough for the star to become visible in the photograph. Figure 1: The picture on the left is a four minute exposure of the Orion constellation. The camera was held stationary on a tripod which led to a crisp image of the tree branches but easily noticeable star trailing. On the right is the same region of the sky photographed on the same night with the same camera and the same exposure time. However, this time the camera was mounted on a barn door tracking device which virtually eliminated the star trailing. 3

4 The device is simple, but tedious to operate. The turning of the screw must be done at the correct rate in order for the picture to be successful. This requires careful timing - and timing in the dark at that! Usually the turning is simplified by designing the barn door to use a 1/4-20 machine bolt for the drive screw and placing it 11-7/16 inches from the hinge. The screw is then turned one time each minute. Some advocate further simplification by only turning the screw one-quarter turn every 15 seconds. 3 This can give adequate results in most cases. However, with the increased magnification of a telephoto or zoom lens, this method of tracking may not be satisfactory. In addition, even this simplified system requires intense concentration on the time making it more work than fun. Since I did not own a glow-in-the-dark watch at the time, my first attempts at overcoming the difficulty of timing in the dark were to use my portable tape player to assist me. I sat for a half-hour in a quiet room and made a recording of the passage of time. I spoke into the microphone every 5 seconds saying, 5 seconds, 10 seconds, 15 seconds,..., keeping this vigil up for a half-hour. Then when using my barn door I would simply put on my headphones and play the tape. This method worked well, but I still found myself tensely concentrating on turning the screw instead of enjoying the night sky. It was at this point that I decided to motorize my barn door. Construction: The motor selected for this project was a high torque, 7 rpm, 3-volt D.C. motor available from Edmond Scientific (Stock Number CR ). There are a variety of 4

5 speeds available for this style motor, but the 7 rpm motor is probably the best choice. This specialty motor costs about $25, but it comes with a free twist-on battery case that you would have to buy otherwise. Another advantage of this motor is that it can be run in the reverse direction by reversing the polarity of the batteries. This means the motorized barn door can be set up and used in the drive orientation that gives the best tracking: driving the boards apart, or driving them back together. The wood used for the main drive boards is half-inch clear pine. Step by step directions for constructing this barn door device can be found at or by contacting the author. A picture of the completed project is shown in Figure 2. Figure 2: The motorized barn door ready for action. 5

6 A single radial line is painted on the main pulley - like the hand on a clock - using glow-in-the-dark (phosphorescent) acrylic paint. The line will glow and act as a marker for keeping track of the motion in the dark. The paint is available from Edmond Scientific (Stock Number CR ) and others. The line can be seen in figure 3. Figure 3: An overhead view of the drive mechanism. The radial line is painted on the main pulley with glow-in-the-dark (phosphorescent) paint so that its motion can be tracked in the dark. A 1/8 th inch wide rubber band (about 7 inches long unstretched) serves as the drive belt for this device. The specific rubber band recommended for this project can be bought in an office supply store in a package called Big Bands, and is made by the Alliance Rubber Company. You may have to try several different size rubber bands until you find one that works well for you. If the belt is too thick or too tight, it may bind or 6

7 work its way off the pulley. If it is too loose, there will not be enough friction to turn the main pulley. NOTE: One problem that I have encountered on occasion with this rubber band drive is that after about 25 minutes of tracking, the drive bolt and main pulley are tipped at an angle and sometimes the rubber band pops off. You may want to try redesigning the main drive pulley to make the groove wider and deeper or consider changing the shape of the groove in the main pulley. Another possibility is to redesign the drive system so that it uses string. The main pulley could be wound with string, then the string could be pulled off the main pulley and wound onto the motor pulley much like a cassette tape functions. The drive rate will need to be adjusted occasionally during a long exposure since the main pulley and drive bolt start to tip sideways. Adjustment of the drive rate is easily accomplished with a 1 kω potentiometer, available at electronics supply stores like Radio Shack. Additionally Figure 4: A schematic of the electric circuit used to drive the motorized barn door. it is wise to add a single-pole, single-throw (SPST) switch so the drive can be turned on and off without removing the batteries. The wiring diagram for the motor, batteries and potentiometer is shown in Figure 4. 7

8 You can go a step further and add a handle and glow-in-the-dark scale to the potentiometer if you wish (see Figure 3). An alternate design would be to mount the potentiometer in a small project box and connect it to the switch and batteries with a long lamp cord. This will make the adjustment something you can control in the palm of your hand and will minimize the risk of bumping the drive device while it is operating. A Learning Experience It is easy to develop lessons for the physics and astronomy classroom associated with this barn door device. For example, one obvious lesson might include calculations regarding the tracking rate. Another may include the relationship between the radii of the pulleys, torque and mechanical advantage. Still another example would be calculations regarding the exposure of the picture and settings on the camera. However, the practical experience gained using the device may be the best lesson of all. When the particular device described here was completed, one of the author s physics students, Ted Luebbert, tested it in the field as part of an out-of-class project. The knowledge Ted gained from his experience was not likely to be learned by listening to a teacher in a classroom. Ted traveled to a rural area southwest of Cleveland for the Thanksgiving holiday in He had never done any astrophotography before, but he managed to take a couple of pictures while on his trip. One of them turned out very nice, showing many stars (see Figure 5). Unfortunately Ted said he was trying to get the method down and this was his warm-up picture. He doesn t recall details on exactly where the camera was pointing except that it was pointing nearly straight up. On that date - 11/27/96 - at that location - 8

9 Wellington, Ohio - at that time - 8:00 pm EST, the camera was probably pointing somewhere in the region of the Andromeda Constellation. Ted learned a very important astrophotography lesson in the process: Never assume one picture will turn out better than another. Sometimes your warm-up picture turns out to be a keeper. Ted also gained appreciation for the meticulous procedure and patience required to do astrophotography. Figure 5: A photo taken with the motorized barn door. Notice the large number of easily discernable stars. The film used was Fuji 800 Super G Plus. The exposure was approximately 20 minutes long with a 50-mm lens at f/1.7. 9

10 Another picture, taken by Penn State York student, John Baer, and the author shows comet Hale-Bopp in the badly light polluted skies in southern York County, Pennsylvania on March 15, 1997 at 4:50 am (see Figure 6). Figure 6: A picture of comet Hale-Bopp taken with the motorized barn door and a telephoto lens. This picture was made on Fuji 800 Super G Plus film with a mm zoom lens, an f-stop of f/4, and a 10-minute exposure. Conclusion: Even when it s not operating at its fullest potential, the barn door device works wonders. And with this motorized system, one need only check the tracking every couple of minutes and make slight adjustments in the tracking speed. This can be done conveniently by comparing the glowing mark on the large pulley with the second hand on a glow-in-the-dark watch. Astrophotography can be fun, educational and affordable! 10

11 References: 1. Gleanings for ATM s: Two Arms Are Better Than One, by Roger W. Sinnott; Sky and Telescope, v.77 n.4, April 1989 (p. 436) 2. Astrophotography, An Introduction (Sky and Telescope Observer s Guide), by H. J. Arnold, Sky Publishing Corp., June 1995 (p. 88) 3. A Homemade Mount for Guided Astrophotos, by John Childs; The Physics Teacher, Vol. 35, March 1997, (p. 186) 11