"Affordable Astrophotography With a Motorized Barn Door”
K. P. Trout
Penn State University - York Campus
(717) 771-4136 (work); 755-8301 (home); 771-8404 (fax);
email: kxt7@psu.edu
1031 Edgecomb Avenue
York, PA 17403
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; 01.90
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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
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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.
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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 CR30418-62). There are a variety of
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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 http://www.yk.psu.edu/~kxt7/Barn%20Door/BarnDoor.htm
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.
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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 CR30318-06) 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 1/8th 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
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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 k 
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.
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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
1996. 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 -
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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.
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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 35 - 200 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!
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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)
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