A sidereal day is 23 hours, 56
minutes, and 4.0916 seconds. As the
Earth takes one solar day to rotate, it
also is revolving around the Sun.
Therefore, a star appearing directly
overhead will take slightly less than
one solar day to appear directly
Referring to Figure 2, at the top
the threaded rod goes through a
pivoting nut made of 5/8 inch Delrin
rod, although other plastics may
work as well. A hole is drilled
through the Delrin rod at a right
angle to the rod’s axis, and is then
tapped out with 1/4-28 threads. The
Delrin rod is mounted in L-shaped
UHMW (ultra high molecular weight)
polyethylene blocks that are screwed
down to the top plate. This material
is somewhat similar to Teflon but has
better abrasion resistance.
At the underside of the bottom plate, the stepper
motor is mounted to a longer piece of pivoting Delrin rod
flattened on one side, and is mounted in L-shaped UHMW
polyethylene blocks. A hole is cut through the Delrin rod
for the stepper motor shaft. An aluminum coupling with
two set screws attaches the threaded rod to the stepper
motor shaft. As the top plate moves up, the pivoting nut
must move to the left. This is accommodated by allowing
both ends of the threaded rod to pivot, maintaining the
geometry of the isosceles triangle.
There are many variations on how to increase the
accuracy of a barn door tracker. This design computes the
angle needed in real time several times a second to
accurately move the camera at the sidereal rate. The
rate at which the screw turns varies with the
distance d, giving rise to difficulties in other designs.
There are different approaches as to how to
accommodate this varying rate.
The use of a piano hinge has been criticized
and can lead to inaccuracies that show up in
exposures as star trails. I used two pieces of brass
telescoping tubing for the hinge with the outer tube
soldered to brass strips, making a hinge that tracks
more accurately. With the hinge pin being a small
hollow tube, this is also a very useful feature in
sighting the north celestial pole. Figure 3 shows
details on how the swivel mounts and hinge were
constructed, and Figure 4 shows the mounted
improved hinge. By not cutting down the 12 inch
hinge pin tube, the longer length works better for
sighting Polaris. The bolts above the hinge mount a finder
Figures 5 and 6 are views of the barn door tracker on
a telescope tripod. Actually, any sturdy mount works as
long as the hinge axis can be pointed at the north celestial
pole, which is very close to Polaris. I used a finder
telescope from an astronomical telescope, but something
like an inexpensive air rifle scope could be used as well.
For the camera mount, I used a ball mount that
screws into the 1/4-28 mounting hole (found on most
cameras) which attaches to the top plate. This allows the
camera to be pointed to the desired position in the sky.
It is important that the threaded rod is straight and
January 2015 27
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■ FIGURE 1.
■ FIGURE 2.