Motorized
Barn Door Tracker
The
Barn Door mount gets it's name from the fact that
it's basically a wooden board with a door hinge.
While there are many commercial mounts to track
the sky, here we will discuss a simple way to
construct a mount that will costs less than $100
complete with a motor to drive the screw.
The mount must turn at exactly the speed of the
rotation of the earth but in the opposite
direction, so the stars appear stationary. The
mount also must be angled for the users given
latitude. There are several designs floating
around on the internet. This design is called a
curved sector drive, because the threaded rod it
uses is curved. The curved sector drive allow for
a constant drive rate, therefore making this
design simpler to use with a motor.
The
movement of the sky is one continuous movement
around a fixed point, the North Celestial Pole.
Since we know the Earth rotates once per day,
which means one full 360° in ~24 hours. This
translates to about 15° per hour
(360/24=15)
or
1/4° per minute (360/24/60=.25).
This is called Sidereal time.
1 Hour star trails
around Polaris
The
sky moves in a near perfect arc around one single
axis. If we angle the 2 hinged boards for our
given latitude and we move the boards at a
constant rate of 15/hour we can now track the sky
very accurately.
Using
some basic math we need to calculate diameter the
worm gear. Assuming we would like the drive nut
to turn once per minute and we use a 1/4-20
thread rod for the curved sector drive. We know
that there are 23 hours 56 minutes in a sidereal
day (1436 minutes), so now we can calculate the
gear diameter.
1426
teeth inch
------------- * ----------- = 71.8 inches in
circumference
1 20 teeth
circumference =
π *
diameter
71.8
Therefore the diameter of the gear is =
----------------- = 22.86 inches
π (3.14)
Now we
have the distance from the hinge to the curved
sector drive to be 11.43
inches.
SolidWorks CAD model
of the Barn Door Tracker
The
next task is to bend the threaded rod to match
the 11.43" radius so that the mount will track
correctly as the barn door mount is rotated about
it's hinge. Using a piece of plywood large enough
to layout a 11.43 radius using a compass, scribe
the radius onto the plywood. Then using some
screws, place the screws every 4" or so on the
inside of the arc. You need the center line of
the 1/4-20 threaded rod to match the 11.43 inch
arc as closely as possible. The length of the
1/4-20 threaded rod will determine the length of
time you can image for without having to
rewinding the mount. 8 to 12 inches seems to work
well. Start out using the full length purchased
of the 1/4-20 thread rod, typically they come in
24 to 36 inch lengths. Work the length around the
form The longer length is easier to form and work
with. Once the rod is the best you can get, then
cut out the best part of the sector matching
desired radius on the plywood.
Form to bend the
drive screw
Now
all we have to do is cut two boards and drill a
few holes and attach the hinge. You can use any
boards but a hardwood might be a bit more stable.
I used some Teak that I already had. The boards
should be 3 to 5 inches wide and about 12.5
inches long.
Side profile show
the hinge
By
adding a 1 RPM motor from Edmund Scientific
and
two sprockets from WM Berg the tracking
process can even be automated!
Photo
of the actual mount
For
alignment on the pole, you can go to a lot of
work to put in some sort of pointer, but any
error in installing it relative to the rotational
axis of the hinge adds to the overall pointing
error. Just sight up along the hinge to Polaris
seems accurate enough for a start. I do have a
Telrad 4 degree finder scope mounted to mine.
Below is what it looks like through the finder
scope, as you can see the NCP is about 0.75
degrees from Polaris. Once I have located Polaris
in the center then I move the center to the NCP
based on the date/time , I use Starry Night for
this.
Position
of Polaris relative to NCP on August 6th around
10:00 PM
Results!
20 frames at 2 minutes each full info on this can
be found here