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