The
design for this barn door tracker is taken from an excellent
article by GARY SERONIK. See his article here.
Note that dimensions differ between Gary's model and mine, as
the pitch of the rod influences the overall size of the barn
door.
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A
recent (2015) version of the mount. The wooden wedge means
that the camera is level above the mount when the mount
is polar aligned.
The
long CCTV type ball head is made up of sections and can
be extended or reduced in length. |
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This shot of Altair is 60 secs
exposure with a 150mm lens at F5.6 - it shows excellent
tracking capability of the barn door. |
the
main drive wheel turns an embedded nut to drive the 4mm
curved brass rod upwards at precisely the amount needed
to track stars as the appear to move in the sky. The main
drive wheel is driven by a smaller cog fixed to the output
shaft of the low RPM motor. Drive ratio is about 3:1
A
spring holds the motor cog in place and elastic bands
can be added to provide tension to keep the doors closed
.
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This image is 4 stacked shots - 60 secs,
150mm lens at F5.6 - centred near 17:45 hrs, 55 deg N |
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The
motor is controlled by a voltage regulator outputting
at around 6volts. Voltage regulation down to 1/10th of
a volt is possible with this circuit. I've recently added
a diode to protect th circuit from a reversal of polarity.
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Andromeda,
100mm lens, 60 sec exposure. |
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Viewed from below, the improved motor on the production
model can be clearly seen. A wood screw firmly fixes the
motor in place while a small spring allows the motor to
articulate and engage the driven cog. This allows the
doors to be opened up, the drive wheel to be spun back
into the start position and the drive train re-engaged
for another 2 hrs of tracking.
Of
course this moves the alignment and the camera will have
to be realigned. |
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Vega
in Lyra, taken 26th Oct 2013. 60 x 4 sec exp. |
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The tracker worked well, keeping good
time. The photos aren't all that good as there's lots
of light pollution and even a 2 min exposure at F11 shows
too much noise.
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Cygnus
60 sec exposure with 100mm lens. Some lens effects near
edge, but centre field is all round stars - just need abetter
camera. |
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- see initial
photos here
Now
available - Kits, Parts and Instructions - click
here
Making
the curved brass rod is the trickiest part of the process, and
the most critical. Rather than bend the 500mm length of rod
by hand or around a formed shape, I decided to invest time and
effort in producing a bending rig to give me more control over
the process.
The
rig allows me to produce a curve which is ever so slightly oversized.
I then use wire and string it like a bow, tightening the wire
until the curve is the correct diameter. The "bow"
is then heat treated to remove stresses in the metal that may
cause cracking later on and to allow it to set into it's new
shape.
As
the 4mm rod has a pitch of 0.7mm the radius of the curve has
to be 160mm. The rod is cut up into (approx) 160mm lengths after
bending. This should allow over 2 hrs of operation from almost
fully closed to almost fully open. 160mm is also the distance
from the pivot point of the hinge, to the centre of the hole
the rod threads through.
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