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For
wide-field DSLR photography, a Barn Door tracker or 'Scotch Mount' can provide
sufficiently accurate tracking to follow the stars. A fixed tripod will only
allow about 15 seconds exposure before star trailing becomes
noticeable.
A small
DC motor drives an M6 bolt at 1.3 revs per minute. The pitch of the thread is
1mm. The thread passes through a small furniture bolt in a perspex housing
which raises a hinged platform. The platform or 'arm' will follow star movement
for a while but the error in tracking steadily increases because the threaded
bolt is straight and provides a tangential motion.
Using the driven arm to raise a
second arm improves the tracking accuracy enormously, and in theory it
could be possible to get good tracking for 45 minutes.
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The
camera is attached to the second arm. The hinge position of the second arm is
carefully chosen to give optimum tracking.
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The
second arm hinge is about 5cm to the right of the main arm hinge. A spring and
bolt arrangement allows the distance to be accurately adjusted because it is
only at one particular distance that accurate tracking is
produced.
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Gravity holds the second arm against the main arm and there is a
sliding contact between the two. To prevent a sideways force acting on the
hinge a small block also supports the second arm, with PTFE between the
contacts to provide a relatively friction free motion.
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The motor is attached by 4 bolts to a perspex square. The bolt
can pivot freely so that the threaded rod is always tangential.
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Underside view of the
motor.
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'L' denotes the threaded rod. The motor pivots at the
lower end of L. The linkage to the main arm pivots at the top end of
L. The main arm is a 'U' shape, and the second arm
makes a sliding contact at the point marked by the arrow. (Diagram
not to scale)
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When the four lengths d, a, c and e are
optimised, the mathematics predicts a phenomenally low tracking error for the
first 45 minutes.
In reality this will never
be achieved. The motor will not rotate at a perfectly uniform rate, and the
distances cannot be precisely set. The arm hinges and motor pivot would also
need to be exactly in line.
Errors of only a few microns have a dramatic effect on the
tracking accuracy, but fortunately the image scale of the camera will be of the
order of 70 arcseconds per pixel at a focal length of 18mm.
The tracking accuracy over 70
minutes. |
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Tracking over 49 minutes. |
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We
cannot start from zero because that would imply that 'L' is zero and the motor
pivot and arm linkage would occupy the same place. A more realistic starting
point starts with L = 23mm |
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19th October 2010
The first
image. 30 seconds exposure with a bright Moon behind the holly tree at the
right. Camera lens at 18mm focal length.
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Cassiopeia at lower centre. M31 over
to the right. Still only 30 second exposure. |
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A 2
minute exposure. Massive Moon glow and light pollution, but the stars are
quite sharp with very little trailing. |
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The
camera at 55mm focal length. The reduced field of view diminishes the light
pollution effect. Perseus should be recognisable at the right. |
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The
bright star is Capella. If your eyesight is good you may be able to make out a
slight green blur as marked by the arrow. This is comet 103P/Hartley 2.
Clouds prevented me taking a more centred image. |
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An
enlargement. This is not the brightest comet! Distortion in the stars is
perhaps due to a coma effect rather than poor tracking, as the comet was right
at the edge of the field. |
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