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 Barndoor
trackers are a great way to let your camera track the sky so you
can make long, wide field photographs. The classic design is a simple
pair of hinged plates with a drive screw between them. The problem
with this design is that a constantly driven screw does not produce
constant angular motion of the platform. A number of designs have
evolved to minimize this tangent error, normally involving more
complex drive systems with multiple arms. My approach has been to
return to the more elegant original platform, and use a computer
and stepper motor to vary the drive screw rate in such a way as
to produce constant angular motion. This results in a theoretical
tracking accuracy of better than one arcsecond in over two hours.
The true accuracy is reduced by polar alignment error and various
fabrication errors, the most significant of which is periodic error
in the drive screw. With a little care in alignment, 20 minute exposures
with a 300mm lens yield no visible tracking errors.
Here are all
the technical details of the project. You are free to use any or
all of this information in any way you wish. I'm happy to answer
any questions, though I don't guarantee the world's fastest response.
Downloads:
Additional Images:
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