A barn door tracker, also known as a Haig or Scotch mount, is a device used to cancel out the diurnal motion of the Earth for the observation or photography of astronomical objects. It is a simple alternative to attaching a camera to a motorized equatorial mount.[1]
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The barn door tracker was created by George Haig. His plans were first published in Sky & Telescope magazine in April 1975. Modified versions of the tracker were published in the magazine's February 1988 and June 2007 editions.
A simple single-arm barn door tracker can be made by attaching two pieces of wood together with a hinge. A camera is mounted on the top board, usually with some sort of ball joint to allow the camera to be pointed in any direction. The hinge is aligned with a celestial pole and the boards are then driven apart (or together) at a constant rate, usually by turning a threaded rod or bolt. This is called a tangent drive.
This type of mount is good for approximately 5–10 minutes before tracking errors become evident when using a 50 mm lens. This is due to the tangent error. That length of time can be increased to about 20 minutes when using an isosceles mount.
A curved drive bolt in lieu of either a straight tangent or isosceles mount will greatly extend the useful tracking time.
These designs were further improved upon by Dave Trott, whose designs were published in the February 1988 issue of Sky & Telescope. By using a second arm to drive the camera platform - thus adding complexity to the fabrication - tracking accuracy was greatly increased, and can lead to exposure times of up to one hour. The most accurate of these designs is the Type-4.
A modified double arm design minimizes tangent error by raising the point of rotation of the arm on which the camera is mounted. This has the effect of tilting the arc traced by the camera arm backwards causing it to follow a better path.
A basic geometrical analysis of the tangent error shows that it can be fully compensated for by inserting a specially shaped piece between the threaded rod and the top board. Such solution was already known for a long time before the original G. Haig publication.[2]
The most basic of these designs are manually operated, although some have added electric motors to automate and improve the accuracy of the tracking process.