Barn door tracker

A barn door tracker—also called a Haig mount or Scotch mount—counteracts Earth’s diurnal motion so that astronomical objects can be observed or photographed. It offers a straightforward alternative to using a camera on a motorized equatorial mount.

History

George Haig invented the barn door tracker, with his plans first appearing in the April 1975 issue of Sky & Telescope. Modified versions followed in the magazine’s February 1988 and June 2007 editions. During late 2002 and early 2003, NASA astronaut Don Pettit, a member of International Space Station Expedition 6, built a barn door tracker from spare parts collected on the station. This allowed him to capture sharper, high-resolution images of city lights at night from the ISS.

Mount design alternatives

A simple single-arm barn door tracker can be made by joining two pieces of wood with a hinge. A camera, often attached via a ball joint for flexible aiming, sits on the top board. The hinge aligns with a celestial pole, and the boards are driven apart or together at a constant rate—typically by turning a threaded rod or bolt—in what is called a tangent drive.

With a 50 mm lens, this setup works well for about 5 to 10 minutes before tracking errors become noticeable due to tangent error. Using an isosceles mount extends that time to roughly 20 minutes. A curved drive bolt, instead of a straight tangent or isosceles mount, greatly increases useful tracking time.

Dave Trott improved these designs, publishing his work in the February 1988 issue of Sky & Telescope. By adding a second arm to drive the camera platform—making construction slightly more complex—tracking accuracy improves significantly, enabling exposures up to one hour. The most accurate of these is the Type-4 design. A modified double-arm version reduces tangent error by raising the rotation point of the camera arm, tilting the arc it traces backward to follow a better path.

Basic geometric analysis shows that tangent error can be fully corrected by inserting a specially shaped piece between the threaded rod and the top board—a solution known long before Haig’s original publication. Most of these designs are manually operated, though an optional electric or stepper motor (considered in Trott’s original design) automates tracking, improves accuracy, and reduces platform vibrations from manual advancement.