The “barn-door” refers to the hinged arms that open (or close) at the sidereal rate of 15.04° per hour  Originally developed by Scotsman G. Y. Haig in the early 1970’s and known variously as a Scotch or Haig mount, there are many variants and claims of invention. I built a heavy-duty version from wood scraps and a wide hinge chosen to have minimal play. The hinge is set to the local latitude by an angled cut in a 4x4 post, and the only critical dimension of the arm is the 11.42 inch (29.0 cm) hinge to knife-edge pivot distance for the ¼-20 screwbolt driven by a 1-rpm timing motor.
 

These images show a 360° view of the Haig mount.

Left: Animation showing the tracking action—westward as the arms close—over about 20 minutes. Center: a close-up of the hinge and polar alignment finder (thumbscrew adjustable ).  Right:The nylon wing nut "knife edge" in a slot routed in the bottom of the upper arm.

... Under the stars, I level the mount, eyeball the hinge on Polaris with a finder scope, and the mount tracks sufficiently well for exposures of several minutes in length. While the pivot-knife edge (a nylon wing nut in a slot routed in the wood arm) and screw does not exactly provide a constant angular rate, the mount works amazing well for such a simple and roughly built device...

Left: Sum of 7, 30-s images. (f5.6, fl = 55 mm, ISO 1600) obtained with Digital Rebel tracked using Haig mount. Above: Single 30-s exposure without tracking shows obvious star trails.

For detailed plans of the many variations of this remarkable device, just do a web search for “barn door tracker.”  Or consult these books for more information: “Astrophotography” (2003, Firefly), pp. 104-107 by H.J.P. Arnold, and “Introduction to Digital Astrophotography” (2005, Willmann-Bell), Chapter 12.9 by Robert Reeves. “Wide-Field Astrophotography” (2000, Willmann-Bell), also by Robert Reeves, has additional useful information: see Chapter 1.13 for barn door tracker details.

 Clicking on the images at right will load theprinted,  full text version of this issue. Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 04 September 2005. Text and images copyright © 2005 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.