Thankfully, wide-angle imaging is much less demanding, and it can even be an easy way into astrophotography. A simple, "barn-door" type tracker is a do-it-yourself hardware-store technology project. Even a camera fixed on a tripod, with no mechanical action, can be made to "track" the stars—in a "virtual equatorial mount"—and can produce great astrophotos of constellations and sweeping celestial vistas. 

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. 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.

A “virtual equatorial” takes advantage of modern imaging processing capabilities. Many images taken in succession, each short enough to give good star images, are digitally de-rotated and combined to compensate for the Earth’s motion. This methodology is featured in Christian Buil's freeware Iris, but could be implemented in almost astroimaging processing software, even Photoshop.

Star trail photos—what would normally result from extended tripod shots—can be obtained digitally as well. Iris can do this, as can a stand-alone program Startrails. The trick here is using a function that retains the maximum value for any pixel position over the entire series of images, recording the trailing stars while the background sky stays dark. Simply adding the images digitally would raise the sky background unacceptably and destroy contrast.

Fortunately, digital cameras with suitably long exposure capability, low image noise, and generous pixel count are readily available. Indeed, amateur astrophotography is popular enough to drive market segments. And what’s next? A dedicated DSLR for astronomical imaging? Feature-laden commercial software? Stay tuned…

 Clicking on the images at right will load animated versions showing the hinge arm tracking action and a full 360° view. 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 28 August 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.