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Collimation Test

Some years back, I encountered binoculars with an alignment problem. I devised a simple, sensitive test for collimation, one that anyone can perform, since it requires no laboratory instruments. Perfect collimation means that both telescopes (the twin barrels or tubes of a binocular) are aimed in exactly the same direction; if they are not, one's eyes are placed under strain to resolve the images.

If binoculars are aimed exactly at the sun (but never look at the sun through them!), it is possible to project an image of the sun's disk on a white screen placed two or three feet (or more) behind the eyepieces. The images can be focused with the binocular's focus adjustment. (Incidentally, it is possible to observe sunspots and see an eclipse safely this way; I saw my first transit of Mercury with my binoculars!)

For the collimation test, hold the binoculars steady against a railing or table edge (or use a tripod) and point them at the sun so that the two images of the sun fall on the screen near the shadow of the binoculars. It may take a little patience to get them aimed accurately -- watch the shadow of the binoculars for a clue as you point them. This may be easier if the sun is toward the horizon. Adjust the focus for a clear image.

Now carefully aim the instrument until one of the solar images is exactly centered on the shadow of the eyepiece. If the collimation is good, the other solar image will be exactly centered on the shadow of the other eyepiece. In using imperfectly collimated glasses, my experience indicates that if the solar image is displaced by 1/4 its diameter, the defect may not be immediately apparent or even noticeable under casual or non-extended use. A displacement of one or two diameters would dissatisfy most people pretty quickly.

Although one certainly would not expect a collimation problem in purchasing a good pair of binoculars (I found a serious one in a popular name-brand product), I would not make a final commitment without doing this simple test. Furthermore, any binoculars can be knocked out of alignment if traumatized severely enough. If you do have an accident, you can tell quickly if your optics need service.