A simple mechanical sight usually takes the form of either a sighting tube or a vee and blade sight. Both of these are amenable to do-it-yourself construction. They can be mounted to the binocular with hook-and-loop (e.g., Velcro) strip (loop on the binocular, hook on the sight) or rubber bands.
Vee and blade sights were common on large naval binoculars up until the 1950s. Of all the options for binocular finders, these are probably the simplest sight to use. You can make a simple vee and blade sight with a strip of metal approximately
25 cm by 1 cm by 3 mm (10 by .5 by .125 inch). One end has a notch filed into it and the other is either filed to a point or, preferably, twisted 90 degrees about a longitudinal axis. About 5 cm (2 inches) of each end is bent at right angles in order to form a U shape. You mount the sight on the binoculars with the vee nearer the eyepiece end. You can align it with the optical axis of the binoculars merely by bending the metal strip. If you use a vee and blade you should ensure that there is no possibility of it coming into contact with your eye!
A sighting tube is merely a straight tube of metal or plastic, about 15 to 20 cm (6 to 8 inches) long with a bore of about 1-cm (.5-inch) diameter. Such a tube will have a field of 2 or 3 degrees, depending on how far in front of your eye it is. It is therefore relatively trivial to position it so that its field is approximately the same as that of the binoculars. It may be more difficult to align it with the optical axis of the binocular, but this is usually possible with a little ingenuity through trial and error. The near end should not protrude beyond the eyepiece and, for added safety, should be edged with some soft material such as foam tape or something like a soft rubber tube pulled over it to form an eye cup. Compared to a vee and blade sight, the tube is simpler to store as it can usually be slipped into the binocular case.
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