THOUSANDS OF PEOPLE ARE USING binoculars with faulty adjustments which they would quickly discover if they were equipped to test them even roughly. The two telescopes that constitute a binocular must be identical and almost perfectly parallel or they will cause severe eyestrain. The formulas in Donald H. Jacobs' Fundamentals of Optical Engineering give a tolerance for non-parallelism in 7X binoculars of only 3.75 minutes of arc for convergence, and 1.S minutes for divergence horizontally and vertically.

Binoculars that leave the factory fully adjusted or collimated often lose their parallelism because of wear in the hinge mechanism, or from the ministrations of the grown-up small boy who feels he is a pretty good mechanic but forgets, as he takes the binocular to pieces, that the factory assembler enjoyed the help of jigs and instrumental aids.

The approximations that result from home assembly are usually tolerated only because most people use a binocular for just a few moments at a time. It would be possible to rate binoculars in terms of minutes. A badly collimated binocular will cause a headache in one minute or less. A "five minute" binocular is one that allows its user 299 seconds B.H. (before headache). A perfectly collimated binocular even if used all day should cause no more headache than your spectacles.

Felix Luck of 651 Lincoln Ave. Orange, N. J., has devised a practical collimator that should improve the performance of many binoculars. It is not perfect. Ideal equipment would be much too troublesome to build for less than a dozen jobs of collimation. The practical advantage of Luck's compromise, which costs less than a dollar for materials, is that it is so easy to build that it will actually be built and used. Though it is made of wood with no more complex tools than a saw, hammer and square, it must be built precisely, with meticulous attention to its rigid geometrical control. Luck describes it thus:

"In my frequent travels around the nation I have been impressed with the large number of tyros who are building or have built binoculars from war-surplus materials and who acutely need a simple means of obtaining a reasonably close collimation. An exact collimation is a laboratory job. The method to be described, if followed with reasonable care, will result in a binocular that can be used for long periods without the eyestrain and discomfort associated with a poorly collimated instrument or even with one that is collimated well enough to appear good for the first several minutes of use. Most of the commonly used methods are simply the 'look, blink, look' kind without accessory apparatus. Unless the user enjoys a natural feel for this work, it often results in a poorly collimated glass, many cuss words and much loss of time. The quick method to be described should give more accurate results than any of these expedients, and give them in minutes instead of days.

"The present article does not deal with the assembly of a binocular. It is assumed that the halves are alike optically and mechanically and that the axes of the tubes and the hinge are parallel. A fairly good test for mechanical parallelism is to stand the binocular on end on a surface plate or plate glass at both the widest and the smallest eyepiece separation. In each position both of the objective tubes should rest squarely on the plate without rocking. This test assumes that the ends of the tubes are square with the sides and axes. The halves are almost sure to be alike if matched optics and metal parts have been purchased and carefully assembled, but great care should be taken to be sure of their sameness if random optics and parts are used. No amount of careful collimation will prevent eyestrain if, for example, the sizes of the images produced by the halves are unequal.

"The first requirement is a source of parallel light rays. The simplest and cheapest is the sun. Next is a projection lens, a simple plano-convex or double-convex of 4 1/2-inch diameter and 10- or 12-inch focal length: the lens from a typical large reading glass. It must be wide enough to cover the eyepieces of the binoculars at their greatest separation. If it is wider, the unused width should be masked off to cut down on heat, since the lens also acts as a burning glass. The focal length will determine the diameter of the image, but this is not critical. Nor will chips on the lens make trouble.

"This lens, covered with a mask with curved slots corresponding in their lower halves to different positions of the binocular hinge, and left open in their upper halves to permit the passage of the solar rays, is mounted in a circular recess in a board. The binocular is attached between this lens and the sun. When the binocular is correctly collimated at all positions of the hinge, the two solar images made by it will be refracted by the lens and will coincide as a single image on the screen at the rear, while the smaller image made by rays passing directly through the upper halves of the openings in the mask instead of through the binoculars and the lens will be concentric with the image from the binocular. The openings in the mask are curved, as shown in the illustration, and concentric with the center of the lens to permit testing the binocular with its hinge at different angles between the bottom of the openings and the horizontal. Their upper parts that admit the sun's rays to the lens are equally curved to insure that the rays will all pass through the same zone of the lens and all be subject to equal spherical and chromatic aberrations, an important consideration when using a single lens for projection. This symmetry automatically eliminates the aberrations from consideration.

"It makes no difference whether the light passes through the whole lens, part of it, or several areas simultaneously, a complete image and only one image will be formed, provided the target is placed at the principal focus of the lens. Theoretically, spherical and chromatic aberrations will create a number of overlapping images, but in practice this effect is negligible.

"When the eyepieces of the binocular are properly focused, the parallel rays of light entering the objective lenses form an image at the focus and pass out through the eye lenses again parallel. But if the optical axes of the binocular halves are not parallel, the two images of the sun formed by them will not coincide, because the bundles of rays leaving the eyepieces will not be parallel with one another and after passing through the large lens will form images at different points on the target. The trick is therefore to bring these two images into coincidence. This may be accomplished in most binoculars by adjusting the eccentric rings around the objective lenses.

"Professional binocular servicemen position the hinge so that its axis coincides with that of the large lens, then bring the halves of the binocular into coincidence. This is done so that collimation will be effected for all interpupillary settings of the hinge. With our simple collimator this would be a complicated process. Therefore an approximation that has produced satisfactory results is recommended.

"The target should be set permanently at the principal focus of the large lens in sunlight and must be adjusted rigidly square with the baseboard. The lens board too should square with the baseboard in both directions and should hold the lens in a similar position on the side toward the target. The support for the binocular should hold its hinge rigidly perpendicular to the face of the lens in all planes, yet permit changing the interpupillary setting without disturbing the position of the hinge. I have found empirically that an interpupillary setting of 63 millimeters, a mean distance for men and women, will serve for settings three or four millimeters on either side. If that setting 51 is used the binoculars will not be too far wrong for most other settings, thus avoiding the necessity of moving the hinge. Yet the less simple approach of collimation at a number of hinge settings is preferable if only as a check.

"At the center of gravity of my gadget I put a nut in the baseboard to fit a standard camera tripod screw. Thus, by using a pan head on the tripod I have an equatorial mounting which facilitates pointing the gadget at the sun as the sun moves. This really helps.

"To line up the gadget, move the target until the diameter of the solar image is single and a minimum. It is well to cover either hole in the mask separately, examining the image with a low-power magnifier to note whether it is circular and whether it has a flare on one side caused by lack of squareness in the position of the target or the lens. Clamp the binocular hinge to its support, point the gadget at the sun and observe the images. There will probably be three, one small image from the lens and two larger ones from the binocular. If there are but two and these are concentric, the binocular is collimated for the one-hinge setting; such luck is uncommon

"A short cut to collimation, even though illogical in theory, is to move the binocular so that the magnified image nearest the center of the target is concentric with the small image from the lens, which must be at the center of the target. Clamp the binocular and cover first one half, then the other, to be sure which half is forming which image, since they may be crossed. Then move the eccentric until all three images are concentric and those from the binoculars coincide and are at the center of the target.

"This provides collimation for a single setting of the hinge, but it should be repeated at other settings. If the lack of coincidence at other settings is not greater than 1/25 the diameter of the magnified image (for binoculars of eight power or less) all is well. If, however, more than a slight movement of the binocular hinge out of square is required, do not move the binocular hinge but, when the small image is exactly at the center of the target, adjust the eccentric rings of both halves to bring the large images concentric with the small and into coincidence with each other.

"It will still be necessary to try other interpupillary settings, as there is no certainty that the hinge is truly in the line of sight even though care was used in setting up the gadget. If the lack of coincidence is greater than 1/25 the diameter of one of the large images, the hinge alignment is too far out of the line of sight. (It was assumed only that the hinge axis is parallel with the axis of the tube, a part of the construction of the binocular.)

"Be sure that the small image is centered on the target before moving the eccentric rings. The center of the target should be marked with a cross. Each time after bringing the large image to its proper place in relation to the small image or to the other large image, repoint the gadget to the sun. The sun's motion will probably not be as large as that due to moving the rings and is of little importance. The large image moves faster than the small one when the gadget is moved and when the sun moves, in proportion to the magnification of the binocular. Disregard the color fringes on the images. If the images from the halves are not equal in diameter the halves of the binocular are not alike."

Luck's gadget would have been appreciated some years ago by the conductor of this department, who is still cross-eyed in one eye and wall-eyed in the other from attempting the method of looking through the binoculars while collimating. On that occasion the parts were finally bundled off to William Waldeyer of 2701C McAllister St., San Francisco, Calif., an amateur telescope maker and professional binocular serviceman, and came back so well assembled and collimated that no eyestrain resulted from prolonged observation at a bathing beach.

During World War II Dr. G. Dallas Hanna, a paleontologist on the staff of the California Academy of Sciences who has an interest in fine mechanism, headed a group of amateurs making roof prisms as a part of this magazine's amateur roof-prism program. He was soon discovered by the U. S. Navy which began bringing him damaged fire-control instruments, and later binoculars. Hanna directed 50 employees reconditioning binoculars in the museum of the Academy, and Waldeyer was the foreman of this force, which reconditioned 6,000 binoculars. Hanna, who became an expert, says that Luck's method of collimating is "perfectly sound and practical."

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