THE LARGE SIZE of a 10.5 x 75mm richest-field binocular made, except the optics, by Henry Paul, 119 N. Broad St., Norwich, N. Y., is shown by comparison with the purchased 8 x 35mm binocular included in Figure 1. The big binocular has 3" objectives and weighs 7 pounds. While it magnifies only 10-1/2 diameters it has great capacity in revealing dim celestial objects.

Although this binocular has proved ideal for terrestrial use it was built mainly for astronomical observation. It is true, optical treatises point out that for astronomical use the added expense and labor represented by making and accurately mounting together two identical erecting telescopes, one for each eye, is not justified, and that the gains thus made are mainly psychological. For example, Bell, in "The Telescope," page 151, describes experiments which demonstrated that an increase of only 5 percent in magnification, if given to a monocular, was alone quite enough to bring its seeing power up to full parity with a binocular. Jacobs, in "Fundamentals of Optical Engineering," debates binoculars vs. monoculars, to the advantage of the monocular, where time and cost are prime factors. Paul concedes this for astronomical research but adds that "for the amateur it has been my experience that the case of observation and the pleasure of viewing the skies are markedly increased by the addition of the second telescope."

Hardy and Perrin, in "The Principles of Optics," page 525, point out that binocular microscopes of the non-stereoscopic type also are growing in popularity because they give increased comfort. In any case, even a scientist (Paul is a nutrition chemist), when off the lot, sometimes finds it irksome to be too scientific, especially when equipped with a fully appointed shop (as he is) and possessed of the uncured itch to make fine things (Paul has made complete telescopes all mechanically high-grade, Schmidt cameras including the optics, and acceptable roof prisms). "Although the optics of this a binocular were purchased," he writes, "the mechanics of design, cementing and the rest, presented interesting shop problems.

"The 3" objective lenses," he continues, "are 15" in focal length. The eyepieces, of 1-3/8" focal length, came in focusing mounts. Their field lenses are 1-1/4" and their eye lenses 1" in diameter. Four regular 1-1/4" right-angled prisms were cemented in a suitable holder to form Porro's second system of erecting prisms (Figure 2), not the more familiar first Porro system used in the majority of binoculars. In this second system the faces of the four prisms are all cemented, excepting the two faces from which the light enters and emerges This compact system has been used by Alvan Clark and by Zeiss in the familiar circular drum-shaped boxes between telescope and eyepiece. For a binocular, two of these prism units are required. One of them must be reversed, the mirror image of the other.

"The light absorption of the 4 to 5 inches of modern prism glass in these erectors is not serious. An ordinary binocular usually has ten air-glass surfaces, two on the objective, four on the prisms, and four on the eyepiece lenses, with total loss of about 40 percent of the light. For astronomical use on nebulae this is a serious loss, not to mention other troubles caused by the reflected stray light. The use of Porro's second system cuts the 10 air-glass surfaces to 8. In the instrument described here all these surfaces except the one facing the eye are coated with nonreflecting films, which were on them at purchase and which were painstakingly maintained. Thus light loss is cut to a negligible quantity and the binocular becomes a highly practicable and pleasing richest-field instrument, free from stray light and ideal for faint nebulae. "Adjustment of prisms and lenses in binoculars must be made with great care, else double images will result. Were the prospective maker not fully informed of this in advance he might well agree with those who claim that binoculars are not worth the effort.

"Scrap magnesium, aluminum, and duralumin were used throughout, helping to reduce the weight to 7 pounds. A similar commercial instrument weighs 18 pounds. The glass of the large lenses and prisms contributes a large percentage to the 7 pounds, and its weight cannot be reduced.

"Materials cost less than one tenth the price of a similar commercial model.

"It was my good luck that the f/5.3 objectives coupled just right with the 1-3/8" eyepieces to give an instrument with greatest useful exit pupil of 7mm (1/3") as is required in the RFT. Also, f/5 is about as high an aperture-ratio as can be used on a 3" lens without too much color error. Both objective and eyepiece were probably designed for dim light, with these limitations in mind, although not necessarily for each other. The large clear eye-distance is convenient for wearers of spectacles, who can see the entire 50 degrees apparent field (4.8 degrees actual) without inconvenience.

"It may easily be overlooked that, in prism binoculars of this or of the conventional type, ordinary crown glass should not be used for the prisms. These should be of light flint glass. The high aperture-ratio of the objectives results in a steep cone of rays that exceeds the critical angle of ordinary low-index crown glass. (Of course, the hypotenuse of a crown glass prism could be silvered, but then it would no longer be totally reflecting, as theory and experiment demonstrate, though it would have the index of reflectivity of silver, about 95 percent.) Prisms may be checked closely enough for this: purpose with a ten-cent protractor and a straight stick, by viewing a bright area such as the sky through the optical axis of the prism and noting how many degrees the eye maybe moved away from the axis in the direction of the other prism face before the critical angle is reached. This is plainly indicated by a line of coloration. This angle is reached at 5 degrees or 6 degrees for ordinary crown, while light flint: gives a reading up in the 10 degrees range.

"A stand (Figure 3) was a great convenience with this binocular because of the weight and because it is difficult to hold steady any binocular magnifying more than about eight times -especially in astronomical use.

"Camera users will recognize in the geared head of the stand the 'Gear Master.' This is very handy since one can sweep various areas of the sky without duplication, and study terrestrial panoramas conveniently.

"This big binocular has added more to my pleasure of 'star gazing' than any visual instrument I have constructed."

TO THE amateur telescope maker glass is the normal material for grinding tools, while the use of metal for such-tools seems abnormal. To the professional the reverse is the case. The key to this apparent anomaly is that, when only one job is to be done, it doesn't pay to make a metal tool even though the metal tool would be worn away less-for the wear on the glass tool will not greatly matter. An incidental dividend is the fact that the glass tool won't be so likely to scratch as a metal tool, into which abrasive particles tend to become imbedded and fixed.

Just how much less metal tools wear away than glass, also crystal quartz (note: not fused quartz) is the subject of quantitative researches made at the Bell Telephone Laboratories Record by W. L. Bond, a physicist, and described in the Bell Laboratories Record, Volume 22.

Figure 4 summarizes the findings quantitatively; it shows the relative resistance to abrasion of the six materials tested, when used against a cast iron tool. The author says: "It will be seen that the four metals have much greater resistance to abrasion than the quartz or glass, and that the resistance is of the same order as the hardness. The very soft aluminum, however, wore 2-1/2 times as well as quartz, and yet on the Moh scale (of hardness) aluminum ranks 2 while quartz ranks 7. This is explained by the toughness of aluminum and the brittleness of quartz. The abrasive particles bury themselves deeply in the aluminum without removing pieces of the surface, while the brittleness oú quartz enables the abrasive particles to break off small pieces, and thus the wearing away is faster. Glass, which is softer than quartz- and even more brittle, wears away still faster."

TWO Seattle, Washington, amateurs, Leonard Hughes, 810 E. 60th St., and R. V. Tomlinson, 8807 Roosevelt Way, made the 8" reflector shown in Figure 5. They write:

"The whole contraption was completed in less than four months of intermittent spare-time work (or should we say play?). The f/10 mirror is of Pyrex and the customary amount of grief was encountered in its manufacture. The tube consists of nine pieces of 1/4" galvanized water pipe thrust through eight cast aluminum rings of 9" inside and 11" outside diameter The thickness varies from 1/2" to 3/4"

"The declination axis consists of two aluminum castings 5" in diameter with two machined faces held together by a double roller bearing and bolt and in such a manner that a bearing surface the full diameter of the axis is obtained [italics by the editor].

"The polar axis consists of a cast aluminum cone revolving in a cast aluminum cone receiver. The cone is held in contact by the axis, which is thrust into a small Timken bearing at the lower portion of the polar assembly.

"The castings were made by the general directions in 'A.T.M.A.' The melting was done in an ordinary coal furnace with plenty of draft and coal. Best results were secured with a 20 percent mixture of foundry clay in fine sand. Trouble resulted if the molds were not dry; ant it often resulted anyway.

'There is a total of 16 major castings in the mounting and tube. All casts were made in open mold, which greatly simplified matters.

"We plan to add setting circles, motor drive, and precision focusing mount.

"The whole telescope cost only $60 plus barrels of sweat.

"After the war we plan a 20" reflector of f/7 for photographic use, also a 4" photographic apochromat and a camera to match for a patrol of novae."

RELATIVELY few telescope descriptions have been coming in of late, yet "A.T.M." and "A.T.M.A." have increased in popularity each year throughout the war. These apparent contradictions seem to say that amateur telescope makers have been busy in war work, also have been unable to obtain materials; but that they are doing a lot of reading and planning for a revival of activity post-war.

WE DISCOVER that the names of two producing members of the amateur Roof Prism Gang were omitted from the summary in last November's number. C. S. Walton, 5975 W. 44th Ave., Wheatridge, Colo., and Anton Bohm, 6815 W. 29th Ave., Denver, Colo., made 488 roof prisms.

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