Edwin F. Bailey, assistant at the Fels Planetarium in Philadelphia, made the 10-inch mirror, and the late John ("Everybody's Friend") Schaud, foreman of the optical shop at the Frankford Arsenal, made the diagonal. The mounting was designed and made by the owner, D. Robert Yarnall, of the Yarnall-Waring Company, Chestnut Hill, Pa., manufacturers of power-plant specialties. "There is nothing unusual about the design," Yarnall writes in reply to an inquiry, "except perhaps the axes, which I made especially heavy because the, amateur telescopes that I have seen and used were inadequate in this respect. The hollow shafts are made of seamless tubing. A smaller tube was slipped into and pinned to a larger tube; then the-assembly was turned to two-inch outside diameter for the smaller size, three-inch for the larger.

"The two yokes in which these shafts revolve are fabricated of 1/4- and 3/8-inch steel plate. The tube is held in a yoke of 1/4-inch steel plate, and may be rotated to turn the eyepiece to the most comfortable position for observation."

This use of steel plate for the parts of a telescope mounting is so direct a method for obtaining a neat and rugged mechanism that it perhaps merits the. special appellation "fabricated steel plate construction."

The Yarnall telescope is semiportable. When not in use it is trundled on heavy casters into a closet near the top of Yarnall's house. When it is used it is moved out of doors to a deck covered with 3/16inch steel plates. These distribute the load. Hand-operated screws on the four feet of the pedestal are run down to the steel plates after the polar axis is pointed to the North Star. Its orientation is conveniently checked by a north-south line painted on the steel plates.

Final adjustment of the polar axis to the altitude of the pole is made possible by a specially designed small-angle.

hinge between the sloping top plate of the pedestal and the bottom plate of the polar-axis yoke. The crosswise hinge pin at the top, retained in its grooves by a small bolt and spring washer, is a rod of 1/4-inch brass. A hand screw near the bottom of the yoke member serves to adjust the vertical angle of the hinge, and an intermediate pair of screws holds the angle chosen. "This has worked out very . satisfactorily," Yarnall states.

"The tube," he continues, "is designed so as to rotate easily in the supporting . yoke, and this too has worked well." Two fixed lengths of Monel metal jack chain, attached permanently to the respective ends of the wyes of the tube yoke, pass around the tube in two deeply grooved brass stiffening rings. At their other ends these chains have eyebolts with wing nuts, and these tighten against slotted faces on the opposite wyes. To rotate the tube, the wing nuts are temporarily loosened. The tube is built up of six light weight steel T-sections.

While the Yarnall telescope has a 10inch mirror, the diameter of its tube is a full 12 inches. The value of this feature has been recognized more frequently since it was embodied in Russell W. Porter's revision of the Springfield type of mounting 12 years ago. It is believed by some that its advantage will be seen if the mirror is Foucault-tested in the tube. According to amateurs in Chicago, a small tube often produces on the mirror the appearance of having a turned-up edge. This is attributed to a layer of cool, dense air adjacent to the metal.

An examination of the telescope and an analysis of its blueprint tend to lessen the initial impression that it may be hard to build. Many amateurs have access to the necessary machine tools. Yarnall, who could easily have turned the blueprint over to his plant for manufacturing, instead made nearly the whole telescope in his small shop at home. He did not, however, have adequate machinery for all the parts. "I am much too fond of doing work with my own hands," he says, "to share all this pleasure with our shop

"When the telescope was completed," he continues, "I soon found that my arms were not long enough to reach the valve handwheels on the worm drive for rotating the axes, so we attached a flexible shaft to each worm spindle and then added another handwheel on the upper end. This convenient extension makes it possible easily to operate both axes during observation.

"One capstan-operated locking screw is applied to each of the axis worm wheels for convenience in major movements of the telescope tube. To facilitate examination and dusting of the mirror a cover plate was added to the bottom of the tube. Although seldom used, it has been found a convenience.

"The telescope has given a great deal of pleasure to our family and to many neighbors in the surrounding area who had never had a chance to look at the stars."

ELLISON'S condemnation of the dry paper, dry-rouge polishing lap for telescope mirrors, contained in Amateur Telescope Making, page 868, three years ago led Father M. Daisomont of Ostend, Belgium, to send this department a printed polemic on the virtues of that ill-reputed method. From his communication the two following paragraphs are abstracted.

"Reverend Ellison argues that paper laps cannot be deformed and that they cause scratches. My mirrors show no scratches, and the claim that the paper, because of its thickness, renders the coincident curves of mirror and tool no longer the same radius by a gross amount is erroneous. Calculation shows that the difference thereby introduced is only 1/250,000-inch. The paper, pasted on the glass tool, is brushed shaggy with bristles and fits after a few strokes.

"Polishing on pitch gives good results. Polishing on paper is at least as good, but far simpler, cleaner, more manageable. Foucault made wonderful mirrors with this, his method. It can produce real gems. As for pitch, send it to the devil. It will then be in its element."

In January, 1947, this department, seeking only the facts, published a theoretical refutation of Ellison's claim that a sheet of paper throws mirror and lap out of coincidence by the amounts he indicated. The amount proved to be about the same as Father Daisomont had stated. This department then invited him to furnish instructions for the paper lap, together with a small sample of the paper he used. These were received two years ago. He wrote, "I send the exact description of how we prepare and use paper laps.

"It is essential to use paper of excellent quality, pure, without defects, unsized. We think well of duplicating paper 1/10-millimeter thick. Make flour paste of soft consistency and strain it through fine linen to avoid lumps. Clean the tool, rub on a light, uniform layer of paste. Rinse paper in water, remove excess water between blotters, lay it at once on the tool, and roll out excess liquid with hand or roller. With a knife tip remove any hard grains in the paper.

"When it is dry, cut around it with a razor blade, leaving a millimeter to fold down. With an old toothbrush rub the paper to raise a light down. With the hand put on a very little dry rouge, uniformly. Remove surplus rouge with a toothbrush. The rouge layer should be so light that the paper may easily be seen. If too thin during work, add a little rouge."

Copies of these instructions, with little samples of the Belgian paper, which a New York paper manufacturer has called ordinary mimeograph paper, have been sent from time to time in the past two years to approximately 25 individuals and groups in the U. S. Most of these declined to make the test. Some of them pointed out that in Amateur Telescope Making Ellison had already settled the matter. However, several did try the paper lap.

In July, 1947, John M. Holeman, of Richland, Wash., reported: "The paper lap is fast but gave a 'lemon-peel' finish, though not so bad a one as a felt lap. Under test my mirror, polished four different times, looked like blistered paint or ripple glass. The figure is easily controlled with paper. Turned-down edge is not so bad as with the soft pitch many use. The drag is great and a 10-minute spell of polishing takes a lot out of one. After fine emery I polished in half an hour. The lap is so fast that it is hard to figure with it.

"Making the lap conform is the chief difficulty. Despite theory, the paper's thickness or something does distort the curve and the edge polished first. This can be handled by watching for it and buffing down the lap with a suede leather wire brush-at the cost of as much work as channeling a pitch lap."

In March, 1948, J. J. Peabody and Dale Bufkin, of Elgin, Ill., reported: 'We strongly suspect that 'Daisomont' is Albert Ingalls in disguise! We battled as follows. With a six-inch f8 mirror we tried eight grades of paper from .002- to .008-inch thick. We polished 180 hours on our machine at various speeds and under pressures 0 to 25 pounds, the lap eight inches in diameter. Also tried it three hours by hand. Extreme caution is needed to avoid scratches. We tried rouge, cerium, Barnesite. In every case, gross lemon peel showed under test. On a pitch lap this vanished at once. Back on the paper lap it reappeared. From theory, you admitted Ellison's error about the fit of curves, but in practice he is right; they just don't fit. Radius of outside zone came out 12 inches longer than central zone."

W. A. Calder, professor at Agnes Scott College, Decatur, Ga., reported: "Tried three times to polish, using mimeograph paper, Scott tissue; rouge, Barnesite, dry; then with kerosene, again with turpentine. Results inconclusive. I finally broke down and put the mirror on a pitch lap, which did wonders in a few minutes."

Rudolph Moulik, of Cicero, Ill., reported that the paper lap seemed to be good when tested on two four-inch convex surfaces. However, since convex shapes cannot be tested by the Foucault method, the state of the surfaces could not have been studied. He attached the paper with thin varnish and found that it was difficult to keep good contact. He polished with Barnesite.

Shown some of these reports, Father Daisomont replied in December, 1947: "There is certainly something wrong with the work of your American friends." His claims, many of them so emphatic and extensive that there is not space to repeat them here (our Daisomont file is now an inch thick), do indeed point toward the conclusion that somewhere there is a large discrepancy; especially in view of the fact that in one of his 11 letters he mentioned that he had just polished and figured a six-inch f8.5 mirror in 10 hours on a dry paper lap and that it showed the image of Saturn very well at 350 diameters.

Therefore this department will pursue the discrepancy further. To that end it has asked Father Daisomont for a little of his rouge and enough paper for a lap, and will try to follow his instructions without the slightest deviation. (It has also sent him Garnet Fines and told him that after the* use, glass on glass, not only the large type named by Ellison in ATM, page 79, but the text type of ATM could be read through the mirror, dry, held seven inches below the eyes and seven inches above the page; also that Barnesite, likewise sent, would then polish the mirror on pitch, its sinful disposition atoned for by speed and its fragrant aroma, in two hours.)

SUDDENLY the images of the stars in a telescope on a rooftop in Washington, D. C., began to dance. They moved rhythmically for four minutes, then stopped. The puzzled telescope owner hurried downstairs. His wife had been beating eggs. Only the blurred images of the stars revealed the resulting microseisms of the house, but the telescope magnified them exactly as many times as it magnified.

Ideally a telescope at the top of a building should rest on a solid masonry pier descending well into the earth and slightly separated from contact with any other part of the structure. For most telescope owners this stiff requirement is unrealizable. In defiance of it they erect telescopes on rooftops. Often the results are satisfactory. (Ideally telescope mirrors should be made in cellar shops with uniform temperatures; but many made in evil temperature conditions prove good.)

Stress analysis of a building should reveal how much a telescope will vibrate on top of it. A far less complex approach is to lug the telescope to the roof and try it there. A collection of actual experiences with rooftop observatories may also throw valuable light on this question of telescope stability.

The following are two extremes. In the first an observatory dome was erected on top of a tall four-legged structure of heavy timbers, diagonally braced. Even then the star images danced whenever the observer batted an eye. Another person, if present, was forced almost to hold his breath. The bracing simply did not work as expected. In the second Russell Porter was asked whether a dog rapidly scratching its ear would agitate the 200-inch telescope. His reply: "Bring on your dog."

In 1932 this department published a description of a dome on top of a large two-story-and-attic frame house, with the telescope mounted directly on the attic floor. Recently the owners, George E. and John R. Pelham, of Newark, N. Y., were queried about the kind of performance it gave. The reply was, "We never had difficulty with vibration."

(A digression to describe the ingenious method of erecting this dome: First a hemispheric dome was built inside the attic, complete, unattached. Next a cylindrical dome ring, the vertical sidewall of the dome-to-be, was built beside it. Finally a hole was sawed through the sloping roof, the sawed-out portion was removed, the loose dome was pushed through this hole and shored, the prepared dome ring was lifted beneath it and the 6ap between it and the roof was sealed. Then the dome was lowered.) Many years ago the late David B. Pickering, a widely known variable-star observer and writer on astronomy, placed a dome with square base on the flat roof deck of his two-story-and-attic frame residence in East Orange, N. J. The weight of the dome and its walls, and also of the observatory flooring, was carried on the studs beneath, while the telescope pedestal was placed on the ceiling joists of the attic below. His son, James S. Pickering, author of The Stars Are Yours, replies to a query: "There was no strengthening of these members, yet there was never any interference at any time from vibration. We live in a fairly quiet street, but one of the main business streets runs about 100 yards from us with all kinds of traffic."

A Chicago architect, Richard E. Schmidt, years ago placed an eight-inch Alvan Clark refractor on his three-story-and-basement residence. "My telescope support," he replies to inquiry, "consists of two 10-inch steel I-beams extending a span of 175 feet from wall to wall, and bearing at their ends on wooden studs Their webs are 24 inches apart and the beams are enclosed full length in Portland cement concrete, 36 inches wide and 16 inches deep. This was done to gain mass. The telescope pedestal at the center of the span is also of concrete, 28 inches square at its base, 30 inches high and 24 inches square at the top. I estimate the weight of the telescope at 2,500 pounds, and of the concrete-and-steel beams at 12,000 pounds. The observatory floor and dome were supported by wooden joists, and the floor is nowhere in contact with the pedestal or the concrete-and-steel beam. William Gaertner set the instrument. He, Petitdidier, Dr Philip Fox and Professor F. R. Moulton, who visited me a number of times, found the installation free of vibration. My house is one of a long row on the sand of an ancient beach. I doubt whether the house would have been free of vibration had it been on clay soil. A factor which doubtless helped prevent vibration was the brick walls of a light shaft built at right angles to those of wood which support the main beam and act as buttress walls."

Suppliers and Organizations

The Society for Amateur Scientists
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Internet: http://www.sas.org/

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