Some readers experienced difficulty in visualizing the plan from a verbal description, such as just given, hence the drawing in Figure 2 was made. You are looking: at the hollow declination axis in the center, which extends to the left from the large framework of the tube (on the right); and at the left you are gazing down the deep well afforded by the interior of one of the two big yokes: couder mirror is at the right, diagonal and an astronomer at left-and the bird cage made by the spokes is near the center. It hardly seems possible that any one will adequately grasp the hugeness of this telescope until on, in and over it a few times; it is like trying to imagine a billion dollars (or, for most amateurs, a mere million! Or why stop there?).

HAVE you a wife? If so, is she a "Telescope Widow"? No doubt she is. No doubt you often become absorbed in practical or theoretical optics and forget to come to your meals, go to bed or perhaps, thinking of ways to solve problems of obstinate mirrors, you start to change your clothes for a meal and find you have undressed and got into bed at 6 P.M., as this writer once did. Telescope Widow Number One lives in Pasadena, California, and she has a sense of humor. She used to live in Vermont and while there she coined that accurately descriptive term for the lot of us, abbreviated as "T.N.," meaning "Telescope Nut." 'Tis said that the clock building amateurs call themselves "Clock Maniacs," and in aviation certain enthusiasts for the lighter-than-air kind are known as "Balloonatics." Well-anyway-here is an item from the Pasadena Post, describing the Flower show of the Pasadena Flower Show Association, which "T.W." No. 1 sent to us; show it to your wife if you own one-she will understand. "For awhile yesterday Association officials had a mystery," the clipping runs. "A woman at the gate requested they inquire at the police station if her husband had been injured in an automobile accident. He had gone to park the car and she had waited an hour and a half for him. Inquiry failed to disclose the whereabouts of her husband, Russell W. Porter. She decided to wait another half hour. At the expiration of this period, Mrs. Porter went looking for the car and discovered him in it, reading a book, having forgotten about the flower show."

The only mystery now remaining is the title of that book. Was it Conrady's optics, "A.T.M.," or one of those old "Pluck and Luck's" we used to read in the days before last year's mail order catalogs were customarily kept where we used to read "Pluck and Luck"? Verily, optics provides a wonderful alibi for the addict.

CORNING is now making 20" Pyrex disks of two kinds-solid ones and 20" replicas of the 200" disk, the latter naturally costing more than the former (harder to make). A number of amateurs have indicated their hopes of obtaining disks of this size; we know of one, for example, whose wife, by the time you read this, will have given him one for Christmas (wives please note). It is no longer correct form to raise the eyebrows more than part way when a telescope maker tackles a job of that size; in fact, who dares set the limit for the amateur? There is some added odor of romance or glamor surrounding the ribbed or non-solid disks, since they are copies of the big 200" disk, and there would be even more odor thereof if some amateur would go the whole hawg and build a tenth-scale replica of the 200" telescope, mounting and everything. So far as we know, nobody has yet started to do this.

C. R. TINSLEY. 3017 Wheeler Street, Berkeley, California, has been working one of the 20" replicas of the 200"-ribbed type-and here is his account of the job:

"To us, Mr. O. A. Gage of the Corning Glass Works had written: 'Your disk is the first one that has been made and sold commercially.' It was a proud day for us and our staff as we gathered around the packing case and the beautiful thing was exposed to view-a perfect 20" replica of the great 200" disk now being ground at Pasadena. With its pearly, alabaster, semi-translucent ribs, it looked for all the world like a gigantic wedding cake (Figure 3).

"The virgin disk had a 3-1/2" hole through the center and, due to its having been cast in a ceramic mold, had a slightly pebbled, frosty surface. The working surface increased in thickness toward the center in such a way that the concave, finished face should be of equal thickness throughout. The glass weighed 45 pounds and it would have bee entirely feasible to grind and polish it by hand in the usual amateur manner. The writer, however, believes that nothing can be done by hand which cannot be done better by machinery (yes, even parabolizing!

"The first job was to grind both faces flat. This was done on a revolving, cast iron turntable (Figure 4), 24" in diameter and run at 300 r.p.m. This apparatus is driven by a 1/3 H.P. motor. The disk was laid on the table between guides which held it somewhat off center. A location was found in which the mirror automatically revolved as the turntable passed under it, and we could stand back and enjoy the job. No. 80 Crystolon was used, and the time consumed was about two hours. The back was finished with No. 240 Alundum. The machine which was to be used for grinding and polishing required that the disk be ground truly circular and have smooth edges. This was accomplished by fitting a wooden plug in the central hole (Figure 4), through which a shaft was fitted and the whole mounted on edge on crude bearings above the turntable. Arrangements were made to lower it gradually as the edging proceeded. Again No. 80 did the job; followed fine grinding with No. 240. Time, about 1 hour.

"The machine (Figure 5) used for grinding and polishing has been a development of several years' experience. There is no mechanism for revolving the mirror, other than the drag of the turntable below. All motions of the machine are adjustable as to speed and the table, in this case, was set at 1.5 r.p.m. The mirror sets freely on the table and is pushed first from one side and then from the other. For the present job this was done at 20 stroke cycles per minute. The pushers are low down near the working surface and, unless weights are used, there is no force that could affect the shape of the glass other than gravity. During the pushing movements, the mirror does not revolve with the table but at the change of direction there is a momentary freedom of movement, at which time the mirror revolves by about 1/4" of its circumference. The pushers are hinged so that they follow the glass as it assumes a rocking motion on the convex tool. The motion is imparted to the pushers through an upright walking-beam leading to the mechanism below. The motor used is 1/3 HP (I/4 HP for smaller glasses). The pulleys and belts are V type. Length of stroke is governed through an adjustable crank.

"The tool used was 1-1/4" plate glass and the abrasive, during rough grinding, was 13 fed through the hole in the mirror. Except to note progress, it was unnecessary to remove the mirror from the tool at any time. No. 36 Crystolon was used and, as the glass sludge accumulated, an overdose of water caused perfect purging without washing out the abrasive. Pyrex, ground on plate glass, practically never grabs, though we used care not to let the abrasive become too thin. The face thickness, though ample, was not sufficient to permit wearing away any of the edge. We were therefore compelled to use long strokes and hog out the center first, spreading the curvature toward the edge later with shorter strokes.

"Just before rough grinding was finished, a 3-1/2" disk was cut from a 4-1/4" Pyrex port hole and this disk was ground to approximately the same concavity as the mirror. The tool was cleaned and oiled (to prevent plaster from sticking) and, with the mirror and plug resting on the tool, plaster of Paris was filled in behind the plug. After a very few strokes, mirror and plug coincided. Thereafter it was necessary to lift the mirror when new abrasive was applied.

"When the focal length seemed correct, the whole was brought to a sphere by using 30 pounds of extra weight on top of the mirror. This brings up a matter which is understood by too few. Concaving action is a function of gravity alone, while extra weight merely causes more wear equally distributed over the entire surface. While a one-third stroke will theoretically produce a sphere, weight alone will do a better job. If the weight is large, in proportion to the tipping due to gravity, the glass will come to a sphere almost regardless of length of stroke. Unfortunately, this principle cannot be applied so readily to a plastic polishing tool, but for solids it works perfectly.

"The grinding had consumed 102 hours, including time out for measurements, adjustments, etc. The fine grinding, including the 600, had all been done with thirtv pounds' extra weight on top of the glass. We believe it due to the use of this weight that on the first Ronchi test, at the end of 500 strokes, the entire mirror gave a bright reflection, and at no time was it possible to say that any part was polishing faster than another. Nor was the mirror ever out of control from start to finish, the final parabolization being the farthest departure {rom the sphere.

"We anticipated considerable difficulty in forming our pitch tool, and probably would not have been disappointed had we followed established procedure. The total concavity was 5/16" and the sides of the tool were as steep as a toboggan slide. We are always ready to try anything at least once and so on this occasion we indulged in radicalism de luxe. The tool was turned bottom side up, with its flat surface exposed, and carefully leveled. A paper fence was tied around it in the usual manner. Then the surface of the tool and the paper fence were liberally smeared with glycerine. In melting the pitch for the tool, care was taken to pour while yet a small solid remained unmelted. This insured the pitch being near the solidifying temperature. When we were ready to pour, it was noticed that surface tension had caused the glycerine to recede from several spots on the tool, leaving its surface dry. A few strokes with the finger cured this and the pitch was poured at once. As soon as the 3/16" sheet could hold its shape at the edge, the paper fence was removed and a long spatula carefully slipped around between the soft pitch and the flat tool face. The pitch slab was quickly slid off on a broad, flat board and the tool turned face up after wiping off the glycerine. The tool was then applied face down to the top side of the pitch slab but not allowed to press. The whole combination was then turned right side up and the board removed. The pitch slab at once settled down on the tool and was wiped free from glycerine on its top side. The HCF was laid on top, the rouged mirror was added, 150 pounds' weight was applied over all and, in exactly one hour from the start of operations, polishing began.

"It will be objected that in the two transfers of the pitch distortions in thickness were bound to be set up; also that adding 5/16" to the top of the tool would offer a different radius of curvature than that of the tool or mirror. The only answer I can make is that, in practice, the darned thing worked perfectly. I have since made a 9" lap by the same method and shall probably use it exclusively in future.

"Pressing presented some problems. The tool, originally l-1/4" plate, was now barely 7/8" thick and found subject to flexure; hence no cold pressing could be used. Upon heating the tool, the curvature was so steep that the pitch sagged in the middle and formed a flat top before pressure could be applied. Finally, the HCF was stripped off and the mirror smeared with turpentine The mirror was then worked on the bare pitch, as in grinding, the turps being wiped off and renewed as fast as it began to become tacky. When the pitch exactly fitted the mirror at all points, new HCF was applied, and the results exceeded any previous experience in pressing. This process was used three times during polishing operations. At no time during the job did the curvature depart from the sphere by an amount nearly so great as the final parabolization. The HCF was never scarified but hosed clean from time to time. Scarification merely prevents the rouge from collecting at undesirable points and, if the lap is cleaned frequently, is not needed.

"I would like to offer a word about turned down edge. Since learning the importance of keeping the edge of the polisher wet, we have scarcely seen such a thing. We believe that 95 percent of all turned down edges-at least, those not inherited from fine grinding-are caused by dry-edged polishers. Consider for a moment that all polishing operations tend gradually to deepen the center of the curve. A dry edge of polisher absolutely ceases to polish. The result is that the part of the polisher that is wet continues to deepen the mirror, while the dry edge of the glass does not change its curvature. This leaves a shelf at the edge the mirror, which is turned down with reference to the deeper center. This does not apply to the outer 1/32". That same slight amount appears at the edge of a Cassegrain plug and is not worth considering. Finish your mirror with as little bevel as possible, and bevel it 1/32" when done. Our replica will have 20-1/2" clear aperture after such beveling operation.

"Parabolization was effected by trimming out the standard rose tool and continuing the machine operation at 1/3 stroke. Results could not be improved by hand.

"Total time for polishing, 73 hours; for parabolization 3 hours."

WHY isn't the present an auspicious time to start planning to make a spectrohelioscope? The sun is now about half way between a maximum and minimum of sunspot and other activity. In two or three years this activity will be very great, and by 1939 or 1940 will be at a maximum-the more active of the pairs of maxima.

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