The richest-field telescope is a stubby, compact instrument usually used without a mounting-simply held in the arms-and it is designed to give magnificent views of the myriad Milky Way stars. No specialized type of telescope has equaled it in popularity since its descriptive data were published in "Amateur Telescope Making - Advanced" in 1937. ,

Paul says he weighed all factors and chose a 5" mirror aperture, with focal ratio 4, which hooks up just right in a 6" Micarta tube to give optimum portability, size, and so on. The 5" mirror was cut from a 6" Pyrex disk. The field of view covered is better than 2° in diameter. When the telescope is rested on the knee the eyepiece comes to just the right height for the eye.

Figure 2 shows detail of the diagonal support, which is easy to adjust. The two curves' locus is at center of the diagonal. The latter is an aluminized solid piece of Pyrex. The other sketch in Figure 2 shows how all adjustments for mirror and eyepiece are afforded: screws passing through sponge rubber.

After taking this telescope with him to the country and using it several nights, Paul writes, "The Milky Way was a bright ribbon all the way from horizon to horizon. I really got more of a thrill from the RFT than from my big telescopes. No complication: Just sit in a chair with a blanket around you and look to your heart's content."

ANOTHER satisfied "customer ' for the richest-field telescope is Charles E. Kratz, 3512 Dennlyn St., Baltimore, Md., whose 4" RFT of 16 l/2" f.1. is shown in Figure 3. "I have had a big kick from the RFT," he writes, "and was surprised to discover how much can be seen with Kratz made the hex tube of 1/4" mahogany, using hand tools, and glued it up.

The telescope sets on a home-made tripod, the head having three pieces of wood each set in so that the grain runs in the direction of the legs, with 1/8" mahogany glued on top and bottom.

The RFT has become so widely established as a telescope type since it was presented in "A.T.M.A." that the initials RFT have now become a word just as they stand, without periods.

MACHINE for grinding and polishing, shown in Figure 4 and made by Robert W. Coulter, 812 Sixteenth Street, N. E. Masillon, Ohio, "works like a charm," he states "with almost unlimited variations of stroke, and requires but little attention while operating." It is a modification of the Hindle type and was built on an old library table of heavy oak.

"Speed reduction was accomplished entirely with V-belts and pulleys. Speeds are: Drive 27; sidethrow, 5 1/2"; turntable 1 1/3"."

The machine as shown worked well on a 6" mirror but when a larger mirror was tried there proved to be whip due to the high extension of the vertical end shafts. When the shafts were shortened, after this photograph was taken, the whip no longer occurred.

ABSTRACT of a correspondence file. A Subject: Outsized mosaic tool for grinding mirror;

February 24, 1943. Coulter (the man named above) to this department. "I am contemplating a 12 1/2" short focus mirror, and would like to make a tool of small glass caster cups mounted on a full-sized circular base, its surface shaped to convex, spherical curvature roughly approximating desired sagitta for finished mirror. I propose to use one central cup surrounded by a ring of smaller cups, and this by a ring larger ones-combinations of sizes and numbers that happen to suit my size of tool. Doubt has arisen, however, whether such a tool would produce a regular sphere on the mirror, or whether zones would result. What do you think?"

Reply, March 1, 1943: "Theoretically it won't work. This tool amounts to a tool made of annular, concentric rings. Stroking should minimize the effect, but not get rid of it all. This is theory, but theory often proves wrong. Theoretically, the Germans had the English licked. So try it, if you are willing to gamble, and after making the experiment please tell us the outcome."

Side comment by Cyril G. Wate Edmonton, Alberta, to whom inquiry was shown: "What's the idea? Why not, instead, make a solid glass lap, bust it on a hydrant, and then glue it together again? In other words, why make such a lap at all? And what a job it would be to bring the irregular glass cups into contact!"

Reply by Coulter, on seeing above comment: "It's much cheaper than a solid tool, easier also to form the with a built-up tool than to work a solid tool to curve, and saves time. Built-up tool also reduces suction when fine-grinding. (While working 12 1/2" mirror a long-winded telephone interruption once led to my mirror and tool being welded together, and bad chips resulted from forcible separating. But I suppose the underlying reason for the venture is to indulge a pet passion for oversized tools. [And it's always fun to try something different. -Ed.] Moreover, I like to be able to use long strokes throughout fine grinding-it goes much quicker and a mirror has to come to a sphere, since the two surfaces are always in contact. Hence I propose an 18" tool for use with the 12 1/2" mirror."

Final report by Coulter, June 22, 1943: "Caster cup tool seems OK. Shadow test of mirror showed evenly spaced, concentric zones-a target out a bull's eye-and turned up edge. But these reduced readily with local treatment by third finger dabbed in rouge. There also wasn't so much suction as with a solid tool. But Wates was right about establishing contact on curved surface; it proved unsuccessfu1, so the tool was made flat, and it worked OK."

A READER of this department inquires: "Can a telescope mirror e made of cast aluminum? If so, where an I obtain aluminum blanks?" The answer is no, but the exact reasons make an informative discussion.

When an optical surface of glass is aluminized, the evaporated molecules, being in a high vacuum, travel, without bumping into other molecules, from the hot metal source to the mirror's cold surface and are deposited in a non-crystalline metallic film having the same degree of polish as that of the lass. As soon as air is admitted, the metallic aluminum begins to oxidize and, according to Strong (Astrophysical Journal, June 1936, pages 401-423), this oxide continues to thicken for about 60 days. It is transparent and is either corundum (AL₂O₃) or bauxite (Al₂O₃.2H₂0). Dr. J. A. Anderson states in a private communication that "the layer of aluminum, approximately 1/250,000" in thickness, is made up of 250 molecular layers. Of these," he continues, "I would guess that within the first month's exposure to air about 20 to 40 layers will have turned into aluminum oxide. The light rays pass through the transparent oxide layer and enter part way into the metal, then turn around and go back again."

The above was shown to Dr. John Strong, who commented as follows: "If thickness of one layer of aluminum is 4 angstrom units, or about 4 x 10^-4, then 250 layers are about 0.1, (or 0.2 wavelengths of green light). The oxide coat is about 100 angstrom units in thickness."

An angstrom unit is a ten billionth a meter. The Greek letter "" (mu) designates one micron, or 10,000 angstrom units A micron comes pretty close to 1/25,000 inch and a wavelength green light is roughly 1/50,000 inch. Hence the coat of aluminum, as originally laid down, is about 1/250,000" thick. After some 60 days about the outer 60/250, say 1/4, of its thickness has turned into oxide of aluminum.

The actual mirror is therefore metallic the same as a silvered mirror, but we still haven't answered the question why all this couldn't be as easily-more easily, it might seem-accomplished simply by letting a disk of plain cast aluminum oxidize in the air in the ordinary manner (the thought in the question which opens this note-a question which others have asked).

Fred B. Ferson, a Biloxi, Mississippi, amateur telescope maker who has inquired into metals and casting metals (see his chapter on molding and casting, in "A.T.M.A."), states it thus: "Aluminum is a metal which absorbs gases readily, and is hard to prevent from taking up impurities when it is cast. Also in castings it cools into crystalline structure, the crystals coarse and full of holes-possibly from absorbed gases driven off."

J. H. White, 20 Burchfield Avenue, Cranford, N. J., a metallurgist and amateur telescope maker who built his own aluminizing equipment, when asked for his comment, added: "Under the microscope the surface of an aluminum sheet, and still more an aluminum casting, shows a great many holes. These are gas holes. There also are black specks which are hard and brittle and are aluminum oxide which it has been impossible to remove from the melt. Even the best aluminum made by the Hoopes process, which has a purity of 99.983 percent, shows these spots. When the sheet is rolled the surface is smeared over and this covers up most of these defects. If a mirror were made of cast aluminum the crystalline structure of the metal, also the oxide particles, would show, and probably would fall out of the surface and leave holes."

Sometimes in popular writing (and some that isn't) it has been said that the oxide coating on an aluminized mirror is sapphire. This isn't literally true, though as a figure of speech it is a relative of the truth. The coating is aluminum oxide, Al₂0₃. If a given specimen of A1₂0₃ is a crystalline mineral it is properly corundum. Corundum has hexagonal crystals and its hardness is exceeded in nature only by that of the diamond, which, however, is very much harder. If black, due to iron impurity, the corundum is emery. It may also be gray, blue, yellow, red, brown, or colorless. If any of the last named crystals are clear and perfect- which is relatively very rare-the corundum is of gem quality. If blue, then it is sapphire; if red, ruby; if colorless, oriental white sapphire. That the coating on an aluminized mirror is protected by aluminum oxide is, therefore, the most nearly romantic (though not romantic at all) claim that can be made, for it isn't a gem and it isn't even crystalline.

Commenting on the above, Dr. Anderson notes an interesting analogy with quartz and fused quartz, the latter being the correct term for a "quartz" (often so-called) mirror: Quartz is a natural, crystalline mineral. Fused quartz is not crystalline, and neither, therefore, is a fused quartz mirror disk properly called quartz.

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