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by Albert G. Ingalls
September, 1935

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"THE fork is weak, and I have learned what tremors are." This is G. O. Bjordal, Box 111, Askim, Ostfold, Norway, and he made his first telescope from the instructions in Krudy-Brunn's "Das Moderne Spiegelteleskop in der Astronomie." This German instruction book lacks the typical German thoroughness, telling how to make a mirror but not telling how to use r²/R "I ask every new beginning amateur," Bjordal now urges, "to make the fork in the mounting so rigid that it can carry a big dog without any bending." Bjordal discovered that his first mirror was very much over-corrected, but on getting hold of a copy of "Amateur Telescope Making,' made not in Germany but in the U. S. A., he says he made a new and fine mirror. He uses his telescope throughout the long winter nights of Norway-15 to 20 hours.

IN contrast with the mounting whose weakness its owner points out, look at one by H. I. Linn, 2737 Humboldt Ave., Oakland, Calif., made for a 6" mirror. "There is no machine work on it whatever," Linn writes, "except to drill and tap for a set-screw in the unions, to hold them in position. There are two types of unions-gasket and ground, and the ground joint should be used, as the other has side play." The stocky saddle of this mounting is a steam-pipe saddle. The pipe fittings are all standard 3-1/2 inch: for declination axis, one cap, one long nip, one union, one T, one butt nip (inside), one pipe saddle; for polar axis, one union, one butt nip (inside), one 45o° el; for post, one nip, one flange. (Add wedge under flange to correct for latitude.) Here is a mounting that will not shiver, for it has plenty of metal at the neck-that place on the declination-axis shaft, between the tube and polar-axis shaft, where so many declination axes are thin swan's necks instead of bull necks. Linn's is one of the best-cleanest and steadiest-small mountings we have seen.

IN "A.T.M.," page 375, there is a photograph of a 6-inch, made by SV. F. Sprengnether, Jr. and its "neat, finished workmanship" is mentioned in the legend beneath the picture. The same Sprengnether now sends us a photograph of another job he has done, and this again looks like professional instrument maker's work. He doesn't say anything about it, but the picture speaks for itself.

HOGGING out the concavity on the average telescope mirror is too brief a job-only a few hours-to warrant setting special equipment for short cuts, but on larger jobs, or on small mirrors having very deep curves, any practicable short cuts are worth looking into. It took Harold Lower of San Diego, plus a grinding machine, 98 hours, and used up all the Carbo west of the Rocky Mountains (25 pounds), to rough out the deep curve on his f/1 Schmidt mirror-the "Soup Bowl" shown on page 295 of the June number. This curve is over an inch in depth. (Incidentally, Lower says such a curve involves literally figuring with Carbo-so closely must it be worked before polishing is even begun.)

Just after Lower had done all that work, something new and better turned up- Borium. Here is the dope on Borium: The Stoody Company, it seems, located in Whittier, California, supplies Borium lathe tools, and with these you turn your curve in the glass just as you would turn a disk of metal. A piece of Borium 1/2" x 5/8" is used, and the glass is pitched to the face plate. Note picture on p. 156, sent by Dr. H. Page Bailey of Riverside, California. Concerning it he writes, "It is a revelation to see the glass scrape off-just like scraping ice with a steel tool. The tool I used on two 10" Pyrex disks doesn't even show any wear, though it is slightly chipped." He roughed out two 10" Pyrex mirrors to a deep curvature in eight hours-12 times the speed of Carbo work.

"In my opinion," Lower comments. "Borium is the greatest advance in the art since the invention of Carborundum." Bailey sent us a Borium point-heavy metal, said by someone else to be borium carbide. It proved easy to write on Pyrex with a sharp piece of it-sign all your mirrors. And the stuff is cheap, at that. Note the shaved "ice" in Bailey's photo, in a snowbank beneath the tool. The metal is shattered in a vise, a selected point is brazed, welded or silver-soldered into a saw cut in the end of a drill rod, and dressed to a broad angle on a Carborundum stone.

HERE is another way of roughing out-a machine which J. H. Hindle of England invented and has been using on a 30-inch he is now making. The picture is almost self-explanatory: rotating horizontal metal mandrel or "torpedo;" slowly rotating chuck for disk on vertical shaft, rope pulley, at bottom; capstan (note handles) for jacking it gradually upward.

"I find this machine works excellently in practice," Hindle writes, "the concavity of the disk retains the water and grinding material, and a great advantage is that, if the disk is first ground on the back, this machine finishes it equal thickness all over. But do not," he continues, "run away with the idea that this machine will finish-grind it; the fine grinding has to be done in the ordinary way." Mr. Hindle, after making the 20-inch shown on page 453 of "A.T.M.," has now tackled a 30-inch job. Fine grinding is being done on another machine of his invention (to be shown later and figuring is done face up with small hand tools, while the mirror is resting on the 18-point flotation system for the telescope.

HERE is an idea for those who like to organize organizations: Let's get up a sort of female relief auxiliary to this hobby, not for the purpose of encouraging "female" telescopes, but to assist us noble male telescope makers. For example, the ladies could spoon feed us while we grind, as Caroline Herschel did her brother, sing to us, even kiss us, and make lots of other noble sacrifices to science. Mrs. Scribe was the first to be invited to join this inspired, humanitarian movement, but ungratefully countered with the proposal to organize, instead, and on a militant suffrage basis at that, "The American Association of Optics Widows." Plank No. 1 in the platform of her organization would be for these suffragettes to fill up all cellars with earth, so that husbands, coming up for air, could not get back to their subterranean optical shops.

"I thought," writes one TN, "that I was the only one who had any differences of opinion with his better half as to the value of telescope building. Mine declares she is a 'telescope widow' and that I don't love her any more." Other wives seem to object unreasonably to finding rouge, not alone on shirts, but on table and bed linen, and pitch on the furniture and rugs.

Mrs. Scribe now threatens to organize all these uncoÜrdinated rumblings into a vast feminist movement which will put all TNs back into the exact rather than the approximate, buzzms of their families. This forces us to reveal our real, original, secret motive in starting this whole telescope making movement, nine years ago. It was not to aid science but to keep 10,000 of you fellows at home and far, far from the old Demon Rum and ultimate Hell-fire. This will give your scribe something to claim some day, when standing before St. Peter, to offset some other things that need offsetting. In the meantime, truly appreciative wives really should send him fudge, for at least keeping their husbands away from platinum blondes, even if they do dive down cellar for most of the night.

IT gets hot in Atlanta. J. J. Stoy, 501 City Hall, Atlanta, Ga., says so, but since this question is not in itself an essential part of our argument, we simply refer it to the Atlanta Chamber of Commerce, with recommendations of mercy to Stoy, who surely meant well. The point is that Stoy has been using, and liking, a new kind of tropical (this gets worse) lap. He says, "The Barbour Asphalt Co., 30 Arch St.. Philadelphia, Pa., processes a pure Trinidad bitumen, designated as 'G-O,' that works well in a hot climate when treated with a half teaspoonful of turps to the quarter inch thick lap for 6" mirror and about 8 percent by volume (not weight) of beeswax. It comes in 5-pound pails. Even after use of 20 pounds weight for cold pressing for 24 hours, with. temperatures ranging between 75° and 85°, no material slumping is visible. After 12 hours of polishing, the channels only require further deepening once, and it does not scratch, as would be expected. Figure shapes up beautifully and normally with 30 minutes' work, then 15 minutes' cold pressing with 25 pounds weight. Mirror always works smoothly. Variations of as much as 15° do not seem to bother the figure in any way."

Stoy's letter was sent to Harold Lower in San Diego, for an opinion from southern latitudes, and his reply was that it never gets that hot in San Diego, and that San Diego, by golly, is not in the tropics! Well- try, it out, somebody.

STOY had another good idea: To forestall bubbles in pitch laps, melt the pitch in a pot having the emphasis on area and not on depth. Deep pots, and deep pitch in them, trap the bubbles. We tried this out and it worked fine. This subject introduces a letter from Edward P. Goodel 5528 Wayne Ave., Germantown, Pa., who says that memories of old days when, as a boy, he dipped bayberry candles came back to him, so he dips or dunks his laps. "Why not dip the tool?" he writes, and then goes on to say: "I spread a little turpentine over its face and, grasping it by its handle, carefully lower it into the pitch until the face is half the tool's thickness below the surface. I then raise it and, holding its face in a vertical position, slowly twist it back and forth for a few seconds to prevent dropping, and then lay it face up, on the table. If, after forming the surface with a soapy mirror, I find that the pitch coating is too thin, it is a simple matter to repeat the process to obtain the correct thickness. I use Pierce's method of making channels, pressing them into the surface by means of a soaped steel scale. The result is the smoothest lap I've ever laid eyes on, absolutely free from bubbles and from the chipped-out places which I so often got when trying to cut my channels. Incidentally, this method makes it an easy matter to form laps on tools without raising them far above room temperature, so that polishing may begin only half an hour after dipping.

Joseph A. McCarroll, 521 Palisade Ave., Teaneck, N. J., the coal-tar pitch mann (who, by the way, will try to handle orclers for coal-tar pitch with different, known melting points for controllable hardness) says he makes his channels thus: A thin stick is shaved to the shape of a crude knife, wetted, pressed in and tipped to either side. After a little practice with the tipping technique the facets made are not irregularly wrinkled but round up clear across the bulge on one side meeting the bulge on the other, blending with it, and giving the lap an effect like a pan of round baking-powder biscuits just out of the oven. No need to press these down: just go to work on the apices of the biscuits and they will gradually come down as you work.

EVERY little while someone discovers independently that if the knife-edge in the shadow test is brought in from the other direction, the pinhole remaining as before, the shadows will be reversed-thus a paraboloidal shadow becomes an oblate. This idea was first reported to us by the late J. C. Critchett of Julian, Calif., in October, 1932, and its use may help in mentally delimiting the areas of light and shadow Mr. Critchett used a double knife-edge, like a broad slit (really two opposed knife-edges), letting the rays come between the sides but using only one side at a time. This set-up facilitates quickly choosing either side, in order to get either reversal. With the reversal a turned-down edge becomes a turned-up edge, or vice versa. Hills become holes, and raised zones become depressed zones. It's fun.

FURTHER with regard to answering other amateurs' letters in pencil informally on the margins and backs, mentioned a month ago: Making it easy for the correspondent is not the only point in favor of this method, for it puts the answers opposite the questions asked, thus giving a closely tied-up record. It is not a job, but fun, to do it in that way-say when riding on a train or leisurely reclining on a soft sofa at home, but if the letter must be answered formally it is less likely to be answered so carefully, because it must then be re-read at the time the answer is dictated or written. The other way really amounts to an informal, natural conversation between friendly addicts of the same incurable habit.

Your scribe often starts reading a letter, sees it is from a telescope maker, says to himself, "That's too good to try to read in this busy place," and throws it down beside his hat, to be picked up and read at leisure on the long way home, where it can be enjoyed and studied in peace. The answers that then come to mind as it is read and are jotted down in the margins, would not come again to mind so readily or spontaneously a couple of days afterward, if it were included in the regular grist of a million less interesting letters to be answered-less interesting because not about amateur optics. The informal, with-pencil-on-the-back style of answering fellow telescope makers' notes was set by Porter, years ago. If you get a reply of that sort, don't think you are being discriminated against: we'd answer Napoleon in the same way.

EVIDENTLY a lot of people have been making good mirrors, adding bad prisms and eyepieces to them, then wondering what can be the matter. A letter from H. E. Dall, of England, bears on this point: "Out of hundreds of prisms I have tested, only a small proportion are sufficiently good not to give a perceptible error on the final image. Some 1-l/2" (name of a noted maker.-Ed.) I have seen ruined the image of a good f/7 mirror. When three surfaces are concerned, it is obvious that first quality extra is wanted." If your mirror is good, use a good prism.

A certain amateur in California has been using his telescope to watch a doctor kissing his nurse, in a building 1.7 miles away. Readers, want the story? Shall we snitch on him?

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