IN THE NEW edition of "Amateur Telescope Making" Porter points out what he calls the "bottleneck"-that point on the declination axis of a telescope where it joins the tube, and which so many amateur telescope builders make too thin to avoid tube vibration. The neat little telescope shown below-a 4"-made by George Sunal of 5046 Eugene Ave., Dearborn, Mich., shows better than usual design at the bottleneck.

The mounting in the center, made by R. S. Danielson, 1711 Ave. N., Galveston, Tex., has a real bull neck in the same place, and is designed all over on the same principle. "Here is a mounting that should be steady in one of our Gulf hurricanes down here on the Texas coast," Danielson writes. Your scribe spent the summer of '16 in that region and lived through two of these Texas hurricanes. Any mounting which will stand up in one of them is sufficiently rigid for astronomical work. This is one of the best adaptations of the Springfield that we have seen. In the drawing, 1 is a concrete pillar, 2 a pillar pipe, 3 a 45 degree el, 4 the worm driven ring gear, 5 the ring key, 6 the coupling ring, 7 a 3" standard pipe T, 8 the worm driving ring gear, 9 the ring key (identical with 5), 10 the saddle coupling ring, 11 the tub clamps, and 12 a cap (machined to suit) Danielson explains that ring gear 8 can be driven by the worm, or can drive the worm so that the tube may be moved quickly by hand, but that in the polar axis the worm drives the ring alone. Bearings 4 and 5 are friction. Ring 10 is rectangular, in order that greater spread can be had between the ends of clamps 11. All in all, this is a rugged, close-coupled, commendable mounting.

THE 8" telescope at the bottom of this page was made by Dr. M. N. Duxbury a Green Bay, Wisc., dentist. There is phonograph motor drive on the pedestal Note the declination slow motion control the handscrew on the tube. One fine feature is the fact that the observer can rotate the tube in order to bring the eye to a desired position, with less exertion than is required in wrestling with a polar bear. "Look closely," Duxbury writes, "and you will see the there is a clearance of 3/4" between the basket circles and the tube for the roller bearings. The tube rolls very quietly and smoothly within the basket without loosening the thumbscrews."

IN "A. T. M.," pages 376-379, Hindle' method of plotting the readings in the knife-edge test is shown. The mirror is divided into zones of definite and know widths, and readings of zonal aberration made at those radii are plotted graphically. The purpose is to ascertain the total zonal aberration and whether the curve is paraboloidal. Nicholas M. Smith, Jr., Box 4 Pulaski Heights Station, Little Rock, Ark. uses a similar method, which is helpful in interpreting irregular mirrors whose shadows otherwise puzzle the tyro. But, instead of measuring the zonal aberration at known radii, he sets his knife-edge either at random distances from the mirror or at such distances that irregularities on it will be best revealed, and then measures the position of the "crest of the doughnut" at those particular knife-edge settings. Then he lets the mirror plot its own curve on cross-section paper.

"It often happens," he writes, "that the study of the shadow and the measurement of the difference between the radii of curvature of the inside and outside zones are in sufficient to give the true nature of the curve-especially to one not too experienced with the various shadows. The only method left, then, is that of making a zonal test, measuring the aberration of the focal point between each zone and the center. This is usually accomplished by placing stops in front of the mirror, that are cut away in the zone to be tested. The knife-edge is adjusted until the two portions are lighted with equal intensity, and this position of the knife-edge is marked as the center of curvature of that zone, and so on, zone by zone-involving a long and laborious process.

"It once occurred to me, while using the Hindle method of testing, to try a correlative procedure in determining the centers of curvature of various zones of the mirror. This was tried with great success. It was possible to make a full zonal test on an 8-inch, f/6 mirror in less than one minute, and all cumbersome stops were dispensed with.

"The method of procedure is this: The knife-edge carrier is provided with the usual straight-edge S, in the figure, and a paper is thumb-tacked to the table under it. A line D, representing the diameter o the mirror, is drawn on the paper, an divided into quarters. The knife-edge is first placed inside the center of curvature of the inside zone (or zone having the shortest radius of curvature) and brought back toward the eye until it stands just at the center of curvature of that zone, as indicated by the uniformity of the shadow at that area (illustrated in the first shadow graph). A line is then drawn along the straight-edge and numbered '1'. Along the representative diameter at the bottom of the paper, the place on the diameter having this radius of curvature, by eye estimate is indicated and numbered '1'.

"The knife-edge is then moved slightly back, a line is drawn along the straight-edge and numbered '2'. The position of thc crest of the apparent bulge of the mirror is judged, and the corresponding position are marked on the line representing t diameter, as shown in the second and this shadow-graphs. [A stick of wood bearing pins at measured intervals (Everest's method) or a strip of metal having teeth filed at the same intervals (Mason's method) may be supported in front of the mirror, and will help in estimating the different radii.-Ed.] The process is continued in as many steps as desired, and from these data-the aberration of the light and the corresponding positions on the mirror-a graph can be drawn and compared with the ideal curve, as calculate from the r²/R values. This graph is of little use in ascertaining the proper procedure for the correction of a badly shaped mirror, but a comparison of the graphs before and after a spell of polishing does show quite clearly the changes brought about- changes that are otherwise imperceptible

"It was found unexpectedly easy to determine the portions of the mirror that were lighted with equal intensity-much easier, in fact, than trying to set the knife-edge so that any two given portions would be lighted equally, as when using stops.

In his letter, Smith also stated that he could see the typical knife-edge shadow without the knife-edge, by using the edge of the iris of the eye as the knife-edge. This was tried out and found to be correct; you get the shadows splendidly after spending about three minutes jockeying around and trying to give your head a filar microscope adjustment with nothing but your neck muscles. For several days, after wringing our neck in the attempt to catch the shadows, we went armed with a neckful of grandpa's rheumatiz, but the stunt is fun-well, anyway, once. Incidentally the various cobwebby effects which wander over the cornea and are known to physiological opticians by long Latin names do not show and the mirror looks better.

AT the right are several drawings done by our artist, J. F. Odenbach, after rough sketches by Ellsworth L. Martinelli, Box 402, Colfax, Calif., whose technic for HCF work they describe. In this technic the mirror is polished on HCF on glass. Then a tissue disk is laid on it, this is wetted, painstakingly adjusted and smoothed, and a plaster tool is cast (Fig. 1). The moist tissue is next rubbed off with the palm, taking pains that the plaster is entirely undamaged.

Figs. 2 and 3 show how underlays of light, unglazed card of about the weight of U. S. postal cards are inserted between tool and HCF, wherever a zone needs treatment. Cut the paper with a sharp knife, to avoid an edge burr.

Parabolizing is done as shown in Fig. 5. The layout is made on paper, transferred to cardboard heavier than that mentioned above, by means of carbon paper, the blocks are cut out and fastened to the original layout pattern with shellac and dried under weight. This makes another underlay for an HCF disk. Cold press and parabolize on it.

The above is an abstract, necessarily sketchy, of a 2300-word paper by Mr. Martinelli, but the figures show most of the data. The original paper will be lent to those who request it. In this paper the author frequently commented on the difficulty of using the HCF strip-and-patch technic, stating, as others have stated, that it leaves irregularities and depressed places at the edges of the strips. This is often true, at least the first time it is tried. But the user soon learns to avoid this by frequently shifting the strips, changing the stroke, mixing in some side motion or else toeing the strips in, to distribute the abrasion; and, in general, finessing. One can pick up the mirror with strips adherent to it, slide them over a hair or two, lay it down and go on working, much faster than one can potter with more precise substitutes. After mussing up about two initial attempts, the technic of removing a raised zone, and then removing every visible trace of its leavings, is soon learned. The matter of using measuring rods, dividers and so on, for spotting zonal locations, may be for gotten if the estimate powers of the eye are used. In short, the method soon becomes one of feeling and intuition and, after that, it is so direct and free from complication that it beats a more exact, more rational, but more complex method.

THE most expert workers seem to get scratches, while some of us who wish we were as expert can't seem to get them, even when we try hard. How they come is one of those impenetrable puzzles-the scratcheés say they take every known step to avoid them. This note from Hindle may explain some of them- though it is improbable that any one cause may be assigned to all cases or even a majority of cases. "Scratches during polishing are almost invariably due to filthy polishers. The rouge is allowed to dry on, and hardens; the polisher changes shape slightly and when next applied is resting on some of those hard points which cut into the polished surface, showing which way the polisher moved. When polishing is suspended, the polisher should be scrubbed down with a strong scrubbing brush, and plenty of water. Pay special attention to the grooves and continue so long as any color comes off. Pour warm water, 100-120 degrees F. over the surface of the polisher before putting it on again to the rouged surface of mirror."

HERE is James L Russell, lawyer, 318 Chester-Twelfth Bldg. Cleveland, Ohio, reporting what he calls a coo-coo idea: "If you can not get the mirror and the prism and the eyepiece in line try this. Set her up in full moonlight and point the tube toward the moon. Then blow a good puff of cigaret smoke down the tube. You will then see the cone of light coming off the mirror, and it will look so real that you can nearly cut it with a knife. The cone hits the prism and bounces right up into the eyepiece. Move the scope a little off and the cone will then miss the prism and it will project out of the end of the tube. It will go from the mirror to a point and then from the point it will again get large. It is a most beautiful sight." This idea isn't coo-coo, but should be added to the data in "A.T.M.," pages 381-382. As Russell writes, the idea in general is old- see top of page 390, for example. But it is a good one.

LIEUT.-COL. Kendrick, U. S. Army, Retired, Jacksonville, Fla., writes R. W. Porter, with regard to testing pitch by biting it, that when he first took up the TN hobby a friend of his, Dr. S., did the biting, as follows: "Took some hot pitch- a piece as big as a thumb-dropped it in a glass of water, removed it and bit squarely into it. His teeth stuck solid. I grabbed a bottle of turps, pried open his lips and went to work. It blistered his mouth, and I Finally had to get some gasoline before I could get him unstuck." Pitch should be bitten in a gingerly, ladylike manner, not like Navy Cut Plug. Just try to imagine this colonel and the doctor-the one wildly running around, the other wildly gesticulating-in the described predicament! Maybe the joke was about 50-50, all in all.

G. MERRILL, U. S. N. Hospital, Annapolis, Md., writes via John Pierce that he made his mirror in a morgue, which he claims is a record, and who will want to tie it! The traditional marble slab served as the test table and was steady as a dead man, while temperature conditions were fine. "Working till 1 A.M. was not bad," he comments. "once you got used to the environment." Can anyone beat that place as an odd workshop? Well, perhaps we can. We have learned that a certain telescope maker in Minneapolis (name will not be furnished on request) found himself inadvertently serving a term in jail, became bored and asked a confrère-confrère, that is, in telescope making-to bring his mirror making outfit to the jail, and turned out an A No. 1 mirror there. File this idea for possible future use.

THE above drawing represents a sitting-down grinding and polishing rig, and was drawn by Odenbach, from a life-sized model sent us by the designer, E. B. McCartney of 76 Roslyn Road, Mineola, N. Y. He calls it his "Invertible, Sit-downable, Synchromesh Grinding and Polishing Dingbat," and our brief preliminary test of it speaks loudly in its praise. The shaded part represents a plank which is to be clamped to a table or bench. Turning the rim of the pan turns the tool, the hole in the pan being shaped to the plan of the wooden knobs, in order to key the two together. McCartney belongs to the Long Island Amateur Telescope Makers' Club, a group which, like the Telescope Makers of Springfield, requires completion of a telescope before considering grant of membership. This club, inadvertently omitted from the new list of clubs in "A.T.M.," fourth edition, should be added to that list. Your scribe, who is getting fat and lazy, will use McCartney's patent dingbat on his next job, and remain seated. Somebody please think up one that can be used from a hammock.

Mathematics hounds may find interest in the following books which have been published: "The Calculation of the Orbits of Asteroids and Comets," 1934, by Kenneth P. Williams. Also "Planetary Theory," 1933, by Brown and Shook. Both are about the methods for the calculation of the general orbit of a planet, and for most mere mortals they will seem deep, deep, deep. A fine new German astronomy textbook, entitled "Astronomie", by Oswald Thomas, has recently appeared. It covers general astronomy- has fine cuts.

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