YEARS AGO John C. Lee devised and made a grinding and polishing machine (see "A.T.M.," page 160) in which the telescope mirror moved in a bath of liquid abrasive. He could start it running, go to bed, and get up to find the part of the roughing-out job finished. Now comes O. Magnussen, 514 Ricketts St., Danville, Va., with the semi-automatic machine shown in Porter's sketch (Figure 1) drawn from original.

Magnussen writes: "I made this machine from worm gears and shafting, and a novel feature of it is the automatic abrasive feeding device. The grease cup fastened to the cast iron lap makes 1/6 turn during each revolution of the lap. The abrasive is mixed with soap, which acts as a vehicle. The parts are easily removed and cleaned between grinds. I use a similar cup for polishing, except that its threads are finer (40 per inch). This method doesn't seem to leave zones, since the rouge mixture is spread through channels in the lap."

Figure 2 shows a neat 6" with drive made from a hand-wound phonograph motor, built by Magnussen, who teaches machine shop work.

H. H. SELBY contributes the following note:

"In using a slit or a pinhole for testing precisely on the axis, the usual method, which I have used for ten years and which Gaviola published a few years ago, is as sketched (Figure 3): A is a fairly plane-parallel, unsilvered diagonal, such as a 20 x 26 x 0.5mm hemocytometer cover glass. B is a plate containing the slit or hole. C is ground or crystal glass. D is the illumination system. E is the eye.

When A is unsilvered, a reflection curs at each surface, giving an over-lapping of shadows which can be confusing. Also, only 0.5 to 1.0 percent of the light at the pinhole reaches the eye when testing uncoated mirrors and, if the diagonal is thick, aberrations are introduced. This doubling of the beam, also spherical aberration and astigmatism, can be minimized by making the diagonal thin, but 0.5mm is nearly the limit for glass, considering that the surface should be plane within, say, 2 to 4 fringes. Pellicles are admirably thin, but are fragile, and interference phenomena within the membrane are troublesome.

"The second surface can be coated with a non-reflecting film of fatty acids or fluorides. The fluoride films are particularly satisfactory and commercially available. If a 0.5mm diagonal, flat to 3 fringes on one side and fluoride coated on the other, is employed, no troublesome doubling will occur and illumination will be uniform but relatively weak. By applying a 50 percent reflecting coating of aluminum or silver to the flatter surface, the light reaching the eye will be the maximum possible-3.5 times that from an untreated surface. The remaining imperceptible residual doubling is also reduced in like ratio.

"From the above, it is seen that the ideal arrangement is a thin, flat-surfaced diagonal, having the flat side 50 percent reflective, the reverse side non-reflective. With such an arrangement the lowest focal ratios found in objectives can be examined with complete freedom from artifacts."

IN THIS department, September 1943, appeared a note by Haviland, on the Barlow lens, and in the May number succeeding there was one by Wates. It turns out that these were partly erroneous. Since an editor is supposed to edit, these errors become automatically your scribe's-which makes three of us for you to boil in oil, so light up the fires and pile on the faggots. Note however how frankly-almost gleefully- we three villians admit our errors and please be good to our families after we have expired.

The detective who spotted the errors is C. R. Hartshorn, 1244 W. 109th Place, Los Angeles, California, and he didn't want his name mentioned. He said he didn't want readers to think him one of those disputatious, contentious persons who miss no chance to jump on others. However, your scribe, who dealt with Hartshorn for a long time when he was in the famous Amateur Roof Prism Gang (where he wanted to make roof prisms gratis for Uncle Sam, just to take a punch at the Krauts and Japs), can testify to the contrary, and gives his name for another reason-because he appears to have had experience with Barlows and should in future be made to work for other amateurs who may need help on them. It is the custom of this department, when questions arrive on some special phase of the hobby, to pass them to other amateurs who are known to have gained experience in those phases.

Hartshorn writes. "I thought some of your regulars would rise to the occasion and point out the errors but, as they haven't, I am sticking my own neck out. Haviland proposes a Barlow of -18" f.l., to be placed 2" inside the focus of a telescope of 48" f.l. He figures correctly that the e.f.1. of the combination will be 54", but then goes on to assume that the eyepiece will move out 6", and that the magnifying power will be quadrupled. Actually, the eyepiece will move out only 1/4" and the amplification will be only 12-1/2 percent. He is just dealing with the tip of a theoretical cone of light 54" long. The ratio is 2-1/4:2::54:48.

"As a check, although the formula Haviland used is entirely adequate, I have just traced the paths of the rays trigonometrically through a hypothetical -18" Barlow and it came out 2.249". I have used a Barlow for years designed and made my own-flint and crown-and observed with it and others with sundry eyepieces.

"Perhaps Wates also is a little off the beam. If you had a Barlow that would amplify X4 at 2" inside focus, the f.1. of the Barlow would be 2-2/3". If you then moved it to 1" inside focus, the amplification would be X1.6. If you moved it 6" inside focus you would see nothing at all through it, because the rays would then be divergent. At 2-2/3" inside focus they would be parallel, the secondary focus at infinity.

"Any particular Barlow has the same amplifying power when placed at the same distance inside focus, no matter what the telescope."

Hartshorn gives some specific figures. For a primary focus at 48", Barlow of 2 2/3" f l.,1" inside focus, amplification is, as already stated, X1.6 and power of telescope with 1" eyepiece is 76.8. Corresponding figures for 2" inside focus are X4 and 192; for 2-1/2'' inside focus, X16 and 768; for 2-5/8" inside focus, X64 and 3072.

Haviland's response to Hartshorn's letter is candid: "Hartshorn is right. I have a string of alibis as long as your arm, but Hartshorn is right. Here is the correct solution:

"Suppose that a Barlow lens (Figure 4) of focus -F is placed p inches inside the focus, A, of a telescope. The new focus, B, will be at a distance q from the Barlow. The relation between these distances is 1/q + 1/p = 1/f.

"The ratio q/p = magnifying power.

"If we have a Barlow of -2-2/3" focus, placed 2" inside the focus of the telescope objective, we get 1/q-1/2 = -1/2-2/3 and q therefore = 8

"In the above l/2 takes the minus sign, in accordance with the direction of light from A toward the left (both images on same side of lens).

"The magnifying ratio then is q/p = 8/2 = 4.

"Caution: In using a Barlow with reflecting telescopes, avoid high amplifications, else the color correction will be poor. A -3" focus Barlow to give an amplification of not more than 1.5 or 2 will be the top useful limit. This works out to p = 1-1/2'' (inside focus), q = 3" (new back focus) and q/p = 3/1-1/2 = 2 (amplification)."

Wates, to whom Hartshorn's letter was next passed, writes: "Hartshorn is right, I was off the beam. I took Haviland's formula as my premise. I must admit I was surprised that the result of moving the lens within reasonable limits is very small, as I had understood that one could produce any amplification by moving the Barlow back and forth."

And thus the sins of Wates pass into the body of Haviland, and then the combined sins pass on into the frail body of your scribe, for letting the errors get by, just as in the 21st verse of the 16th chapter of the biblical Book of Leviticus ("The Devil can quote scripture."-old proverb): "And Aaron shall lay both his hands upon the head of the live goat, and confess over him all the iniquities of the children of Israel, and all their transgressions in all their sins, putting them upon the head of the goat, and shall send him away . . . into the wilderness."

Your goat, only too glad of a pretext to hunt up a good wilderness, goes willingly-tail up in the air.

Right in the middle of a war there has been a rush among Englishmen to join the British Astronomical Association, mainly by persons who, because of the blackout, have discovered that above modern city streets there are stars. The new List of Members recently received in this country, contains about 1200 names mainly British but also scattered all over the planet. Of these, only 30 are Americans but there no doubt would be more if more of us were familiar with the Association's good publication, The Journal. To obtain it, the easiest way is to seek to join. Nominations must be signed by one member from personal knowledge and your scribe would try to contribute that help for any except convicts or those who should be. The costs are about $6 for the first year and about $4.25 a year thereafter. Over there they really go in for astronomy and a few of them for telescope making, about which there are occasional articles. The monthly meetings of the Association are reported verbatim-oral discussion by this member and that in each number.

COARSE diffraction gratings may be made at 10 or 20 cents apiece if made in quantities, by a method described in a short article in American Journal of Physics (57 East 55th St, New York, N. Y.), April 1944 by Warner W. Schultz, Reed College, Portland, Oregon.

He knew that gratings had been made by photographing a series of parallel lines but getting such lines is not easy. Easier, it occurred to him, would it be to stack up alternate layers of black and white sheets of paper and photograph these.

A simple rack was built, one face of it being of glass, tilted at 45 degrees, and the papers were stacked 6" deep against this. It then was placed vertical, clamped, and the glass removed.

Focussing the camera was critical, so a finer screen was made by grinding the glass with No. 600 Carbo or finer, and focussing was done with an eyepiece (details of an even better method of focussing are described in the original article). Special film of high resolving power, lantern slide plates, or microfilm, may be used. It is also hoped that gratings of 10,000 lines an inch may be produced using a new Eastman film of high resolving power.

Such gratings may be used for direct observation of spectra.

AUTHOR of the well-known book "Applied Optics and Optical Design," Professor Alexander E. Conrady, died June 16 at his home in London, aged 78 years. He was formerly Professor of Optical Design at the Imperial College of Science and Technology, London and previously lens designer at W. Watson and Son, Ltd., of London, manufacturers of microscopes. He is survived by three daughters, one of whom, Hilda G. Kingslake of Rochester, N. Y., the wife of Professor R. Kingslake is herself the author of several optical papers. She is also the mother of Professor Conrady's only two grandchildren, both of whom were born in the United States.

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