Figure 1: India, Calcutta, 28 Gangadhar Babu Lane, Hari Charan Datta. 'It was an auspicious day indeed when I chanced upon the advertisement of your book 'Amateur Telescope Making.' The book was a god-send to me, Please accept my sincerest thanks and salaams.

"The mirror was ground from a disk of plate glass 3/4" thick and 5.65" in diameter, on a similar glass, using the inverting device of Mr. Porter, as illustrated on page 288. Time consumed, about 18 working hours using Carborundum powders. The mirror was polished on a rosin and beeswax lap. The best proportion. after many trials here at Calcutta at that time (middle of October), as found to he l in 20 and the best method of cutting the channels was found to he sawing, as advised by Mr. Pierce. The hack saw blade was kept rubbed on soap. It took me about 40 hours to polish and 10 more to parabolize. I made all the tests, including the Ronchi test, which was done by 8 strands of hair spaced one tenth inch. The total cost of my instrument was about 24 rupees [about $9.- Ed.]. my sincerest thanks and salaams for distributing the knowledge by means of which a novice like me on the other side of thc globe has made a telescope."

Figure 2: Japan, Yokohama, 140 Mamegochi-dai; R. M. DeMuth. The capstan for turning the head of the tube is interesting. No other data were received.

Figure 3: South Africa, Capetown, Mowbray, Clifton Road, "Tircrevan," W. G. Andrews, 'I was able to get 6" disks in Capetown but could not get Carborundum grains till I mentioned it at the Astronomical Society, when I received any amount It was only after the mirror had been finished that I discovered that I had been using paving asphalt, but had not the slightest trouble with the lap. I had been reading in 'A.T.M. 'about the many different figures I might strike but had beginner's luck, as almost the first test was paraboloid; so, to see them, I rubbed several holes in the mirror, and then I did see them! It took several hours to get it parabolized again. The 2-1/4" finder was salved from a wreck in False Bay. The total cost was under £1."

Figure 4: Manchu-kuo, Harbin, Slavianskaya St., P. Ramensky. "The mirror is of 6" diameter and gives magnifications up to 300 diameters. The mounting is from Ford rear axles."

Figure 5: New Zealand, Auckland, W. 2. Grey Lynn, West End Estate, 161 Garnet Road' Alf R. Martin. "I enclose a photo `of a 12" reflector built by myself. It is mounted on Ford back axle housings. There are quite a number of amateur observers here. Recently, in this city, we have formed a club and meet once a month for discussion."

Home, James.

ABOUT making camera lenses: From time to time since this hobby of amateur optics was got going in 1926, requests for instructions for making camera lenses have been received. A number of years ago we addressed letters to some of the photographic journals, asking for light on this situation and parts of the replies, published below, still make interesting reading; they show that the job surely is not a beginner's job, also perhaps that the ability of at least some amateurs may have been a bit underrated. It is true, however, that at that time there were few who could be called advanced amateurs, as there are today.

The first letter:

"I am afraid you are up against a tough proposition. The photographic lens is entirely different in conditions from the telescope lens in that it must cover a very wide field at a large opening, say 80 degrees at F/4.5, with minimum errors.

"Photographic lens computation is not easy. Lens designers are only able to compute about one lens per year. Even Dr. Rudolph, one of the most famous, calculates only one in about two years."

The second letter:

"We have not published material of this kind for three reasons: first, the calculations involved are over the heads of the majority of our readers; second, even if the calculations were understood, few, if any, readers would have the necessary grades of optical glass or the mechanical equipment to produce the lenses, and, third the manufacturers keep their lens calculations a trade secret."

The third letter:

"Personally I am averse to exploiting making things at home which can be bought better made and often at lower cost in the shops. This is not wholly a commercial expedient, either. It is based on two facts, the mass of readers cannot make complex things, and the price of such things is usually lower than the cost of material and waste through spoilage. The optical stores are so generally equipped for grinding lenses and the optometrist is usually a college man trained in computations that the layman could do no better than formulate his needs and have them filled by an expert.

"I gladly tell you that never more than in photographic objectives is the refractive element a factor. Even further, the refraction and diffraction enter into the computation of curvatures. Flare spots and lateral catch of light show on the sensitive emulsions where the eye, looking through a telescope, might not see them or mentally ignore them." [That one sounds snooty.-Ed.]

The fourth letter:

"The making of a simple camera lens, such as a meniscus or rapid rectilinear, is not beyond the capacity of an amateur worker. When it comes to the question of making an anastigmat, however, the problem is a difficult one and it would require an excellent machine shop equipment and the making of so many tools that the cost of a single lens would be far above the retail purchase price. The anastigmat consists as a minimum of three elements, but those which would be easiest to construct would have four or six and it would presumably require a grinding shell and a polishing and test block for each surface, and as the utmost accuracy is required in the polishing, even a skilled optician needs much experience to make anastigmats which will pass the necessary tests.

"The principal difficulty, however, lies in the fact that, even if the amateur can get optical glass of approximately the proper constants-which he might be able to through the courtesy of one of the few lens manufacturers-the manufacturing optician finds that every batch of glass varies, so that in practice every lens has to be recalculated every time it is made from a new lot of glass. Of course, the optical firms do this rather empirically, and have in their possession data arrived at from many years' experience, which enable them to make these changes without much difficulty.

"We have had under consideration the possibility of publishing a book on lens making and could command the services of one of the best lens makers in the country for the technical details, but we have hardly considered the publication feasible for the reasons set forth above. Telescope making is child's play compared with photographic lens making."

These letters were shown to an amateur telescope maker who both calculates and makes his own camera lenses and he commented: "The first one tells the true story, the attitude shown in the second is justified, the cackle in the third gives me a -, and the fourth is about right." He added, "Do not trust examples given in patent specifications of lens designs. Impossible glass types are frequently specified and residual aberrations or flare spots are said to be deliberately introduced as jokers." Some years later-that is, a year ago-we invited the writer of the above comments to prepare for Vol. II of "A.T.M." a detailed chapter on photographic lens making, for we were still hypnotized by the desirability of making such data available to the amateur and hoped there might be some possible way out. This is his reply:

"I believe it would be impossible to write a chapter on the general subject of photographic lens construction, because the mathematical treatment of comatic, astigmatic, sinical, chromatic, spherical, distortion and flatness errors would occupy an immense amount of space, even if the rudiments and one example only were taken. The absolutely necessary discussion of glass types. and the graphic methods of 'bending' compromises would consume more space, as would the discussion of the relative merits of the various corrections for different purposes. And, even if you were to give me 500 pages, quarto, I should have neither the time nor the ability to write such a treatise. You know I'm neither a Frauenhofer, a Rudolph nor a Conrady.

"I can compute for you, however, some examples, giving glass types, radii, thicknesses, separations and iris positions for, say an f/4.5 Tessar, an f/4.5 Cooke triplet, an f/6.8 Goerz Dagor, an f/6.3 Rudolph Protar, or some such popular combination. However, when the amateur had read the dope, he would find that the glasses he could obtain from B. & L., Chance, School or Goblin would not have the precise constants called for. Values would be sure to differ, by several units in the fourth and possibly in the third place. He would then have to recompute his system for the new values, which would mean that he would need to be quite proficient in the art.

"In a doublet astronomical objective, it is easy to work to the 'V' figures of a pair of glasses in order to get fair chromatic correction, and then to figure one surface in order to approximate spherical correction for visual rays, since the axial image is all that matters, In a photographic lens, though, 20 or more compromises must be made, and few errors are completely eliminated, since, in order to cover a large field sharply at a large aperture, on panchromatic plates, relatively minor errors must remain in order that major faults may be eliminated. In order to cover 40 degrees on 35 mm. film at f/2.0 apochromatically within only 0.01 mm. of true sharpness, Lee had to use ten components, two of them unstable in air!"

Well, there we are-where? Nowhere, apparently. Incidentally, the man who wrote that letter is not a professional in any sense he is a chemist who makes telescopes and so on as a hobby. But he dislikes answering letters and that is why we cannot given his name here-from past experience, he would probably shoot us. In fact, any mention here of any man's name always brings him a big batch of those six-page letters (without eve return postage being included!), using up all his spar time and more, if be is kin to all.

We replied to the letter "Your answer gives me a good laugh: you talk yourself into the proposed writing job and then right out again. But your logic is watertight: to give the background stuff would require a book; some examples might be given; but then, the exact glass not being obtainable in each instance, the amateur would be just where he started. I shall publish this sometime and if the readers who have asked for full instructions for making an anastigmatic lens don't still think I am in the Ananias Club they will stop teasing for them and materially lengthen my life." He replied: "Too bad about the wet blanket I pulled over the photo lens flame, but facts are facts. My own computations have been performed solely to fill some personal need. I don't do outside jobs.'

The same man also wrote this "I can't 1 sit down with a couple of reciprocal and log tables, and a glass list and design, say, an anastigmatic magnifier. Each job has meant looking up forgotten equations and eikonels, brushing up on a differential or two, making a dozen graphs to assist in balancing compromises and then beginning algebraic third order work, which must be followed by trig. ray tracing. The tracing takes five to 100 hours and may show higher order faults which make it necessary to re- compute some of the system. In other words, I stumble along, consulting Steinheil, Hastings, Gardiner, Conrady and Taylor from time to time. A good computer would be finished when I was half through."

Another amateur who has done much lens designing writes in the same vein: "The stuff you send me [just quoted. -Ed.] clicks with what I've always said-that you can't give 'simple rules for calculation of optical instruments.' They are invented, not calculated. Every new item is a new problem. After you've traced rays through 100 or more sets of lenses you learn about what to expect, but you couldn't tell anybody how, any more than you can tell why you like the little red-head on the end better than the blonde in the middle."

Not, however, that all this will stop the requests for "detailed instructions for photographic lens making," since they come mainly not from amateur telescope makers but from outside. But with these various comments now in print, all we shall have to do in future is to tell them which back number to buy. We inserted them here merely to save ourselves future hard work! But even yet we harbor a bull-headed hope that, someday, somehow, someone will find a way to impart the "dope."

WHILE telescope making is not generally a juvenile hobby, a few keen lads have made good telescopes. Wilson Hole, 540 South Greenwood Avenue, Pasadena, California, has arranged a double yoke in "wheelemout" fashion (Figure 6). His use of a deep section for the yoke sides is pleasing to look at and lends steadiness. His carriage has four corner screws-1/2" bolts- which screw down to give a solid stance, much as a wrecking crane lets down corner screws or a dredge pokes down spuds and ceases merely to float. Hole, a young friend of R. W. Porter's, writes that he has made about five mirrors and completed three telescopes in the past two years. He hopes to join the A.A.V.S.O. His age is 14.

Figure 7 shows a mounting of angle iron, made by Eugene Shafto, 1114 Corlies Ave., Neptune, N. J. The yoke, with split ring is arranged the same as the one on the 200 telescope at Mt. Palomar. The ring is of hard wood, backed with iron. The maker submits a list of expenditures, which totals $16.75 for the 8" telescope.

A second keen lad who has made good as a telescope builder is George Economy, 108 Auburn St., Manchester, N. H.-see Figure 8. He is 13 and did the job in three weeks making a model described in earlier editions of "A.T.M." by John M. Pierce. The entire cost, he writes, was $13.

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