IN THE FOURTH edition of "Amateur Telescope Making," in his chapter on the design of mountings for telescopes, Porter wrote (page 142) that be knew of nothing better for a stable polar axis than the crankshaft of an auto engine, together with the engine block itself. Several telescopes have subsequently been made, using such axes.

Before going into Army service, Daniel Langpaap, 200 Kessing Ave., Porterville, California, sent in a description of a 6" reflector (Figure 1) mounted in that manner. He states that the engine unit cost 75 cents and the whole telescope, including mirror kit and aluminizing of the finished mirror, $16.81.

"The hour circle is graduated on the flywheel, and the declination circle is a protractor. Parts of the starter assembly are used as the slow motion."

Commenting on Figure 1, R. W. Porter says, "I've never seen a mounting that showed so much strength and weakness. If Langpaap would stiffen his slender fork with struts to the flywheel this would altogether alter its stability." When shown this comment Langpaap replied, "The Army was on my tail and I wanted to get the job done before I was inducted and they didn't leave me time to do that. This is my first telescope, but it isn't going to be my last. I am now in the army (aged 18), so this ends my telescope making for the duration.

"I think," he adds "there is too much warning the beginner about the difficulties of mirror making. My mirror came out an oblate spheroid but figuring this to a paraboloid took only half an hour."

By all odds the most blood-curdling warning to the beginner has been Ellison's paragraph at the opening of Chapter V in "Amateur Telescope Making," where he ominously says: "We now come to the crux of the whole process. Grinding and polishing are purely mechanical processes, which any handy man should be capable of learning in a few lessons. But the man who can produce a perfectly true paraboloidal curve right up to the edge of a mirror is not a mechanic, but an artist; and the artist is born, not made." He continues: "Volumes might be written on the art of figuring and the reader of them would be no nearer being able to produce a true curve after reading them than before, if the talent were not born in him."

Something has now been done about these awesome sentences. From time to time, as stocks of "A.T.M." are sold out, new printings are made and, while a new printing is not a new edition, minor textual changes may be made on these occasions. One change that has just been made for the current fifth printing of the fourth edition is the insertion, on page 95, of the following Editor's Note, directly bearing on Ellison's bogey.

"Many old-timers, advanced amateurs, vividly recalling the misgivings caused when they were novices by the opening paragraph of the above chapter, have urged with feeling that it be struck out of the book. It also must have discouraged many from even starting.

"It is probable that many-perhaps most-novices run away with the belief that a telescope either has a perfect mirror or it will not perform. At the risk of seeming to encourage slovenly standards it ought to be explained by the Society for the Prevention of Cruelty to Beginners that a mirror has to be pretty bad before it won't perform; also that a mediocre mirror will usually seem, at least for some time till your observing acuity has been educated, to be about as nearly perfect as your first grandchild; and that the reasonably good mirror you I should aim to make on your first try will probably seem about perfect for a while. Thus Ellison's paragraph misleads beginners by omission.

"Many, before beginning, think of making one telescope and one mirror, which must of course be tops or else, and thereafter using it indefinitely. More common is something like this: On trying out your first mirror you exclaim, 'I did better than I thought I could.' After a few weeks you add, 'but I know its faults and can do better, and while I'm at it I'll make a bigger telescope.' A year or so later you build a third, again larger, and your third mirror will be on nodding terms with Ellison's described ideal. All along, as your observing eye has become more exacting, your manual skill has improved to keep pace. (About this time you go back and, out of curiosity, re-test your first mirrors- the ones you once thought good-and maybe grin a little.)

"As for 'talent,' Ellison doesn't specify how much talent is needed but everybody has talent, which is mainly perspiration. The words 'artist . . . born, not made' have been a real bogey to some, but read Ellison closely; he applies those terms to perfect mirrors alone.

"All sorts of plain average folks, thousands of them, have made successful mirrors, even editors. Cheer up."

Experienced amateurs who recall the howling of the banshee in their dreams after their first reading of Ellison's paragraph, quoted above, may now perhaps feel satisfied that this specter can no longer make the tyro's hair stand on end.

"While home on a furlough," writes another Army man, Pfc. James B. Downs, Jr., 14164282, Co. N, 847th Signal Training Battalion, Camp Crowder, Missouri, "I got into a discussion with an Atlanta amateur telescope maker. As always, the discussion arrived at 'A.T.M.,' 'A.T.M.A.,' and your column 'Telescoptics.' We remembered somewhere you had mentioned that you expected an average of one or two mirrors per copy of 'A T.M.' Our little club in Atlanta, appropriately called 'The Cloudy Night Observers,' has in its possession copies of 'A.T.M.' and 'A.T.M.A.' that have 13 mirrors to their credit and one in process.

"The books belong to C. B. Moore, Jr., who broke them in on a 6" f/8, a 6" f/3.5, and an 8" f/10. From him, the books went to D. E. Bowers and myself, where the following were ground: 4" f/16, 4" f/8, 4" f/3.6, 4" f/1.5, 6" f/2, and an 8" f/6.3. [Note wide focal ratio spread of the four 4" mirrors: excellent experience for the worker who would put himself through the mill.-Ed.]

"The 6" f/2 turned out better than was expected. It necessitated the use of an extremely large diagonal (Figure 2) which cuts its effective light gathering power very much [and adds to diffraction.-Ed.]. Nevertheless, the 12" focal length and 6" diameter is very useful for nebular observing. Some of the very faintest nebulae show up. The power is low but the field is very wide.

"Figure 3 shows the 6" f/2 in the pre-polishing stage, using levigated alumina [and, as a most useful catchpan, an angel cake tin exactly like one your scribe uses.-Ed.]. We carried the pre-polishing until the glass was almost polished before changing to rouge. The total time for rouge and levigated alumina was about two thirds of the time rouge alone would have required.

"It is amazing what a sturdy portable mounting (Figure 4) can be made from some 2 x 2's, 2 x 4's, a dozen nails and a common lawn chair.

"From us 'A.T.M.' and 'A.T.M.A.' next went to C. H. Kitchens and John Brown who built a 6" f/3.8, an 8" f/9, and a 3" f/4.4. The 3" is pocket sized and Brown is carrying it with him in the Army. The books then went to Conrad Meaders, who ground a 6" f/8. Then they went to Rolf Sinclair.

"Thus for two books we have 13 completed mirrors and one more in process."

INTERMITTENT mirror makers, who can work a week but then must go away for a month, often return to find their pitch laps in bad shape because of slow flow and filling in of channels. N. J. Siegel, Box 116, Forest Hills, N. Y., obviates this difficulty by suspending the lap, face down, over water in a receptacle. This keeps it moist and cool.

What Siegel actually does is to make the lap on a round metal plate much larger than the mirror, and uses a cardboard collar to confine the melted pitch to the desired diameter. Then, when leaving it for a protracted period, he inverts it over the water, the metal plate tightly sealing the receptacle at the same time. Since the lap is made on the flat backing, rather than the curved one of the usual glass tool, the pitch' is thicker at the center. When suspended upside down it therefore tend to flow downward more at the center and this, he says, is easier to cold-press back.

To dispose of the mirror while absent he submerges it, face up, in the same receptacle and marks it "Don't Touch." In some households this is probably sufficient but in others, where the lady with the grim, determined face, the eager eye, and the waving dust mop goes on a tidying-up binge once a week, a burglar-proof safe, large enough to take the whole kit, is recommended. Mysterious "spooks" often scratch mirrors. One such finally proved to be the Everest's cat. Roaming the cellar on nights it tracked across coarse Carbo on the floor and then tramped over the mirrors left between working sessions.

CARBORUNDUM grains are obtainable two series, the familiar numbered series, like Nos. 80, 120, 220, 400, 600, and a lettered series, F, FF, FFF. Many workers have been puzzled about the reason for the two partly overlapping series and the relation between them. Here is the answer, from the manufacturers.

The approximate numerical grading of the F series is F:280; FF:320; FFF:500.

The numerical powders are more closely graded.

SOME thermal expansion coefficients per degree centigrade.

"Not long ago I recall saying 'I wish I could get hold of a mirror with a perfect edge, in order to know exactly what I am supposed to shoot at.' Not long after that my eyes fell on Hindle's paragraph on page 371, 'A.T.M.' For the first time I realized what it meant. Eyes that don't see! Here was the very thing I had been hunting. It took only a few seconds to cut a mask concealing the edge of the mirror, and the diffraction ring which appeared around the inner edge of the mask was evenly illuminated on both sides, just as both edges of Everest's straight-edge ('A.T.M.' page 371) are illuminated. On removing the mask I was able to see that my right edge was brighter than the left, and so my edge was not perfect. I set to work, and finally brought it right. When I had done all that seemed possible, I found that the edge looked the same with the mask as without it."

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