PERFORATED sheet iron makes an attractive telescope tube, and one which is well ventilated and thus not subject to the pocketed air current effects that sometimes are troublesome where the tube is solid and the mirror closed in--effects that often cause a deterioration in seeing. Rev. J. F. Peat, Bellville, Ohio, a retired China missionary, who used to do star gazing in China, used this material, bought at an ordinary hardware store, in the 6" reflector shown in Figure 2.

His mounting, a double yoke type with pipe-fitting declination axes, has elements of originality and he says the telescope gives complete satisfaction--"I really have a grand little telescope." It has a celluloid protractor declination circle.

THIS magazine's stock of copies of the book "Amateur Telescope Making" having become once more

exhausted, a new printing was made and the opportunity was used to make a few minor corrections and to revise entirely the last 36 pages of the book. Yet we don't think this is enough of a change to justify calling this a fifth edition--maybe you'd feel a bit cheated--so it is still the fourth edition. Fourth edition, third printing, to be precise.

The old book reviews on pages 469-476 needed revision, likewise the Directory, and, especially, the list of astronomical and telescope making clubs. Wates, of Canada, also didn't like the index; nor did various others, including your scribe, who originally made it. So practically the whole back-yard part of the book was chucked out and a new one made.

Much more space than before has been given to the book reviews, both for their number and detail. Practically all the important astronomical and telescoptical books in the English language are now not merely listed in "A.T.M." but described in some detail. The new printing reviews 76 books (and it was a real job to do the reviewing). Not all are new books--mere newness isn't, in this field, the universal criterion of usefulness--but mainly the books have been published within the last few years. Nearly a page, for example, is given to telling just what's in Conrady's famous "Applied Optics and Optical Design." Few can afford this expensive work, not very many can even understand its mathematics, but nearly all would like to know approximately what it's like. Or just what's in the Ephemeris.

There is also a new list of astronomical journals, a new list of astronomical societies, a list of 70 local groups of amateur astronomers and telescope makers. To compile this and obtain the correct names, also local addresses that are likely to remain relatively permanent, required a large amount of correspondence and sleuthing. This list of clubs--the most complete now available anywhere--should be valuable to many users of "A.T.M."

The "Last Word to the Beginner" was left in and, with misgivings, that odd portrait at the bottom of it.

Wates volunteered to make another index, and now your scribe therefore has a goat on whom to lay future peeves, if any, about the index. (Probably Wates didn't think ahead that far!)

Here are the minor corrections for the main part of the book.

Page 16, line 34, after "flat is," add

"one of." (We believed that--years ago.)

Page 27, line 4, delete "our greatest of all mirrors."

Page 34, two thirds way down, substitute "often" for "usually," with regard to Huyghenian eyepieces (when that was written they usually were) .

Page 54, third line above bottom, delete "need not be exactly" and substitute "ought to be quite." (This shows how far the amateur standards have moved since this was written.)

Page 58, second line below Figure 47, add "at first." (That, too, was written in 1926 but won't get by today).

Page 60, in line 8, substitute IV, Figure 37.

Page 111, near middle, formula below "Substituting," the minus sign in second term should be an equality sign.

Page 344, line 30, "over" should be "one."

Page 370, fourth line above bottom "tool" should be "disk."

Page 381, kill final sentence.

Page 382, in figure, alter denominator of formula to F--l/2 (A-a), which Wates points out will be a little more exact, especially where large prisms are involved, as in an R.F.T., for example. Big help, Wates. And now--

NEW method of zonal testing is offered by Cyril G. Wates, 7718 Jasper Ave., Edmonton, Alberta, Canada, in the following discussion:

The increasing popularity of short focus mirrors is making the subject of zonal testing of great importance to the amateur. Of all the mistakes into which the beginner can fall, perhaps the most elusive is the completely erroneous notion that, because a given zone or series of zones tests correctly, therefore that part of the mirror requires no further attention; in other words, that figuring may be confined to those zones which are, by zonal tests, undercorrected.

For the sake of simplicity, consider a spherical mirror, 8" in diameter and divided into eight half-inch zones. Assume that all zones have the same focal length; that is, zero aberration. In Figure 3, A shows a cross-section of a true sphere. B represents the same mirror with all zones of exactly the same focal length or, to state the case more correctly, with all zones having exactly the same center of curvature. Than this, there can be no clearer demonstration of the fact that zonal tests are not the final criterion by which a mirror must be judged, but that each zone must be considered in relation to the zones on either side of it.

A step-by-step surface such as that shown in B is, of course, never encountered in actual practice. In fact, it would be impossible to make such a surface, since our method of polishing insures that each zone shall blend into the adjacent zones. We do, however, meet with such surfaces as that shown in C, in which zones 2, 4, 6, and 8 are exactly the same as the same zones in B, while zones 1 and 5 are undercorrected (short) and zones 3 and 7 are overcorrected (long). Obviously the entire surface must be worn down to the level of zones 4 and 8, and it is also clear that zones 2 and 6, in spite of the fact that they are apparently "correct," must be polished away; and that, in a lesser degree, zones 1, 3, 5, 7 must be polished down until the entire surface coincides with the dotted line.

In an "all-over" test on a long focus mirror, zones 2 and 6 would show as raised rings, zone 4 as a depressed ring, and zone 8 as a turned edge. In the case of a short focus mirror, when it is necessary to depend upon zonal tests alone, the amateur is hardly to be blamed if he finds himself confused by the apparent "correctness" of the even-numbered zones, and is in doubt as to the practical interpretation of the readings for the odd-numbered zones.

Careful study of an excellent chapter by F. B. Wright, on page 257 of "A.T.M.," will enable the worker to draw a correct graph of the actual surface of his mirror at any time, and will prevent him from falling into the error I mentioned in the first paragraph. The results of Wright's method are quantitatively exact, but the present writer ventures to suggest a somewhat simpler method which will enable the amateur to draw such a graph without the use of mathematics.

The following table refers to an 8" mirror, f/4, the radius of curvature, R, being 64", divided into eight zones as before:

Referring to Figure 4, draw the line AA to represent the axis of the mirror. Lay out the arbitrary scale on this line, each division representing .01" aberration. It is suggested that AA may be 10" long, and each scale division 1/8". Find the point B on the scale at one half the total correct aberration; in this case .110". With B as a center, describe the arc AC, and lay off the equidistant points a, b, c, and so on to represent the zones. On the scale suggested these points may be about 3/4" apart. Draw radii from these points toward B.

Refer to the table and note that the correct aberration of zone 8 is .220". Set one point of the dividers at .220 on the scale and the other point at any convenient place h' just within the circular arc. Prick h' and g'. Now set one point of the dividers at .165", the aberration for zone 7, and adjust the other point to rest at g'. With this radius, prick f'. Continue this process until each radius has a prick-point according to the table. Join these points with a straightedge and pencil. The resultant "curve" represents the correct parabola.

Referring again to the table, note that in Case 1, zones 1, 2, 7, 8 are correct, but that the other zones are all over-corrected. Starting at point h', prick off the points as before, taking the readings as shown under Case 1. The resultant "curve" is the actual surface of the mirror. It will be seen that the actual surface in zones 7 and 8 coincides with the parabola, but that the surface in zones 1 and 2, for which the readings were also "correct," is parallel to, but not coincident with, the parabola. It is apparent that glass must be removed from the center tapering out to f', and that something in the nature of a small star lap, and the use of a stroke which will bring the points of the star to f’, is called for.

Referring again to the table, Case 2, it will be seen that zones 1 and 8 are "correct," zones 2, 3, 4, 5, greatly under-corrected, and zones 6, 7, overcorrected. When the curve is plotted, as shown in Figure 4, it will be found that zones 1 and 8 actually coincide with the parabola, but that the rest of the surface constitutes a raised band, which is highest at zone 5. A ring lap seems to be the remedy, with a stroke which will confine the abrasive action between a' and g'.

It will have been noted that in both these cases the graph was started at the edge, with the idea of getting all the surplus glass outside the parabola, but in some cases it may be necessary to start at the center, or even at some intermediate zone. A little experimenting will soon make this point clear. If the scale and radii are drawn in ink, the various graphs may be drawn in pencil and erased as often as desired.

CLUB of amateur telescope makers--The Sixteen-Inch Club --who would, by clubbing together, obtain a greatly reduced price on 16" Pyrex disks, was announced in the July number, Clyde W. Tombaugh, Lowell Observatory, Flagstaff, Arizona, being the sponsor. Since then the club has gone over the top--gained its necessary 20 members. This reduced the cost of disks to only $35, tools to $7.80. A 16" is a nice size--big, but not too big. As we go to press we don't know whether it is, or is not, too late for others to get in on this club. To find out, if interested, write to Tombaugh.

TEST: "While doing my second fining down with Levigated Alumina," A. M. Mackintosh, Inspector of Police, 70 Arnold Road, Kingston, Jamaica, B. W. I., writes, "the following test occurred to me because the film between the mirror and the tool is so thin with Levigated Alumina that it is almost completely transparent. Draw the tool slightly over the edge of the mirror and look through it at the edge of the mirror. If there is the slightest thickening at the edge, turned edge is present. Of course this will show only a turned 'mountain'."

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

Internet: http://www.sas.org/

At Surplus Shed, you'll find optical components such as lenses, prisms, mirrors, beamsplitters, achromats, optical flats, lens and mirror blanks, and unique optical pieces. In addition, there are borescopes, boresights, microscopes, telescopes, aerial cameras, filters, electronic test equipment, and other optical and electronic stuff. All available at a fraction of the original cost.

SURPLUS SHED
407 U.S. Route 222
Blandon, PA 19510 USA
Phone/fax : 610-926-9226
Phone/fax toll free: 877-7SURPLUS (877-778-7758)
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Web Site: http://www.SurplusShed.com