TELESCOPIC observation of the moon by amateur astronomers can be pursued at two levels. In the first, the novice examines the moon through the telescope he has just completed. He is thrilled by the splendor of his first magnified view of it, and astonished at the almost infinite variety of fine detail he increasingly discerns as he pores again and again over the brilliant, image Truly the moon is, as the astronomer Henry Norris Russell has said, "the finest of all telescopic objects."

Within a few weeks the new observer can identify the principal "seas" and mountain ranges of the moon. But then, after following it through one or two lunations, he may think the show is over and stop, hardly realizing how much further he could go. Some amateurs, however, continue observing the moon and studying its map. These enthusiasts soon enter the realm of the selenographer, to commune with a limited but world-wide circle of inveterate observers.

At this stage, amateur selenographers seek a detailed map of the moon, but find that Walter Goodacre's famous map of the moon is no longer available. Luckily there is another great map of the moon even larger than Goodacre's: the Wilkins map, drawn 95 feet in diameter by H. P. Wilkins, the director of the Lunar Section of the British Astronomical Association. It is reduced for reproduction to a diameter of 100 inches and is cut into 25 convenient sections 20 by 21 inches each. The Wilkins map is obtainable from its author at 127 Eversley Road, Barnehurst, Kent, England, postpaid anywhere for two pounds sterling approximately $8. The 25 sheets constitute the most detailed map of the moon ever drawn.

The accompanying illustrations are full-size samples of one sheet of the Wilkins map. The larger represents 1/4400 of the entire lunar area, all of which is drawn on the same faithfully detailed accurate scale. This area includes the 25-mile crater named Porter; the name was recently inked in by Mr. Wilkins as a lasting memorial to the late Russell W. Porter. The fine detail depicted on the map is exemplified in the smaller sample by the little circle above the terminal "o" in the word Tycho. This circle represents a lunar hillock only a little more than a mile in diameter; the tinier circle to the right of the "o" is therefore less than half a mile in diameter. A single dot might represent a feature the size of the Capitol at Washington, which would be about the limit of visibility in any telescope.

Represented on the Wilkins map, in addition to all the major features, are an unnumbered host of craterlets, crater pits, crater cones, clefts, ridges, light streaks (dotted lines), and a few spots designated as variable areas. These alleged variable areas are kept under observation, mainly by amateur astronomers, because they seem to some to undergo change.

Is the moon a totally dead world? This is an open question of long standing among astronomers. Apparent minor changes in the formations Aristarchus Atlas, Eratosthenes, Kepler, Linné Manilius, Pico, Plato, Theophilus, Tycho and a score more may or may not be real. In 1926, at the end of a discussion of this question, Russell, R. S. Dugan and John Q. Stewart stated in their textbook, Astronomy, that the consensus was that no real changes have been detected on the moon. In saying this, however they do not dogmatize. They add that the disputed question is difficult to settle.

In a 76-page booklet entitled Does Anything Ever Happen on the Moon?, devoted wholly to this fascinating question, the planetary astronomer Walter H. Haas of Albuquerque, N.M., has pointed out that the moon has been greatly neglected by professional astronomers. One observer who saw changes on it had made far more extensive studies of the moon than had his professional critics. The booklet by Haas, a series of articles reprinted from The Journal of the Royal Astronomical Society of Canada, brings together widely scattered material on the study of lunar changes. There are severa1 types of changes: permanent, irregular, periodic, and color changes.

Is the darkening of some small spots the crater Alphonsus, seen at each 29-1/2 day lunation, due to vegetation, as H. Pickering believed? Are the changes in the crater Eratosthenes, to whose description Pickering devoted several articles in Popular Astronomy 30 years ago, real? What of the crater Linné, whose tiny white area appears to change in size, and what of Plato, where new craterlets seem to have appeared, and where mists were seen by Haas? In Tycho, Haas claims to have seen a curious, milky-looking luminosity on the east outer wall, strongly suggesting the existence of a lunar atmosphere. Some of these changes have been discussed in the periodical Haas edits, The Strolling Astronomer.

To participate in the observation of fine detail on the moon and not merely gaze at it, the amateur needs a telescope of six-inch aperture or larger, a minutely detailed map, and persistent effort. He soon learns to recognize many of the small details and comes to think of some of them almost as his own possessions.

The Wilkins map, now in a second, revised edition, was compiled from three sources: photographs taken with the 100-inch telescope, drawings by eminent lunar observers, and its author's observations at the eyepiece. Since 1909 Wilkins has observed the moon with telescopes of various apertures, chiefly with a 12-1/2 inch reflector. Goodacre has pointed out that the details which can be obtained on photographs with the 100-inch telescope barely tax the powers of an amateur-size instrument of about that aperture, in his own case a 10-inch refractor.

A NEW BOOK, Engineering Optics, by K. J. Habell and Arthur Cox, both formerly employed by the British optical firm of Taylor, Taylor and Hobson, Ltd., has just been published by the Pitman Publishing Corporation, New York, N.Y. It deals with the principles of optical methods in engineering measurement. The opening two chapters give the theoretical background for understanding the subsequent chapters on light and illumination in optical instruments, on microscopes, telescopes, optical projection and profile microscopes, and on miscellaneous optical methods. In all these the emphasis is on practical applications: for example in the contour projector, the profile microscope, the alignment tester. At $7.50 its addition to the amateur's library is well justified if his interest is mainly practical.

PLEASURE in the successful attainment of a high degree of precision is believed by some to be the force that keeps the amateur telescope maker going back to his optical work when he should be using the telescope he has just made. If this really is the underlying motivation, it would explain the fact that the lover of precision optics is usually a lover of any kind of precision work.

The most exacting and difficult of all high-precision work is thought by many to be the construction of a ruling engine, and with it the actual ruling of diffraction gratings, especially those of the larger sizes. For some months past there has been an unprecedented ferment in the small and very limited world of the ruling engine. Whereas all the world's demand for diffraction gratings has until recently been satisfied by two or three makers, that demand has increased so rapidly since wartime that several laboratories are now hard at work building new ruling engines for making more gratings. The chief cause of the greatly increased demand is the widening discovery by industry that spectrographic analysis is more rapid than chemical analysis, and that grating spectrographs have greater advantages than prism spectrographs; also, in the realm of physical science, the need for large gratings having high resolving power for use in basic research.

In a monumental article on "The Production of Diffraction Gratings," published in the June number of the Journal of the Optical Society of America, George R. Harrison, Dean of Science at the Massachusetts Institute of Technology and editor of that journal, discusses the present status of grating ruling.

Two-inch gratings, such as are used in many spectrographs of the industrial type for chemical analysis, can be produced almost on demand by any of a number of engines now in operation.

Four-inch gratings can be produced by ruling engines at Johns Hopkins University, at the Mount Wilson Observatory, by M. Siegbahn in Sweden, and by Baird Associates in Cambridge, Mass. The Bausch and Lomb Optical Company of Rochester, N.Y., and the Jarrell-Ash Company of Boston, Mass., also are expected to operate within a year on gratings of this size.

Six-inch gratings can be produced at Johns Hopkins University fairly regularly and at the Mount Wilson Observatory occasionally, with Siegbahn, and Bausch and Lomb, probably coming in soon as sources, and perhaps Baird Associates and the Jarrell-Ash Company.

Eight-inch gratings are not at present available anywhere on earth. There is some hope of their production by Siegbahn, possibly by Bausch and Lomb, by Johns Hopkins University and M.I.T. Gratings of this large size are needed for research in pure science, but very little if at all in industry. It is on this size that the rivalry to get into production first is greatest. It is hoped by the more optimistic that 1949 will see the actual beginning of ruling by several large engines, some of them newly designed on principles that depart from the conventional engines of Rowland. (These are still in operation at Johns Hopkins University, and still produce virtually all the larger gratings made in this country.) Since grating ruling is beset by many problems, none of the friendly competitors in the grating race has issued a schedule of production. None has even promised to produce gratings. This is not evidence of faintheartedness, but of wisdom born of others' experience.

EYEPIECES made by Sir William Herschel with tiny lenses only 1/45-inch in diameter, one third that of a pinhead, that gave a magnification of 10,000 diameters on a six-inch reflecting telescope, were described in this department last August. One modern writer was quoted who wondered how a present-day optician would proceed if required to duplicate such extremely small lenses.

It has been learned that several years ago H. E. Dall of Luton, England, succeeded in slightly surpassing Herschel. The respective diameters are Herschel .02 inch, Dall .016 inch. The foci are: Herschel .0111 inch, Dall .010 inch. Even when used alone, Dall's lens gives a magnification of 1,000 diameters.

THOSE who make Cassegrainian telescopes must first make a concave spherical primary and alter it to a paraboloid, and then make a convex spherical secondary mirror to another radius of curvature, and at still greater pains alter it to a hyperboloid. Some years ago Dall, and also Alan R. Kirkham, decided that this method demanded much unnecessary work, and that it had been practiced for two centuries only as a bad habit. So they left the convex mirror spherical and, by altering the concave mirror to an ellipsoid, obtained as good a telescope as the Cassegrainian. This "Dall-Kirkham" telescope, sometimes misnamed "the spherical secondary Cassegrainian," is now a firmly established timesaver.

The amateur designer James H. Wyld of Denville, N. J., has now proposed a modification for which the name "wyldalkirk" is suggested, representing Wyld's special case of the Dall-Kirkham telescope. "No one," he writes, "seems to have thought of the equal-radius Dall-Kirkham. The advantage that I see in it is the reduction in the number of grinding tools. The secondary, having a radius equal to that of the primary, would be used as a subdiameter grinding tool for the primary, or as one of the grinding tools; a larger tool might also be used for smoothing.

"The primary," he continues, "would be polished to a spherical figure, then used, as suggested by James G. Baker, as a test plate to figure the spherical secondary. Finally it would be figured 'flat' (null) by the Dall-Kirkham test, with the light source at the proper distance to give the right correction. Then the whole would be assembled and collimated.

"I've worked out the proper corrections, focal lengths and so on for several different designs."

No way is known to make fascinating reading of the bleak specifications that follow, except to telescope making enthusiasts. To them they will be as interesting as they are significant.

In the table on the opposite page, unity is the radius of curvature of the primary and, therefore, of the secondary. All other dimensions, keyed by letter designation to the diagram above (which is not to scale), are multiples of that unit.

"Column 1," Wyld writes, "gives separation of primary and secondary mirrors along the axis.

"Column 2 gives the distance of final focus from the secondary.

"Column 3 gives the effective focal length.

"Column 4 gives the minimum ratio of secondary mirror diameter to primary mirror diameter. (Add to D₂ a correction equal to Sd/e.f.l., where d is the diameter of the field lens of the largest eyepiece to be used; this correction is usually approximately one half inch. )

"Column 5 gives minimum value of R/D₁ (for an OSC coma limit of .0025).

"Columns 6 and 7 give positions of the source and knife-edge respectively when using the Kirkham direct-focal test described in Amateur Telescope Making.

"Column 8 gives the primary correction as a percentage of usual parabolic :correction (for use in zonal testing of primary).

"A sample design for a 16-inch wyldalkirk follows:

"Take R/D₁ as 10, to be on the safe side in regard to coma.

"Take S as .34R, to avoid too large a value of D₂. (Since l' is less than S, a diagonal flat must be used to divert the image to the side of the tube.)

"R is 10 X 16, or 160, inches radius.

"S is .34 X 160, or 54.4 inches separation.

"l' is .2353 X 160, or 37.65 inches from the secondary.

"Minimum diameter of secondary is .32 X 16, or 5.12 inches.

"Correction to be added to this is .2353/.7353, or .32 inch per inch of eyepiece field-lens diameter. Taking the latter as one inch, D₂ may be made 5-1/2 inches.

"The total effective focal length is .7353 X 160, or 117.6 inches.

"The over-all focal ratio is 117.6/16, or f 7.35.

"The diagonal flat should be set forward of the secondary focal point by a little over half the tube diameter–say 10 inches in the present example. This places its center about 27.7 inches back of the secondary. The size of the flat is best determined by a graphical layout of the rays, to scale. In the present case it will be about 2.3 by 3.3 inches, assuming a one-inch eyepiece field lens.

"If the Kirkham direct focal test is used to test the figuring of the ellipsoidal primary, the pinhole or slit should be set 6.378 X 160, or 1,020 inches (85 feet). The knife-edge or Ronchi grating should be set .5425 X 160, or 86.8 inches from the mirror, or a corresponding 'dogleg' ray path by way of the diagonal flat used for diverting the beam. If the primary is tested by zonal tests at the center of curvature, the knife-edge motion between center and marginal zones (for a fixed pinhole) is .71 X 82/160, or .28 inch. Similarly, the knife-edge motion for the 50 per cent zone is .71 X 42/160, or .07 inch, and so on.

"A 16-inch mirror could be ground to size using the 5-1/2 inch secondary blank and a larger (10- or 12-inch) grinding and smoothing tool. These tools would be used alternately, to maintain good contact.

"The primary and secondary must be polished on separate laps. The primary is brought to accurate spherical figure first. Then the secondary is tested against it by interference and brought to a sphere. Finally the primary is brought to an ellipsoidal figure by the Kirkham test or zonal testing."

Who will pioneer the first wyldalkirk?

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