LAST MONTH in this department George P. Arnold, a graduate student in nuclear physics, State College, Pa., offered extensive and definitive data on design, testing, and performance of the Dall-Kirkham spherical secondary telescope. Figure 1, squeezed out of that layout, shows his telescope, a 10".

So well was the spherical secondary (why not simply the "s.s."?) principle regarded some years ago (1938) when Kirkham referred it to those in charge of the 200" telescope, that it probably will be used on the tenth-scale (20") model. Before the 200" was built a 20" model was made and tried out, and this very interesting model was not afterward tossed on any junk heap or given to any of us TNs to get it out of the way (maybe we should have asked!) but instead it was mounted on the roof of the Astrophysical Laboratory at "Cal Tech" where it will be used in its own right. Last January, during exchanges of mixed telescoptical lore with Porter such as have gone on by mail every few days since 1926, he wrote the following note in pencil on the bottom of a page: "In our tenth scale Cass-our guinea pig-the primary was and still is spherical as Dr. Anderson was in a hurry to get it working, and the secondary was corrected to fit it. Dr. Anderson is now computing the changes required in the primary if the secondary is made spherical and it seems that his procedure follows Kirkham exactly."

Your scribe recalls that at various times when "Kirk's" ideas were shown to Dr. Anderson via Porter they, and evidently Kirkham himself, were very well thought of. Telescoptics lost much when the genius Kirkham died after prolonged pathetic vicissitudes revealed in letters in files still preserved. "Kirk" was one day suddenly made highly aware that he had been ill with pulmonary tuberculosis for years, and it is a fact that his attempts to help other amateurs less well favored with a knowledge of design optics were a contributing factor in his fatigue and ultimate death. He wrote in 1938, when working on an invention: "If I can dope out a way to make diffraction gratings 10" long in a few minutes, I can go to the sunny southland-which I realize is about the only hope I have of living to see 1940." A proposal to collect a fund from amateurs was then made but Kirk apparently was not in favor of it. He died in 1943, at Pendleton, Oregon.

Returning to the Dall-Kirkham or, as Dall prefers, the Kirkham-Dall: In 1672 William Cassegrain proposed a telescope having a paraboloidal primary and a secondary to suit, which called for a hyperboloidal convex. It is passing strange that he thus was able to set the mode for almost three centuries, when the s.s. telescope with spherical convex secondary and elliptical primary was just as obvious and simpler. Probably, however, he asked for the hyperboloidal convex simply because he had the thought fixed in his mind that a reflecting telescope necessarily called for a primary which was paraboloidal.

This leads to a new question. Since Cassegrain didn't, himself, invent the actual idea of a telescope having a secondary mirror and perforated primary, for Gregory had already published it in 1663 (Bell, "The Telescope"): and since he failed to find the spherical-secondary-elliptical-primary idea only recently found by Dall and Kirkham, is the Dall-Kirkham telescope then a Cassegrainian telescope of any kind or at all? Why isn't it simply the Dall-Kirkham telescope? Thus deposed as a child of the Cassegrainian, it becomes instead a brother of the Gregorian and Cassegrainian. If this is logic then this is advanced as the declaration of independence or the Dall-Kirkham telescope. In time it is likely to supplant the Cassegrainian. The poorer form was discovered first.

That it took nearly three centuries to find a thing so simple and so obvious (after the event) is perhaps a reproach to the past personnel of telescoptics. Another such reproach is the obvious and beautifully simple Schmidt principle, only recently hit on. During the two decades of this department's telescoptical life many proposals for new principles have been submitted and many of them have proved to have been anticipated by someone or other. One gets the feeling that every possible avenue has already been explored and exploited, with nothing left to discover. History shows, on the contrary, that in such situations the unborn souls of more discoveries are flitting around invisibly overhead, eagerly waiting to be discovered and named for the discoverer. While most searchers evidently look for complexity, the best finds usually prove to be simple, like the s. s. telescope and the Schmidt.

BUILDERS of compound telescopes should, though they usually do anyway, work to a tight tolerance. Least squares treatment shows that when n surfaces reflect in a series, the tolerance should be 1/8. On the 200" telescope six surfaces sometimes will be used in series, calling for 1/20 wavelength. On a two-mirror telescope the tolerance is about 1/11 wavelength.

CHAMPION member of the American Association of Variable Star Observers, Cyrus W. Fernald, of Wilton, Maine, with 7216 observations to his credit for the year, also Dr. William L. Holt, Scarboro, Maine, and Winter Park, Florida, who made the Association's millionth observation since its organization, both use telescopes having Springfield mountings.

In the Springfield mounting, due to the added reflection of the second diagonal, the star field is reversed and, as Porter himself comments, "disoriented" for any kind of chart work. Asked how he was able to reconcile this difficulty Dr. Holt replies: "Fernald and I would not trade our 8" Springfields for Clark 6" refractors. He has a geometrical mind and the reversal does not bother him. It does me, and so I have used, since I began variable star observing in 1932, a mirror chart box (Figure 2) which, when the chart is placed face down above a mirror, normalizes it. The box is shaped like a wedge and the upper, left-hand face as seen or, rather, not seen in the photograph, is a strong sheet of window glass. It has a hinged cover to keep the charts from blowing away. The level bottom has a plate glass mirror 12" square. The side shown at the right is mostly open, to permit viewing the reflected image of the chart. The closed part below the opening serves to shield the eyes from the weak red flashlight installed within.

"The chart box," Dr. Holt states, "is attached to the end of a swinging, extended bracket and can be put in just the right place on either side of the telescope pier, so that one looks down into it at about 30 degrees (parallel to the glass back) and sees the chart, placed face down on the transparent glass, reversed. Thus the top of the chart becomes the bottom, but right remains right and left stays left. Finally, one has to adjust the chart on the glass back so that the east-west line corresponds with the east-west line in the sky as seen in the telescope.

"All this doubtless sounds complicated but it really very simple, both to make the box and to use it. It enables me to find and estimate magnitude rather carefully of 10 to 15 variable stars an hour. With the Springfield one wastes no time or energy and has no discomfort from nearly lying on the ground for a star near the zenith or mounting a stepladder for a star near the horizon. I observe by the hour, sitting comfortably on the high stool shown at the lower right-hand corner in the illustration. This is an advantage of the Springfield mounting that Fernald and I swear by. We believe we are the only A.A.V.S.O. members using Springfields."

Dr. Holt, a year ago, was runner-up for the A.A.V.S.O. championship. He observes summers in New England, winters in Florida, carrying the mounting and tube with him and attaching it to permanent pedestals having tops beveled to suit the respective latitudes. Continuing in praise of the Springfield, Dr. Holt writes: "The most important of all advantages and the one which provides the chief explanation of the miracle by which Fernald finds estimates, and records magnitude for 30 or more stars an hour, is the great ease in reading the setting circles; they can be read without getting off the stool. I have often worked for an hour or more and get 15 stars without getting off. Neither Fernald nor I use our finders or look at the sky at all in finding our stars; we just have the correct local sidereal time for the night and place, instantaneously (nearly!) subtract the star's R.A. from sidereal time or vice versa (I use a sidereal watch hung on the chart box), and set the star's hour angle and declination with help of the convenient slow motions, the whole process taking from 20 to 60 seconds."

CORNING casts mirror blanks up to 12-1/2" diameter in permanent iron molds and tries to keep them in stock but above that diameter they have to build up a special, temporary mold from firebrick and the make-ready costs them more than the glass and the pouring. Thus a single 20" blank, 3 1/3" thick costs the buyer $225 and a 16" blank of same thickness $140. But if 25 or more orders can be accumulated, so that the same make-ready can suffice for that many blanks, the price can be cut more than a little. Today the quotation is $37.50 net for each of 25 or more 16" blanks.

In 1941, Clyde Tombaugh sponsored a Sixteen-inch Club in which Corning agreed to provide 16" blanks at similar price, provided 20 orders received before pouring. This department (July and November 1941) lent its help and eventually Tombaugh was able to round up 33 members who ordered 37 blanks (this department, October 1942). These were delivered but the war sent most of them into temporary storage and only a few have yet emerged. Such holdovers tend to happily to deaden the spontaneity of a group where each member knows others are doing the same job at the same time and knows who they are.

This department now propose a new and additional or post-war Sixteen-inch Club of its own-though the Tombaugh group will be revived by its sponsor-and will take much interest in its progress and outcome. It obviously cannot undertake any kind of middle-man responsibilities of an actually business nature and therefore, pending accumulation of the orders necessary to put the Club over the hill, orders and cash must be deposited with Corning Glass Works, Corning, N. Y. Two potential orders are now known of as a starter. Please keep this department posted; it cannot keep posted through Corning since while C. F. Henkel, Jr., Corning's Manager of Optical Sales, is a friend of your scribe's (summers, we both bathe in and drink from the same lake and what more do you want? ) this does not make it ethically possible for any outsider to snoop into company orders to see how things progress in order, if necessary, to do a little supplementary selling to reach the quota. The Tombaugh club started dubiously but in the end 17 more orders than were necessary came in. But those same orders partly saturated the market, also. Sights are now set for 50 in order to hit the 25.

Tools: In 1941 some of the Tombaugh group bought 16" tools of plate at $7.80 each from Pittsburgh Plate Glass Co., Ford City, Pa. Many prefer to work 16" disks (about 50 pounds) face up with sub-diameter tools on a modified Draper or other extemporized machine. If a 16" tool is used it possibly should be channeled as described by Ferson in "A.T.M.A." (printings after June, 1944) to avoid risk of sticking and for other advantageous reasons there described.

TEST for mirrors, described by H. E. Dall, Luton, Bedfordshire, England: "First, if a mirror is f/7 or longer focus a careful center-of-curvature Foucault test will be good to 1/20 wave-length. I favor using masks with 1/2" holes. Then put two short vertical scratches, about 1/2 mm apart in black enameled glass, in front of the test lamp and arrange a good 1/2" Kellner or similar eyepiece in front of the lamp on a measuring stand capable of measuring to and fro movement. Adjust the eyepiece inside focus to give four equally spaced bars of light. Mark the position and repeat outside focus. The center of these two positions is the zonal focus and this method is capable of better accuracy than most."

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