CONCRETE, offered as a material for telescope making by Russell W. Porter in "A.T.M." (chapter on mounting design, Figure 7, "Porter's Folly") has not yet met with general favor, perhaps because there is a feeling that concrete is crude. Frederick H. Minard 834 Thayer Ave., Los Angeles, California, with Edward A. Carson of Los Angeles, has now constructed "Porter's Folly" of concrete in 10" f/10 size (Figure 1). The conical polar axis is made of 16-gage steel attached to a 27" flywheel and poured full of concrete. That part alone, with the concrete-filled, 16-gage steel fork, weighs 1250 pounds.

Asked to particularize, Minard (Figure 2) writes:

"After carefully studying the two volumes on amateur telescope making we decided that 'Porter's Folly' was not at all, as he nicknamed it, a folly but a feasible idea. So we set to work and it took us the better part of a year to complete the entire installation. Edward A. Carson was associated with me; in fact, the telescope was constructed and installed at his estate in Bel-Air, this city. The results proved the soundness of Mr. Porter's idea and it is a most satisfactory instrument in every way.

"Upon viewing the instrument it would never be suspected that the main elements were constructed of concrete. The basal block, containing three adjusting screws on the bottom, is a plain concrete block, poured in the usual way in a wooden form. The latter was removed and it remains a concrete block, but it is all below the observatory floor level and is not visible. It is covered by removable floor sections.

"Now comes the disguise of the visible polar axis-the right ascension circle is a flywheel, 27" in diameter, into the hub of which is fitted a long 2" shaft, trued in a big lathe. On the lower part of the shaft is welded a 16-gage steel disk about 12" in diameter. Then a 16-gage truncated steel cone was rolled and welded on to this assembly, the large end fitting inside the rim of the flywheel and the smaller end on the 12" disk.

"Into this assembly were placed the long rods which hold the declination bearings and around these rods were placed shaped 16-gage steel forms and these forms make the outline of the yoke, which shows in the photograph.

"Having this concrete-metal structure all set and trued up, the entire layout was stood up with the small disk end down and the entire form was filled with concrete.

"The only concrete surface that shows is in the center of the flywheel around the hub top and, as this is painted black, no one would suspect there is any concrete in the construction. Not even a trace of a crack has ever appeared on this small exposure. In fact, the entire polar axis looks as though it were turned out of steel.

"One can kick the polar axis to the extent of almost breaking a toe and there is no vibration of the telescope tube.

"The telescope driving mechanism is located at the lower end of the polar axis, entirely below the floor level, in a concrete vault. Consequently, the observer is always in a comfortable observing position and even at the meridian has only to mount two steps above the floor level.

"The weights are: Tube (16-gage rolled steel), 55 pounds; mirror and cell, 29 pounds; lead counterweight ring, just above cell, 350 pounds; tube bearing ring with trunnions (one-piece aluminum), 26 pounds; upper end ring with flange and revolving head (aluminum), 26 pounds; revolving cone, steel-jacketed concrete, 1250 pounds; concrete three-point bearing block, 1500 pounds. Total, 3226 pounds.

"It is a very stable telescope and free from vibration, yet the part showing above the floor does not look massive.

"The observatory is fitted with flanged wheels on the bottom of the dome and the entire dome revolves on a 20pound circular rail on a concrete foundation. The observer can easily push it by hand. The floor of the observatory is of concrete on which is laid heavy linoleum."

Porter, who went to see this telescope and its handsome and practical dome, reports: "'Porter's Folly' is at last vindicated. It's a mighty good job and it works. Steady as the Rock of Gibraltar. A concrete telescope mounting!"

True, this telescope is largely concrete but it still is not quite a concrete mounting unashamed. Yet, though so much of the concrete work we see here and there looks ratty, it remains nevertheless possible to do concrete work that looks finished and refined. To accomplish this let the worker forget most of the working technique he has picked up from sundry laborers, foremen, and contractors. Let him then obtain scientific instructions from a sound source (for example, the Portland Cement Association, 347 Madison Ave., New York) and carefully study those instructions and the principles behind them. And then let him follow them to an extent that seems fussy, taking special pains to obtain low water content, to ram the placed "mix' well and cure it really adequately. Such work should be free from the cracks and spalls and other flaws commonly seen in concrete.

SOME seeds gestate a long time before sprouting. The short chapter o "Dealing with Spider Diffraction," in "A.T.M.A.," reached notice in 1941 in the July-August number of The Journal of the Royal Astronomical Society of Canada (198 College St., Toronto, Ont.) in an article by C. H. Werenskiold, from which the following is quoted:

"This method consists in placing, in the open end of the telescope, a diaphragm having four elliptical openings, in such a manner as to cover or hide the four straight arms of the spider and in effect provide them with a curved outline. A modified procedure is also described in which the spider arms are covered with small plates or screens of curved outline. The spikes or cross-bars in the star images were eliminated in both cases.

"The use of diaphragms or screens naturally causes a certain loss of light, and it has occurred to the writer that this loss can be reduced to a minimum by curving the spider arms themselves, which would render the use of diaphragms or screens unnecessary.

"Several methods for accomplishing this are possible, of course, such as suggested in Figure 3, at 1, 2, and 3, and where E indicates the position of the eyepiece and P that of the assembly supporting the prism or diagonal.

"Up to recently the writer has employed the 'straight' spider construction of 4, made from brass strip approximately 1" x 1/16", in an 8-1/2" reflector, and no substantial mechanical weakness or vibration due to the omission of the fourth spider arms has been noted. The star images with this spider naturally exhibited the usual spikes. In considering the various possible curved spider designs, the one shown in 1 appeared most attractive from a mechanical standpoint, since its arch-like design offered strength and stability, together with a reduction of the number of spider arms to two. A curved spider of this type, made from brass strip approximately 1" x 3/32, was substituted for the straight spider formerly employed and has been found both mechanically satisfactory and effective in eliminating the spikes from the star images.

"Furthermore, the definition obtained in the observation of planets, such as Jupiter and Saturn, appeared to be considerably improved. This is readily understood when we consider that the bright disk of a planet as seen in a telescope constitutes a pattern composed of smaller light elements. Each of these, depending on its individual brightness, gives rise to more or less luminous cross-lines or spikes when a straight spider is employed, with the total result that the image of the planet is accompanied by a hazy, more or less noticeable, cross-band of light, approximately as wide as the diameter of the planet and oriented in the manner of the usual star spikes. It seems obvious that the definition would suffer under such circumstances, the fine detail originally rendered by the mirror being again partly obliterated by the superimposed cross-band of light. Since no such cross-band is formed when a curved spider is used, the definition is improved accordingly.

"As to the practical construction of the spider 1, it was found desirable to prepare an exact drawing beforehand and to calculate geometrically the overall length of the spider to determine the exact position of the bend adjoining the telescope tube. In order to obtain satisfactory results, one must make certain that each spider arm is curved continuously without any straight sections. As the work progresses, the shape of the spider should therefore be compared carefully with the drawing. A liberal additional length of strip was allowed for fastening the spider to the telescope tube with short bolts. The holes for the bolts were elongated lengthwise of the strip, and the corresponding holes in the tube were elongated at right angles to this, that is, lengthwise of the tube, to the extent required for adjusting the spider to its correct position.

"The more strongly curved forms shown in 2 and 3 may have certain advantages in construction, such as lessened danger of straight sections but whether these modifications are otherwise practical has not been investigated.

The same subject is discussed in mimeographed Mailing 20, of the "Astronomical Information Sheets" (a service for placing information on new comets, novae, occultations, and so on into amateur astronomers' hands quickly: 20 mailings, a dollar, G. B. Blair, Dept. of Physics, University of Nevada, Reno, Nevada, Editor) by Clarence W. Parham Sr., 2354 Woolsey St., Berkley, California, who has had excellent results with the streamlined spider in his 6" reflector and will be glad to answer any questions proposed by interested amateurs. He made his spider of a single strip of 3/32" x l-1/4" brass bent as in 2. Where sharp bends were needed at the ends, the strip was grooved with a hacksaw, then bent, and the groove filled with solder

In Mailing 44 of the same "Information Sheets," Carl E. Wells, 419 Oak St., Roseville, California, states: "Mr. Franklin B. Wright, 155 Bret Harte Road, Berkeley, drew my attention to the fact that this spider must be curved in multiples of half circles-that is, one half circle, one circle, one and one half circle, and so on. Also, your prism holder should present a full circle to the light rays. A square prism holder will destroy the effect you are trying to get with the streamlined spider."

TO THOSE who are designing Maksutov telescopes, the following, from the author of the articles in the October and December numbers, should be of interest. "I wish you would kick me good and hard, as there is an error in the December article, page 285, first column, at bottom. The figure .122 should instead be half that, or .061," and the other figures mentioned in relation to it are wrong to the same extent. I picked up the figure for a diameter instead of an aperture radius My apologies to all and sundry."

The Maks are still delayed because those who had nearly finished them suddenly took war production contracts.

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