A BRIEF STUDY OF Lieut. Col. Alan E. Gee's unconventional telescope, shown in Roger Hayward's drawings below, is enough to reveal its simple secret. It would be a conventional refractor if its tube were not "broken" and jointed at the center. At this point it has two cubical box castings connected by a swiveling joint. In each box is an optically flat mirror. The eyepiece half of the tube remains pointed downward, parallel to the earth's axis, but the objective half is free to dip or turn sidewise. When it turns sidewise the eyepiece end rotates in its fixed bearings.

Thus the Gee telescope is an equatorial coudé, or "elbowed," refractor similar in principle to the widely known type devised in 1882 for the Paris Observatory. It differs from that instrument, which is diagrammed in Louis Bell's book The Telescope, in a number of excellent practical modifications which are of interest to all planners of small telescopes.

"A refractor of this size on a conventional mounting would be extremely bulky," Colonel Gee points out, "weighing hundreds of pounds if it were to equal this one in stability. Even using the eyepiece on the coudé as the handrest when getting up or down from the observing chair will not disturb the image in the least. The reason is the location of the eyepiece at the highly stable apex of a broad pyramid. The instrument performs beautifully and is well worth the very considerable time and effort it took to build it.

"Without its two removable counterweights the whole telescope, which is made almost entirely of aluminum alloy, weighs only 40 pounds and can be folded nearly flat and carried by one person.

"The tube can be moved at will to a new part of the sky, and the slow motions take over from there. The right-ascension slow motion near the eyepiece is a 360-tooth 18-pi diametral-pitch worm gear. I cut this 6.4-inch gear on my Globe miller with a hob that was made from drill rod and hardened. I made the hob for a 14 1/2-degree pressure angle instead of using a standard tap because I have noticed that worm wheels cut with taps are not very satisfactory.

"The declination slow motion at the elbow is a positive-action tangent screw without a spring, working on a floating friction plate in typical Springfield mounting fashion.

"The right-ascension bearings consist of a 3 1/4-inch bronze sleeve bearing at the eyepiece end and a one-inch brass sleeve bearing in a forked 'mermaid's tail' resting on the ground. The declination bearings consist of a pair of seven-inch-diameter bearing plates that are integral parts of the two boxes containing the diagonal mirrors. The 3 1/2-inch-diameter hollow central stud has a threaded, removable retaining ring.

"The rack-and-pinion focusing tube has a 1 3/4-inch clear aperture and permits the use of low-power, wide-angle eyepieces. Three eyepieces are used- a 1 1/2-inch wide-angle modified Erfle, a one-inch Erfle and a very good two-thirds-inch Brandon orthoscopic, plus a 2X Barlow lens. These three provide powers from 60X to 260X. The telescope handles the high power nicely.

"The Barlow lens is a Laboratory Optical Company (Plainfield, N. J.) product of minus 4-inch effective focal length, mounted in a tube to provide 2X magnification. Various war-surplus Barlow lenses have appeared on the market in recent years with elaborate claims for impossible optical characteristics. Those I have examined are simply negative achromats of less than minus 2-inch focal length, far too short for optimum performance. Actually, minus 4-inch is on the short end of desirable focal length and minus 6-inch would be better. Barlows I have made for my own use are of minus 6 1/2-inch e.f.l. and leave nothing to be desired in performance.

"The mirrors at the elbows are mounted on push-pull screws for adjustment and can easily be aligned by autocollimation. They are one-tenth wave aluminized optical flats and are highly important to the image quality. They must be mounted completely strain-free, and gave me much trouble until they were given three-point suspension on little cork pads that pinch them between the metal backing and the hold-down lugs. To give satisfactory results they should check within one-tenth wavelength of flat when mounted.

"The finder consists merely of a rear peepsight with an eighth- to a quarter-inch opening (the exact size is unimportant) and a forward stud whose end is coated with luminous paint. Such a finder is very simple to construct, and is superior to the usual optical finder of considerably more complexity. With it an object can easily be placed in field of view of the telescope.

"I did not make the objective.

"Were I to build another telescope this type I would use ball bearings place of the upper sleeve bearing, whose inevitable play made it very difficult to adjust and maintain the fit of the right ascension drive gear. Ball bearings would alleviate this. Probably a pair of retainerless 'propeller-type' bearings or a single 'double-row' bearing, would best. In any case the two rows of balls would be necessary for stability of worm seat.

"It is true that there is a loss of light at the two elbow mirrors of this telescope, but the loss is negligible."

Amateur telescope makers of long standing will recognize Colonel Gee as the co-builder of the 12 1/2-inch reflector shown in Amateur Telescope Making-Advanced, pages 319-322, and more recently of a 20 1/2-inch Cassegrainian Portland, Ore. He also is the proponent of a practical method of testing Cassegrainian secondaries and the author of theoretical studies of that problem. Colonel Gee recently completed two years' study of optical design at the University of Rochester's Institute of Optics, and is now Chief of the Fire Control Division (which includes optics) at the Frankford Arsenal, Philadelphia, Pa., but he remains one of the widespread and enthusiastic fraternity of amateur telescope makers. Wherever his vocation sends him his lathe and other tools, like Mary's lamb, are sure to go. He has made numerous mirrors and flats, and also a dozen identical Barlow lenses of 1 3/8-inch diameter minus 6 1/2-inch e.f.l.

IN THE DRAWING in Figure 3 Roger Hayward has worked out the effect of a coudé telescope on images of star groups or of the moon in four different attitudes for the four cardinal directions. (The images are represented by a letters.) If the objective tube of the telescope is shifted to the opposite side of the eyepiece tube, these different attitudes change still further. Thus it all seems to add up to confusion. In practice, however, the user comes to know his star groups well no matter how they are thrown at him-whether inverted, erected, perverted or reverted. In this respect the coudé is closely related to the Springfield, each having two added reflections, but with a difference. in the Springfield these follow a course "up-across-up," in the coudé "down-across-up."

Brandon orthoscopic eyepieces, no longer available from the original Brandon source, are said to be obtainable through the Valley View Observatory, 106 Van Buren St., Pittsburgh.

USING the grinding strokes described in Amateur Telescope Making, the beginner usually can excavate about 95 per cent of the desired concavity of his telescope mirror in less than two hours, preparatory to the much slower remaining stages. A few, however, encounter a baffling, often maddening refusal of the glass to change from flat to concave. In final desperation and before considering suicide, some of them send poignant appeals for help to this department.

These SOS calls are always regarded with sympathy. On his own early mirrors this writer encountered the same total depravity of inanimate objects. The following entry has been found, dated November 7, 1926: "Half hour spell. Increased stroke cadence to 120 full strokes a minute, sweating copiously, but curve still refuses to deepen."

The frenzied speeding up to 120 strokes per minute was intended to force the curve to deepen, but the cause of the failure in this instance was precisely too rapid strokes! A note concerning this source of frustration was latest inserted in Amateur Telescope Making, page 244.

The following are related problems submitted by other sufferers similarly at their wits' end. No two workers describe identical circumstances, which is an added difficulty in diagnosis and prescription.

From Ralph B. Saunders of Detroit Mich.: "After 20 hours of hard work with the coarsest abrasive and the use of every known stroke from Amateur Telescope Making and Amateur Telescope Making-Advanced, the curve on my 6-inch mirror is still only one thirty-second inch deep, but the thickness of the disks is vanishing into thin air. The glass is plate and the tool is Pyrex."

From Abdulazim Aziz, Nicosia, Cyprus: "In grinding an objective lens 4 3/4 inches in diameter the glass wear was abnormally high, with the result that the disks now are too thin for specifications. I ground dense flint glass on top of medium barium crown 10 1/2 hours. Result: thickness removed was more than 200 per cent of depth of sagitta on the upper disk and 87 per cent on the lower. I also ground white plate on dense flint 20 1/2 hours. Result: thickness removed, 267 per cent of sagitta. Strokes, cross and back, from the center of upper to edge of lower disk, 16 per minute."

Puzzled as a physician would be if called upon to diagnose by mail, the writer sometimes passes such problems around among those in the mirror-making fraternity having more than average experience. In reply, Albert H. Johns of Larchmont, N. Y., an amateur who worked in wartime as a professional precision optician, commented as follows: "In answering the second inquiry I would mix in a little professional dope. The upper disk should be pushed forward until its center is over the far edge of the lower, and on return strokes brought back center over center; in the early stages, not so far as that. For a very deep curve the same result is obtained by grinding the center of the upper disk over the edge of the lower at the side, though this tends to hollow the center too deep; it is too local.

"A way to avoid excessive wearing away of glass," amateur-professional amateur Johns continues, "is to observe the following simple procedure. Suppose we are using coarse Carbo. If the center of the upper disk has been contacted by the farther edge of the lower disk, then those are the only areas that are ground away. Stop grinding with coarse Carbo when the lower disk shows at the center an untouched spot of, say, one-inch diameter, as shown at 1 in the drawing at the bottom of this page, and the upper disk has an untouched rim of, say, one-fourth- to one-half-inch width. This diameter of untouched area works out well on f/15, but shallower curves should have a larger untouched diameter. If the worker is a beginner or is roughing out with coarser Carbo than No. 120 (which gives little or no net gain anyway), a 15-inch untouched spot should be left on the lower disk.

"Proceed with the next size of Carbo, and when the pits left by the coarsest size are removed a central spot of about one-half-inch diameter should remain as shown at 2, with a decrease in width of the untouched ring on the upper disk. The central spot should always be perfectly round and exactly centered. Any departure from this indicates astigatism, but can be corrected by working on one side to bring the spot to center. Similarly, on a concave surface press on a wide margin while grinding. The use of regular strokes, however, should avoid these difficulties.

"Continue as above through finer grades until No. 600 or equal has been reached. Regular one-third strokes there develop a complete spherical curve, and the entire faces are fine ground, as shown at 3.

"This method of grinding ensures that a maximum thickness of glass will be left. Professionals use cast-iron tools and high speed-200 to 300 revolutions per minute on spindles-but in grinding down the blanks a spot is left as described, to ensure full thickness at the finish."

The attentive reader may inquire, "In what essential, if any, does the stroke described above differ from a stroke from the center toward the worker?" States Johns, "Theoretically it would be the same. However, pushing forward gives better control of pressure and of general manipulation while turning the disk. Pulling would probably bring it into one's lap. Exerting pressure while pulling the disk toward the near edge would be awkward. Also, for long grinding periods one applies frequent dabs of abrasive to the lower disk without removing the upper disk, but the substitute stroke would require placing dabs on the far side while the disk was being drawn toward the worker. By selecting the proper stance, arm's-length work should bring the center of the upper disk to the far edge of the lower where it can't be pushed off."

Commenting on the case first described, that of Ralph B. Saunders, Johns wrote: "Using a Pyrex tool might lengthen the time consumed." Saunders soon reported: "I have discarded the plate-glass mirror disk for one of Pyrex, grinding it against the same Pyrex tool as before, and it is coming beautifully to curve without complications other than poundings on our door by the occupants of the apartment below us, who complain no less than before about the reverberations that resound from above them. What would Mr. Johns suggest as a remedy for this?"

Many, including the writer, have ground Pyrex mirrors on top of plate-glass tools with no complications whatever; why then should the reverse create difficulties? In any scientific experiment it is a basic principle that only a single circumstance should be changed at one time: there should be but one variable. A big source of difficulty in the diagnosis of mirror-making ills is the fact that, by the nature of the work, more than one variable is often introduced without the knowledge of the worker. He may then attribute the outcome to the wrong factor and make erroneous deductions as a result. According to his temperament, he regards the baffling situations then produced as either a "headache" or a fascinating contest of wits.

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