"Not content with merely suggesting a new and untried mounting for a telescope, as I did in the fourth edition of A.T.M.' I have taken the bull by the horns and made one (Figure 1). Members of the 'Hundred-to-One' Shot Club (100^-2 Club) recently took it out into the Mojave Desert for a night's try-out. [Editor's Note: The Club named has been heard of before, though our resolving power fails to define its exact outlines from this distance. It appears to be a conglomeration of TNs and other highly intelligent people supposedly addicted to things scientific, who trek to various godforsaken places within motor radius, fry steaks, eat smoke and ashes and put up with one another's theories about the secret of the universe, measure the sun's altitude with sticks and strings to make sure that orb is not getting lost, concocting new mountings, sleep in the sand, sit on cactuses dodge sidewinders, and get all tired out resting up. They do these things well in California.] Surprisingly, it developed no serious drawbacks, and proved at once its convenience as a support for a Cassegrain tube assembly.

The optical parts are an 8" primary and secondary contributed by Byron Graves, slightly modified by an aluminum coat applied in vaco by Dr. Strong. The mirror is about f/3.5.

The mounting is portable and may be quickly assembled or taken down and packed into a box that goes in the car.

'The leg (arm of fork) A was hewn of sugar pine. In assembling, it is slipped over the tapered end of the polar axis B, and drawn home with butterfly nut C. Likewise, the tube goes on the other end of the leg, but here the connection is the stud D and circular track which makes the declination axis.

The polar axle consists of two parts that telescope namely, F made of 2" Shelby tubing, and G of 1" solid steel rod. The steel point H at the end of the solid shaft defines the lower (thrust) bearing of the polar axis, the north bearing being merely a cradle of V-block I attached to the two detachable levers J, J. With the telescoping polar axle and open upper V-block bearing the lining up of the polar axis with that of the earth is easily accomplished on any terrain. The steel point found that a dimple in a boulder (shown in the sketch) made a reliable south bearing.

"Slow motions in Dec. and R. A. have long tangent arms bearing against screws K and L. The R. A. arm, having a hinge at M can be transferred from one leg to the other as desired.

The instrument is completely counterpoised. The advantage of getting rid of one arm of the fork (but at the same time doubling the strength of the other) is in the room thus made available in the vicinity of the gooseneck eyepiece when the telescope is directed to the northern heavens.

Graves supplied the tube complete. [Graves in the picture (Figure 2) is the second figure from the left, under the western hat who is doing the heavy looking on while several members of the aforementioned club are on their knees getting the latitude within a mile by means of shadows under Porter's guidance and using only a piece of string, Porter reclining.-Ed.]

"The only machining on the mounting itself was done on a simple drill press.

"We detected some vibration and by tapping different parts of the mounting thought we had located it either in the wooden arm itself or the two legs supporting the V-block. These vibrations can be eliminated by increasing the cross-section of the arm and legs.

Now that the one-legged mounting has gone through the fire of a Mojave Desert test, and survived, I feel no hesitancy in recommending it to any one who may lack a machine shop but wants a convenient portable support for his Cassegrain telescope."

FROM time to time puzzled workers have asked us why "Pyrex" mirror disks usually come with a ring on the back which either stands above the rest of the back surface, like a, Figure 3, or below it, like b; also why the sides of the disks are slightly tapered instead of parallel; and why the face of the disk is usually concave. Many ask why there are bubbles in the glass. Mere inspection of the disks themselves is likely to leave the worker without the answers, but a few minutes' observation at the shop of the Corning Glass Works, when disks are being molded, would quite readily provide them.

(1) The ring: The disks are pressed in a cast-iron mold having a cross-section essentially like that shown at c. The "gatherer," as the workman is called, tries to pour into the mold just enough molten glass to fill it. Now it is quite easy to fill a receptacle rather exactly with some ordinary liquid, such as water, but not so easy to do it with molten glass. Still more difficult is the "Pyrex" brand glass from which the smaller telescope disks are made, for this has a relatively high melting point-about 2800 degrees F.-and it is impracticable to reduce it to the convenient fluidity of water. Instead, it is "worked" as a thick, slowly crawling mass, the amount poured into a given small mold being better describable as a gob than a liquid. However, the experienced glass worker, even then, can estimate the size of gobs fairly closely-he has been estimating gobs all his life. So he pours in his gob.

Next, this gob man claps on the metal ring d, and then down comes the plunger e, to force the reluctant molasses-in-January to fill out the mold by means of pressure. Now, if the gatherer's gob was estimated just a trifle too small, we get what is shown at a; if too big, we get b.

(2) The non-parallel edges of the disk: These represent the "draw," just as in a molder's pattern. If the metal mold were given a parallel side the cooled disk would be difficult to remove.

(3) The concave face of the disk: This represents shrinkage of the glass after cooling. Shortly after the plunger is raised, the disk, already pretty cool-for while it is difficult to bring "Pyrex" glasses up to the temperature where they work easily they drop in temperature with alacrity-is slid out by inverting the mold, and what we now see is a disk that is dark all around the exterior but still has a red hot lens-shaped part in the middle, something like f. As the disk, now lying on its back, goes on cooling and contracting, the center "falls," just like an unlucky cake. Now, if this "cake" would only be so kind as to fall the same amount in each case, the fall could easily be compensated by making the inside of the mold c a bit deeper at the center, thus attaining the desired flat-faced disk. But glass isn't a much more obliging performer than pitch-and you know what that means.

(4) The bubbles: In a low-viscosity fluid like water or, say, beer the bubbles rise rapidly to the surface, but in the viscous molten "Pyrex" glasses the smaller ones cannot make the grade fast enough, and so they get frozen in before they reach the top. A lot of good scientific gray matter in the glass industry has been worn down thin on this old, old problem but the bubbles we still have with us. Even in the much more easily melted crown glass used in fine photographic lenses it is impossible to eliminate them all.

Speaking of these same bubbles, what to do about them when they are broken into in grinding is something else again/ Obviously their fragile edges ought to be reamed out, so that fragments of glass will not become detached and cause bad scratches. Just how to do this is something we cannot advise, never having had the experience. If a dozen workers who have actually done it would report what they learned, the published accounts would be useful to the next fellow and the next.

NOW for some of the cussing about ordinary plate glass disks. Four years we have heard complaints that these were not truly round; one man claimed his was a triangle. Here is how these disks come to be sub-polygonal. First you have a slab of plate glass. On it the glass worker scratches a circle. Next, he puts the slab in a sort of clamp, so that it is held horizontally with an edge overhanging. He grabs a man-sized, pair of tongs-about a yard long-and begins rapidly breaking off hunks of glass. In 60 seconds or so he has a sub-angular disk with bulges around its periphery. He takes this to a slowly rotating horizontal metal plate, like g, on which a stream of water steadily brings down coarse abrasive' and, holding it in his two hands he rapidly grinds off the bumps. He has no gage but his eye, though that is pretty fair. It could of course, be done on a centered arbor, giving a true circle, but then the disks would cost more. Naturally, a truly circular disk is more satisfying to the user's sense of neatness. Optically the shape, whether round or the shape of Texas, makes no difference. Mechanically a round disk is not liable to flexure due to its shape, but an irregular shape, if bad enough, may be.

One amateur claimed a disk that wasn't quite round-and they may vary a sixteenth inch or more-caused flexure that altered in different positions of the telescope. He was from Manchuria-and we are from Missouri.

THE Dr. Calder around whose work Professor Russell's article is built this month, is one of the amateur telescope making fraternity-a former physicist who, after making a telescope, made more, and shifted from physics to astrophysics for good.

PRESENT and former radio "hams' abound among amateur telescope makers and these, as well as others, will take interest in a telescope drive (Figures 4, 6) described by Wilbur Silvertooth, 273 Ximeno Avenue, Long Branch, California.

"The driving system to be described was designed in an effort to secure all of the desirable refinements essential to a perfected unit without recourse to the usual complicated mechanisms. While constructed for a moderately sized telescope, the principle is equally applicable to larger instruments where its conveniences would be more readily felt.

"The gear train has been reduced to the simplest form. It consists of a 100-tooth worm gear connected with the polar axis through a friction plate; the worm is shafted in common with a 144-tooth spur gear which meshes with a 10-tooth pinion on the shaft of the motor. This synchronous motor, which turns one revolution per minute, is available commercially in both 50- and 60-cycle models.

"With such a gear system the polar axis will rotate once in 24 hours when operated on the regular line, the accuracy being a function of the stability of the supplied frequency. By interposing some method of frequency control between the line and the motor, the rotational speed of the telescope may be varied. Numerous complicated systems of motor-generators, tuning forks, pendulums, etc., have been tried; all having the disadvantage of added moving parts and limited flexibility.

"By substituting for such devices an electronic frequency changer, these drawbacks are eliminated; while such decided advantages as remote control, complete flexibility, inexpensiveness, and silent and vibrationless operation are secured. The only moving parts are the control dial and the motor armature.

"The small remote control contains three potentiometers. Two are employed to set the range of frequencies to be covered, as well as the location of those frequencies in relation to the one being altered. The third is employed as the actual control, covering the band as set by the initial dials.

"Considerable experimentation was done in order to secure a stable oscillatory circuit. The one described and finally decided upon shows no tendency to drift when compared with either the line frequency or harmonics of standard laboratory oscillators. Additional refinements of switching arrangement make it possible to change from the line to the unit from the remote control. Other switches on the unit make it adaptable for various purposes. The circuit was designed by Ed. Sawyer. The gearing arrangement being made by Mr. Edward Lester. Any further information or details of circuit constants will gladly be supplied by the writer. "

In further explanation of this drive, Mr. Silvertooth adds the following, in a letter.

"The speed of a synchronous motor is dependent on the frequency (oscillations per second) of the voltage supplied to it. If this frequency is changed, the speed of the motor will also vary. In the described electronic frequency changer this is done by changing the frequency of an oscillating circuit. Variable resistors are employed to do this. The rest of the equipment is merely to amplify the output of the oscillator sufficiently to run the motor. With a different amplifier a larger motor could be used if this were desirable. The condenser C₃, and choke T₅, must be selected to yield the desired frequency. This is the only part of the device that might require adjustment. It is not necessary to use the fundamental frequency of the condenser and choke: any one of a number of harmonics may be employed.

"Since sending you the description 1 have completed another driving unit of the same general description, and it functions equally as well as the first refined system."

ABOUT that book-"Amateur Telescope Making-Advanced." more can be said next month than now as there has been delay due to difficulty in getting proofs back from some of the authors. We begin to think authors are almost as bad a lot as editors, but this is not meant as an insult. The chief difficulty in making announcement of a book in a magazine is that you cannot do it as of the time the reader will receive it. As this is written, on Nov. 25, the last re-mailing author writes from England that the "next ship" will bring his corrected proofs. All the corrected galley proofs must then go back to the printer for alterations. Back come page proofs, again to be read (over 300,000 words). Only then can one say to the printer: "Print, bind, ship." It is a complicated business. Even the above omits most of the slimy details.

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