"So there you have it," Walton writes. "Motor to thread-rod; traveler to brass strip to pulley; pulley to split ring; split ring to telescope." When the traveler has crept the length of the thread-rod, returning it to the beginning is a simple trick. And now for a little refinement that affords small manual adjustments, as in guiding. The left end of the thread-rod thrusts or butts square against the right end of another thread-rod. Now, if this second thread-rod is rotated in its own fixed screw, the effect will be either to add to or to subtract from the total motion of the telescope tube. This feature works as follows: On the nearer end of the second thread-rod is a simple pulley. A foot or so below this on the nearer post is another little Hansen motor belted to that pulley. Seen thrown loosely around the horseshoe, in the photograph is a heavy wire. This is the flexible distant, hand control for slowing or speeding the drive by operating that motor at will. A double switch, made from spring clothes-pins and held in the hand, does the trick. To advance the tube slightly, you give the little motor an electric kick. To retard it, you cut the current of the main motor. "No gears to throw in or out, no clamps, no nothing," Walton says. You push the telescope to a star and the drive takes charge from there. He sent some photographs-Pleiades, comet, nebula-made with this guiding control and these speak well for the whole equipment, including the 12" mirror of the telescope.

Walton's neighbor, Bohm, mentioned above, took one look at the drive and ran home to cook up one for himself (Figure 2). It varies a little from Walton's; few amateurs like to copy slavishly. It has no micro-adjusting feature for guiding, hence it is only for visual use. Main motor at right (cost $2) drives screw-rod (cost 5 cents) at 2 r.p.m. through simple clutch. Since the screw-rod has 32 threads per inch, the nut traveler which tows the telescope tube along moves toward the left at the rate of 1" per 16 minutes, and Bohm points out that it is a matter of simple arithmetic to find how far out from the center of the polar axis to attach the little rod that connects the traveler to the lever that drives the tube final adjustment may then be made exact if there is a slotted hole in this lever. Bohm also has a friction connection between his lever and polar axis, this being the equivalent of a friction disk drive, and this permits quick large shifts while the motor is running.

The weight seen pendant in Figure 2 pulls on a silk fishline running over two small pulleys and to the traveler. This helps the motor and holds the rod against the thrust bearing near the extreme left. What at first appears to be a flywheel, at the left, is simply the convenient handle of an end-thrust screw for the screw plug which takes the end-thrust of the thread-rod.

After about an hour's running the traveler reaches the end of the thread-rod (what it actually reaches just before that, however, is a little limit switch which shuts off the motor in case the observer forgets, thus forestalling a jam). At the end of the hour's run the handwheel is unscrewed and removed, the thread-rod slid far enough to the left to disengage the "clutch" connection at the right. Then, by means of the little; crank handle visible below, the traveler is spun back to the starting point in a hurry, ready for the next hour's driving. It sounds complicated, but actually it all works quickly.

Bohm says that, with this drive, it is possible to keep an object in the center of the field with a high-powered eyepiece for the hour's run on the thread-rod.

IN NO place in "A.T.M.," unfortunately, is it clearly pointed out that the handle to be attached to the mirror disk for grinding and polishing (unless the worker prefers to omit it entirely, as some do, and simply take hold of the disk itself ) is not intended to be grasped in the hand in the typical manner of a handle but is rather a convenient centering device, There is evidence that some beginners do, however, grasp it in one hand, full length, and tightly, throughout grinding and polishing. The result often is a badly turned edge since the pushing effort is usually too high, also since it is practically impossible not to introduce undesirable lateral force components when working this way. (You can pick up a cat by the tail, close to the body, and swing it without a protest, provided you don't bend the tail in any part of the swing, but the cat will tell you from experience that, when you say you don't, you only think you don't.)

Probably the whole trouble into which many beginners are misled derives from the unfortunate use of the term "handle." In "A.T.M.A.," Everest discusses the effect of pressure applied too high and shows one excellent form of centering device: a 4 1/2" by 1 1/2" wooden disk, for working, carries on its top a 4" by 1" handle for lifting the mirror. This keeps the working pressure low.

STELLAFANE convention, Springfield, Vermont, Saturday, August 2.

SPHEROMETERS for measuring sagitta, or depth of curvature, r²/2R, of a mirror, or the curvature of a lens, may be made from a ten-cent-store micrometer caliper, as shown in Figure 3, which is redrawn from a sketch submitted by D. Everett Taylor, 192 Prospect St., Willimantic, Conn. Saw off the anvil and frame part and substitute the two arms and legs shown. And don't forget that the r²/2R now employs the r of the spherometer, not that of the mirror!

WHILE nearly all amateur telescope makers test their mirrors at the center of curvature, nearly all professionals test at the focus, with a flat as an accessory; and it is a rather amusing commentary that, even granting the superiority of the test at the focus, some professionals have tested in this manner for so many years that they have come to think, and one of them even to say, that it isn't even possible to test at the center of curvature. It does require a little more mental effort, it is true. However, after one has provided the set-up, the test at the focus is a big convenience, and there are other considerations: more rigorous, for example.

William Buchele, 2832 Sagamore Road, Toledo, O., sends us the photographs shown in Figure 4 and says:

"This is a gadget for testing at the focus with a flat. Light source is a 100-watt projection lamp. Its housing has cooling flanges to prevent the lamp from overheating. A thin silvered diagonal reflects light through a hole in the flat, it returns from the glass under test, and passes through the diagonal, thus permitting the light source and the eye to be in the same train simultaneously. The gadget has a micrometer screw feed. The dark upright strip in the center is a graduated system of fine and coarse pinholes and slits. There is also an eyepiece, knife-edge and Ronchi grating holder, with lateral rack and pinion feed."

TEST for short focus mirrors, used 1 by telescope-making members of the Detroit Astronomical Society and reported by Eugene G. Brown, 4404 Vermont Ave., Detroit, Mich., is shown in Figure 5. At b is the light source, c the perforated mirror under test, d a paraboloidal test mirror, e the knife-edge or Ronchi grating. "The image produced," Brown writes, "is identical with that produced by a sphere under the Foucault test. We therefore work to this flat image and we interpret our high and low zones exactly the same as we would with the Foucault test.

"Such a test is necessary to produce a good figure in an extremely short focus, such as f/2 or f/2.5 (unless we use the Gaviola test)," Brown continues, referring to the fact, still not sensed by all, though Ellison explains it in "A.T.M.," that while you usually can get by with a visual estimate of the smoothness of sweep of the curve of a medium or long focus mirror between inside and edge zones, provided only the latter are correct, you cannot safely depend on this on a short focus mirror. Even when the intervening shadows then look smooth, they are so dark that they may easily mask local irregularities which you therefore may let go in ignorance of their existence.

Brown adds that Ralph Tozer of Detroit is the first there who used the test described above. He points out that there are other applications and variations of this test. For example, if a point-source of light is placed at the focus of the paraboloid, a parallel beam is projected from the latter, and this artificial star may be used to test any astronomical instrument, refractor, reflector, or camera.

Another wrinkle, suggested and used by C. M. Davenport, of the Detroiters, is piped-in light for this test. The object ab is a bent rod of solid l/4 " or 3/8" Lucite, a transparent plastic [obtainable from E. I. duPont de Nemours, Inc., Plastics Department, Arlington, New Jersey and bent in very hot water or oil. Ed.]. Instead of placing a primary light-source at b, where its heat would interfere with the test, this is at a, well out of the works, and the light follows through the Lucite to b. The end of the Lucite rod may either be left straight but beveled, or bent, but in either case tinfoil is wrapped around its end and touched with a finely pointed needle ("A.T.M.A.," page 89).

W. S. Bohlman, 823 West Street, Wilmington, Delaware, has similarly used Lucite to pipe light from a removed primary source to the pinhole, thus avoiding a burned nose, also permitting the pinhole to be placed close to knife-edge. He used the Lucite rod from a common throat light such as those commonly on sale at drug stores. This served as well behind a Ronchi grating as in place of the usual pinhole. He suggests a piece of Lucite bent to 90°, its end covered with foil having a pinhole for testing secondary mirrors.

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