Figure 1 is an 8-1/2", f/6 reflector made by Harvey H. Boven, 304 Seventh St., Red Wing, Minn. Tube, galvanized iron. Cell, a Chevrolet hub crown filled with lead (for counterweight). P.A. bearing, an 18-1/2," split wooden pulley on two non-bearing 3" brass rolls "which give me all the friction needed," the maker states. This brings up the fact that too little friction may be a won t stay-put nuisance unless there is perfect balancing and a clamp. Boven also invites correspondence.

Figure 2 is a 7-1/4" f/8 made by Alfred Bryant, 516 Egleston St., Kalamazoo, Mich. His P.A. bearing is an old gasoline engine flywheel and the P.A. shaft is l-3/4" in diameter. To it is attached the gearing of an old phonograph motor with an extension hand rod which is used as a slow motion. Base of fork, heavy washing-machine casting. Sides, boiler-plate. Tube, 17 pounds, 3/8'' thick, spirally wound paper with five coats of paint, "strong enough to hold an elephant."

Figure 3 was made by L. H. Strum, 232 First St., Petersburg. Fla., and has a 15" mirror. The P.A. bearing is braced with four struts.

FOR solar, and often for lunar, observation a telescope often needs some kind of light and heat reducer. In the following paragraphs D. Everett Taylor, 191 Prospect St., Willimantic, Conn., author of the ATMA chapter on the construction of the metal parts and mounting of a refractor, also of various items previously published in the present columns, and always a finished worker, tells how to make a Herschel wedge.

"In 'Amateur Telescope Making', third edition, page 147, and fourth edition, page 179," Taylor says, "the late Prof. Charles S. Hastings closes his chapter on astronomical oculars, with the following paragraph: 'I venture to add the following, under the impression that the Herschel wedge is not nearly as much used as it should be. With it Venus, so unsatisfactory an object in a dark, or darkening sky, is a delightful study. Then the moon, also, except when a rather slender crescent, is much pleasanter to view with this accessory. Ordinarily this object is so brilliant that the pupil of the eye is contracted so that only part, perhaps a small part, of the objective is effective, which may he the cause of a prevalent impression that the moon is too easy an object to afford a test for the excellence of a telescope.'

"Bell, in 'The Telescope,' page 166, briefly describes the solar diagonal devised by Sir John Herschel. He shows a schematic plan of this solar diagonal or Herschel wedge and says, 'In viewing the sun only about 5% of the light (and heat) is reflected at this upper surface to form the image at the eyepiece.' On an adjacent page, in describing a star diagonal, Bell also says, 'The right angled prism is replaced by a simple elliptical prism of small angle, 10 degrees or less, with its upper face accurately plane and at 45 degrees to the axes of the tubes.'

"The above quotations comprise all the published data known to this writer. Albert G. Ingalls states that he knows of no other literature on the subject, aside from the references here given.

"A Herschel wedge is a most satisfactory accessory and, if constructed after the accompanying drawings, without sacrificing any of the required accuracy, the result will be most gratifying and the performance will leave nothing to be desired. Like the star diagonal, the Herschel wedge is especially suited to and indicated for the refractor. There is, however, no reason why it cannot be used successfully on a reflector-in which case the position of the eyepiece would be changed, and the normal focal and tube lengths of the reflector should permit of being shortened the three or less inches which the light travels through the accessory. There is no appreciable heat from the Herschel wedge when viewing the sun-in fact, with it there is no difference in comfort between sun gazing and star gazing. Attention is called to the glass filter disks (Figure 4). These are used when viewing the sun but are removed from the tube when the wedge is used for viewing the moon, Venus, or the landscape during daylight. The latter is an interesting pastime because of the views one enjoys, especially of trees and their foliage.

"When using the conventional methods, in photographing the sun and sunspots, the work must be hurried because so much heat is generated at or near the focus that a camera shutter which is not of metal is likely to be damaged or destroyed, but by using the Herschel wedge with its filter disks removed there is no necessity for hurrying the work, because the heat is so completely dissipated that this question is no longer a consideration.

"The following notes on construction describe materials and practices used in making the Herschel wedge shown in the drawing, Figure 4.

"Stock: Brass throughout, except the glass wedge or prism. Tubes machined from suitable sizes of brass pipe.

"Prism: Made from plate glass 1/4" to 5/16" thick. Angle of finished wedge to be 8 to 10 degrees. Upper surface which reflects the image, if plane to 1/2 wavelength is of acceptable quality; 1/4 wavelength however, is better but is sufficiently good. The prism was cut to its cylindrical shape, which looks elliptical, by the method described by Selby, page 126, AT\IA.

"Construction: The surfaces between parts A and B were first machined to a fit, the then contacting surfaces were carefully tinned and the parts joined by sweating, which after the standard eyepiece O.D. of 1-1/4" was machined on part A.

The circular intersection on the end of part C was machined to fit the circumference of part B. It is a safe practice to fill part C, before machining the curve, with a turned hard wood mandrel, which will support the wall of the brass pipe when the latter is clamped in a two-jaw chuck. The next step is to bore the inside diameter of part C to a shoulder which will support the filter disks: The internal diameter of C should be the standard 1-1/4" for a standard size of eyepiece. Locate and bore a 1" hole in part B, over which part C is to be centered. The contacting surfaces between parts B and C should be tinned. Part C is centered over the hole in part B and held firmly in place with several turns of fine iron wire, thus binding the two parts together. By placing three or four pieces of solder the size of a pinhead on the inside of the tube at the joint, then heating the tubes in a Bunsen flame until the solder flows, a neat soldering job will result, leaving little or no solder in evidence. Working a thin burnisher around the outside of the joint, after soldering, will finish the joint to a high state of perfection.

"Parts D and E are tubes which assemble inside of part B. If preferred, parts D and E may be combined as one tube. The two-part construction, however, is more complete because it facilitates adjustment and adds to the outside appearance in clean design.

"Part D telescopes into part E. The friction or fit of joint should be sufficient to hold the parts firmly in adjustment. One end of part D is machined to an angle (preferably on a milling machine); this angle, plus the angle of the glass prism, should make 45 degrees. In other words, if the angle of the glass prism or wedge is 8 degrees, the angle of the tube should be cut to 37 degrees, if the wedge is, say, 10 degrees the tube angle should be 35 degrees. On the diagonally cut end of part D sweat a piece of 1/16" brass plate to cover the entire angle or elliptical end of the tube. Finish the margins of this newly added piece flush or parallel with the tube, as shown in the insert drawing.

"A hole is to be bored through the angle face of part D, longitudinally with the tube. Therefore, center and chuck part D in the lathe. The diameter of this hole should be approximately 1/8" smaller than the diameter of the glass prism. It is assumed that the prism has been made after the method presented on page 126, ATMA, which gives the prism a cylindrical shape with elliptical sections or faces. Therefore, its diameter is the greatest dimension of the prism's width or minor axis.

"Before removing D from the chuck, locate the prism on it over the hole, with equal width of brass showing around the prism. Scratch the outline of the prism on the brass base. Outside of this scratched line drill four 1/16" diameter holes, parallel longitudinally with the tube. These holes are to be spotted as shown in the insert drawing, and they are to carry the 1/16" diameter wires which will hold the prism. This method of holding the prism was chosen in preference to several other methods, because it is simple and direct to make, holds the prism securely without pinching it, and simplifies the assembly and disassembly. These four wires should be long enough so that, when in place, they can be seized with pliers through the rear and open end of the complete assembly. Each of the two lower wires which hold the point of the wedge or prism should have a small piece of soft brass or copper soldered on the end which is to be bent over the prism when in place, thus making a hook. A paper-thin piece of cork should be placed between hook and prism. The other two wires are straight pieces which will easily be understood when the parts are assembled. The clamping ring for holding the four wires in place (also a similar ring for holding the filter disks in place) should be a 1/4" to 1/2" length of thin-wall (.030") brass tubing of suitable diameter. Open this piece of tubing to make the spring, and cut out a piece of the tubing to permit snapping it in.

"Part E requires no comment except to call attention to the knurled collar which should be sweated on the end of the tube, also the locking screw with nut in front of the collar, as shown in the drawing.

"Assembly: Telescope part D into part E. Put the four wires in place and clamp them with the clamping ring. Adjust the upper wires so that they extend through the face of part D 1/8" or more. Adjust the lower or hook wires to extend beyond the face of D an inch or less: Now place the prism under the upper wires and with pliers pull the hook wires through the clamping ring until the prism is held in place with the hooks. Remember to place the thin piece of cork under the hook just before the hook is made up snug against the prism. The remainder of the assembly is of course obvious, as is also the necessary adjustment. To preserve the outside finish of the brass parts, lacquer them with a thin coat of Bakelite lacquer No. B13128 or BH1805. Allow this to dry for one half hour. Then bake in the kitchen oven for one hour or a little more at 200 degrees F."

IN Figure 5 the Herschel wedge is shown at the left. Near it is a three-lens Ramsden eyepiece made by Taylor and at the right is another of his gadgets, a micrometer focus control. This may be used on any telescope having a standard 1-1/4" diameter eyepiece fitting and it moves the eyepiece assembly in or out of focus, similar to a rack and pinion. It employs the helical slot principle instead of a thread and Taylor says it is a sweet device in operation. It is also large enough to get hold of.

Figure 6 shows Taylor's RFT refractor recently completed; in fact, he states that it is pretty close to the RRFT specifications in ATMA. It is a 2-1/2" f/6 and the knurled band is the rotor of the micrometer focus control shown in Figure 5.

THE following is taken from a letter from H. E. Dall, 166 Stockingstone Road Luton, Beds., England, a co-author of ATMA: "I've made a strain viewer, using Polaroid, which is so easy and comfortable to use that there is no excuse for working on any untested glass. It consists merely of (1) a lamp in a blackened box having a hole covered with a Polaroid disk, (2) a ground glass screen on a stand (mine is 10" x 10"), (3) a pair of American Polaroid goggles worn by the viewer. It shows up the strains so brilliantly that there is no need even to test in a darkened room. The glass to be tested is just held between the screen and the observer. The analyzing Polaroid has its axis at right angles to that of the goggle.

WE learn that the Bailey and Sharp Co.,

Hamburg, N. Y., who handled Chance Bros. optical glass, have sold their manufacturing facilities to the Optical Glass Products Inc., of Hamburg, which will mold lenses, prisms, and other optical parts. The Ednal Company, 160 Fifth Ave., New York, N. Y., are now the American agent for the glass of Chance Bros. and Co., Ltd., Birmingham, England.

CIRCULAR dividing engines are discussed in ATMA, which shows a picture (page 297) of an "ultra" type at the Bureau of Standards, capable of dividing a circle within an accuracy of about one second of arc, about a millionth of the complete circle. The late Dr. Ambrose Swasey, the professional telescope builder, made one of comparable refinement and the following, taken from an article in the Journal of Applied Physics (New York), written by the astronomer, Prof. J. J. Nassau, of the Case School of Applied Science, Cleveland, may make readers take in their breath.

"Last May Mr. Swasey was asked, 'What in your opinion is the greatest thing you have done?' All at once his eyes sparkled, his face brightened more than ever, and it seemed that a war veteran was getting ready to describe a famous battle. 'The highest type of construction and piece of work I ever did,' he said, 'was the dividing engine. When you can take a spindle 4" in diameter and about 25" long with 5/8" taper to the foot and make that spindle fit into a bearing easily and when you drop it a thousandth of an inch it goes hard, you are getting down a refinement of which we knew nothing in those times. Dividing engines had fallen down on the spindle-could not get a spindle in the bearing that would fit.' The dividing engine was built primarily for "graduating circles of astronomical instruments used for fundamental star work as well as for instruments in geodetic surveying. It has an error of closure of one second of arc and required three years in building."

Feats like this rank with making ruling engines-ne plus ultra.

ACTIVITIES on 20" reflectors: The New York group-the Optical Division of The Amateur Astronomers Association- have worked a 20" Pyrex disk (20-1/4"), solid type, to f/2.4 curve and are boring it out for a Cass. Their optical workshop is in the basement of the Hayden Planetarium, which provides spacious and ideal quarters. It is even air conditioned-what sybaritic luxury! In Philadelphia the Amateur Astronomers of the Franklin Institute are "exploring possibilities of building a machine to grind the 20" blank," according to their monthly publication, The Observer. Their headquarters are in the Fels Planetarium. They have a luxurious machine shop.

A group of advanced engineering students at the College of Engineering, University of Kentucky, Lexington, Ky., is said to be designing and is to build a 20" reflector. Earl G. Welch is one member of this group. At the College of Liberal Arts, University of Louisville, Louisville, Ky., a similar project was under way some time ago, according to Walter L. Moore, but we hear no recent news.

In Boston, the Amateur Telescope Makers of Boston are also planning a 20", and drawings of the proposed mounting were recently published in The Telescope (Cambridge, Mass.). The mounting is unique and distinctive: it is half German equatorial, half double yoke and half fork. The tube like all Gaul, is divided into three parts, the first of which is closed, the middle part absent and the top part latticed. The space occupied by the non-existent middle part is filled up with the ether if there really is an ether, and the top part is held aloft by this ether and or hypnotism. The Pyrex disk is to be half solid and half ribbed, because there are two schools of thought in this club just as in all others. Similarly, there is to be half a hole in the disk, representing a sensible compromise between the Newtonian and Cassegrainian factions. All this once more shows the marked flair for compromise which we Anglo-Saxons (or what are we?) usually do more bragging about than exercising. The telescope, however, is still in the paper stage and there are hopes that a single type can be settled on-either peaceably by Boston methods, or by a gang fight à la New York.

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