IN HIS CHAPTER ON THE warmed observing room in Amateur Telescope Making, Russell Porter discussed a collection of ingenious arrangements by which an amateur astronomer may sit cozily indoors and direct his telescope outdoors through a partition. Porter built several of these indoor telescopes, and all his life sought to originate new arrangements of the same sort for others. Probably no one else in the long history of telescope making so searchingly explored the possibilities of new types of mountings. To this favorite activity he applied all his versatility and inventiveness, remaining uninfluenced by the cramping conventionalities which, as he often remarked, cause many astronomers to look with suspicion on novel mountings. Porter also noted with the keenest interest the combinations that were submitted to this department by other amateurs. (He always considered himself an amateur.) He never ridiculed an idea because he had failed to think of it first, though he did turn thumbs down on some that seemed impractical. When one of these was sent to him by the editor of this department, Porter would usually return the proponent's letter with the simple notation: "Nothing to it-P." His trademark was a circle drawn around the P. Unhappily most of the proposals proved to be original with the inventor but old in the lore of telescopes. Sometimes, however, Porter would remark cautiously and hopefully: "I almost believe this is a really new one." He was pleased that so many minds were exploring where once he had worked almost alone.

Frank McCown, Box 176, Holtville, Calif., has submitted the proposal for a telescope in a warmed observing room that is shown at the upper left in the illustration below. On preliminary analysis this arrangement appears to contain elements of the Porter turret telescope at Stellafane, shown in Amateur Telescope Making at VI in Figure 42 on page 51. Yet McCown's telescope includes other details.

The polar axis is expanded into a cone with end-thrust bearing at the south. This cone, and the telescope mounted at one point on its periphery and counterweighted at the opposite point, are out of doors. The eyepiece and observer are inside the warmed enclosure. Of course this warmth will radiate outside, to the detriment of the image, unless the enclosure is well insulated and not too thoroughly heated. Even then the image will be somewhat affected; all telescopes involve many compromises, as every telescope maker knows.

The design is not well suited for either too large or too small a telescope. Its proportions are virtually dictated by the physical proportions of the observer, unless he aspires to become a contortionist. The illustration shows a V-shaped pier that provides leg room in rather the same manner as a kneehole desk.

McCown's design for an eyepiece mounted eccentrically in a sleeve which can be rotated to bring the finder focus into view is shown at the upper right of the illustration. An erecting eyepiece stretches out the cone of rays from the primary mirror to the eye, obviating the need for an unduly large diagonal mirror. As an alternative a Barlow lens has been suggested by Henry Paul. The smaller central inset drawing indicates the analogous eyepiece scheme for a refractor. The design is offered for others to mull over, modify, improve and perhaps construct according to their lights.

The two drawings in the middle of the illustration show two versions of a method that permits a double-yoke-mounted telescope to reach the celestial pole. It was devised by Alan McConnell of Millstone, N. J., who suspects, however, that it is not new. His own telescope follows the first version, but he proposes the second.

A. WALLACE EVEREST of 85 Ridge Avenue, Pittsfield, Mass., writes as follows: "To remove zones on a mirror there is one procedure I forgot to mention in Amateur Telescope Making-Advanced. Zones are usually related to channel spacing in the lap. For a central bump use strokes such that the center of the mirror travels over the centers of the facets in the two rows nearest the center of the lap. [This is shown at the lower left of Figure 1.] Occasionally change the direction of your strokes 90 degrees to prevent 'lemon peel.' For a hole in the middle use similar strokes, with the center of the mirror traveling along the two channels nearest the center of the lap.

"Don't overdo these strokes, as they work fast. Any slight hangover from this treatment will disappear with the zigzag-stroke which should be used during the rest of the figuring. The width of the zigzag patterns (path of center of mirror) should be equal to the channel spacing. That is, work out to an overhang of one half the channel spacing on one side, then back to the same overhang on the other, and so on, back and forth.

THE drawing at the bottom center in the illustration reveals the principle of an automatic tracelet or toolhead used by professional instrument makers in England for graduating circles on surveying instruments. A brief description of this was recently discovered in The American Machinist, Volume 41, page 813. By varying the heights of the bumps on the drive wheel desired sequences of long and short lines can be cut again and again without error. This supplies a more elegant alternative to the method depicted in Amateur Telescope Making-Advanced, page 290, Figure 3, and renders superfluous the anvil shown there. In an alternative design found elsewhere the graver is lifted by an eccentric mounted on the same shaft that carries the bumps. Possibly the user can further simplify this mechanism. In numerous instances those who have graduated their circles by the ordinary, or memory, method reported the fallibility of the human equation in the form of single slips in line length that threw the system of the entire circle out of correctness.

IN THE lap-channeling dingbat sweepstakes J. William Wright of Baltimore, Md., has entered the wooden ring with cross-wires shown in the drawing at the lower right of the illustration. First the lap is poured in the usual manner. When the lap has lost most of its fluidity, its surface is dipped briefly in cold water to form a skin over its fluid interior. Only the surface can be dipped because otherwise the hot glass tool would be broken. Then the channeling tool, which has previously been soaped, is pressed clear through to the glass and left several hours to cool before removal. The secret of success is in correctly judging when to press in the channels. The substitution of flat strips of sheet metal (resistance wire from an old toaster) in place of thick round wires enabled Wright to make laps having the desirable thickness (twice the wire diameter plus your favorite lap thickness). Two courses of heavy round wires resulted in laps that were too thick.

Making Focograms: Dr. C. P. Custer 155 East Sonoma St., Stockton, Calif. has made many of these. For a camera he uses a cigar box with film taped inside one end and a 1/2-inch hole, placed about where the eye was, in the other end There is no lens. The source of illumination for the pinhole lamp is a 7 1/2-watt, circular, frosted refrigerator bulb 1 1/2 inches in diameter, which can be screwed into a standard-sized socket.

First Custer adjusts the knife-edge Then he places the camera on its narrower edge on top of another cigar box. This has three nails along the sides and front of the camera for stops, so that the hole is in proper position.

Now the room light is turned off and the camera is loaded by taping inside its end a 2 1/4 by 3 1/4 Eastman Super Press Type B film, with the concave emulsion side facing the hole. A 10-minute exposure is tried using a 1.25 millimeter (1/20-inch) pinhole. With the smallest pinhole, about an hour's exposure is needed; for paraboloids, about two hours. A very sharply turned down edge may require a narrow mask on the mirror, probably because of light reflected from its back surface.

The films are developed by using the 10-cent packages of Eastman developer and fixer according to directions. The image is 1/2-inch wide and enlarges well.

Time can be saved if the camera is carefully removed after the exposure is finished, and the image examined by leaning over from the side. If it has turned black or brilliant white, a movement has occurred and development of the film is useless. Correct the knife-edge and re-expose. Vibration caused by people walking about the house will ruin the exposure.

"My camera is a home-made box with a 2-inch lens and a plateholder for 4-by-5 film, which I originally made for a camera to photograph star fields. To make focograms the lens is removed and the box is placed some distance behind the pinhole. Thus during exposure there is room to place the eye behind the knife-edge occasionally to check the setting-a necessary precaution when the local steel mill causes the earth to vibrate.

"The extended path to the film makes a large image at the expense of longer exposure. About one hour is right for my 10-inch mirror, varying from 20 minutes to three hours according to the figure and the focal ratio. The nearer the mirror is to spherical, the longer the exposure required to produce an image of sufficient density

"With a 1/25-inch diameter pinhole I use a 25-watt oven lamp, but this is too weak. On long-focus mirrors it is difficult to see the cone of rays to set the knife-edge."

Theoretically the time of exposure should be increased in proportion to the square of the focal ratio. Thus a mirror of f/8 should require increased exposure over an f/4 as (8 x 8) / (4 x 4), or 4.

INSTRUCTIONS for the adjustment, alignment and collimation of telescopes do not make lively reading, but are most welcome when they are needed, which is every time a new telescope is assembled. One telescope maker recently wrote that this task, for which he had scarcely made allowance in advance, proved to be a major one; another worker said that it would be impossible to publish too wide a variety of instructions for the job. Walter R. Redmond of Greenwood, N. Y., a teacher of science and mathematics, contributes a rigorous new method of aligning reflecting telescopes:

"Many an amateur has been ready to throw out his mirrors lock, stock and barrel because of poor results, when in reality lack of proper mirror alignment has been the chief reason for his distress.

"It is hoped that the method to be described will prove helpful for those who have some difficulty in making a quick alignment of the optical system of their Newtonian reflector.

"Preparation for making the first alignment by this method will require some time, but for all realignments thereafter the optical and geometric alignment of the telescope should take a matter of minutes. It will be assumed that the primary mirror can be centered with respect to the telescope tube, that there are adjusting screws for tilting the primary in relation to the body of the tube, that the adapter tube is perpendicular to the telescope tube, and that the secondary mirror has provisions for both rotational and tilt adjustment.

"The first step is to make a circular hole in the primary mirror coating, about 1/8-inch in diameter and in the exact center of the primary. This hole will of course lie within the silhouette of the secondary mirror and therefore can in no way affect the light-gathering power of the primary. To locate this hole precisely the mirror must be carefully measured at its diameter. With a compass the radius can then be found, and a pin-point mark made on the primary coating at the center of the mirror.

"Next a circular piece of cardboard is cut out with a diameter of about 1 1/2 inch and a 3/16-inch circular hole in the center. This cardboard is placed over the primary so that the pin point in the coating is in the center of the 3/16-inch hole in the cardboard.

"Then with the tip of a pencil eraser which has been somewhat pointed with a knife an opening is made in the primary coating by rotating the eraser in the hole in the cardboard [drawing at the upper left-hand corner of the illustration at the left].

"The next step is to remove the eye piece from the adapter tube and fix cross-hairs over its end, using white thread attached to friction tape wrapped near the end of the tube. As shown in the upper right-hand drawing, one cross hair should be placed longitudinally with the telescope tube, the other perpendicular to it. Some may desire to make a permanent piece of equipment for this part of the test. In my case a 6-inch extension of lightweight tubing was obtained which fitted snugly in the adapter tube, having the same outside diameter as the eyepiece. At one end of this tube cross-hairs were arranged as described but wrapped more securely by friction tape. With this prepared tube, the eyepiece can be removed, the prepared tube inserted, and the telescope alignment checked in a matter of seconds by this method to be described.

"This will be a convenient time to check the position of the secondary mirror, to make sure that its effective reflecting plane is centered with respect to the center of the adapter tube, which will now be marked by the intersection of the cross-hairs. This can be done, by measuring from the open end of the tube to the top and bottom of the secondary, taking an average of these two readings, and then measuring down the inside of the tube to a point in line with the intersection of the cross-hairs [drawing at upper right]. If this last measurement agrees with the average of the other two, the center of the secondary will be opposite the adapter-tube opening. If it does not agree, longitudinal movement of the spider assembly will be necessary until it does.

"The secondary mirror should now be removed from its support and the primary mirror and cell placed at its proper location. The worker should then step to the open end of the telescope [drawing at left center] and sight back to the primary mirror through the small central opening in the spider assembly left by the removal of the secondary mirror. If the opening in the spider assembly is e not circular it may be made so by pasting over it a small piece of paper with a hole of about 1/8-inch diameter. This opening must be accurately centered with respect to the diameter of the telescope tube. Then when the primary mirror is properly oriented with respect to the tube, the opening in the spider assembly will be reflected directly back to the pupil of the eye, and will appear coincident with the opening in the coating on the primary. In fact, when the primary has been properly aligned by adjusting the screws on its cell, the reflection of the pupil of the eye will appear to be exactly cut out by the opening in the coating of the primary.

"This alignment of the primary is basic and must be carefully done. When the described conditions are met, the optical axis of the primary and the geometric axis of the telescope will be coincident.

"Next it will be necessary to sight from the back of the mirror [drawing at right center] through the opening in its coating and through the central opening in the spider assembly toward some small shiny object from 150 to 200 feet away. When the sun shines this object may also serve as an artificial star for testing. I use the silvery portion of a discarded radio tube with good results. When this object is sighted in, the telescope is left without further movement until the alignment is complete.

"Now the secondary mirror is placed n its assembly [drawing at bottom left] and, while the worker looks through the center of the adapter-tube opening, it is rotated slowly until the reflection of the pupil of the eye and the opening in the coating of the primary mirror fall upon the cross-hair which is in longitudinal alignment with the axis of the telescope tube [drawing at bottom right]. Then the spider is adjusted until the reflected image of the pupil of the eye and the same opening meet at the intersection of the cross-hairs. At this point the following conditions may be noted: the intersection of the cross-hairs and of their reflection, the pupil of the eye and the opening in the primary are all coincident. If the extension to the adapter tube was prepared as described above, the foregoing conditions may be quick]y checked at any time in case bumps, jars or temperature changes alter the optical alignment.

Suppliers and Organizations

The Society for Amateur Scientists
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Phone: 1-401-823-7800

Internet: http://www.sas.org/

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