IN ASTRONOMICAL telescopes the image appears upside down, backwards, or both. The astronomer soon comes to regard this as normal, and would be confused by a correct image of the moon or a planet. Leaving the image uncorrected saves added lenses, prisms or mirrors that would cause needless aberrations and loss of light.

In terrestrial telescopes, on the other hand, most users prefer normal, erect images. Vern E. Hamilton of Inglewood, Calif., has worked out an arrangement for turning the image right side up in the ordinary Newtonian reflector, although as he says, the idea is probably not new. Below is a drawing of Hamilton's arrangement by Russell W. Porter, one of the last drawings Porter made before his death. Hamilton writes:

"In this telescope the conventional Newtonian diagonal is placed a little nearer to the principal mirror than usual to allow the focal plane to extend farther beyond the side of the tube. After the cone of rays emerges from the tube it is turned back toward the principal mirror by a roof prism in the position shown.

"This arrangement completely erects the image, with top on top, left on left, and–much to be desired–with the observer facing the object. It also permits the telescope tube, if it is not too large to be rested on the observer's shoulder instead of on a tripod. This design allows the relatively- easily made reflector to compete with the more difficult refractor in such instruments as hand telescopes, spotting scopes and super-binoculars."

Porter added an insert to his drawing to explain the reflections that take place within a roof-angle prism invented by the Italian astronomer-microscopist, Giovanni Battista Amici, 1784-1863. This prism, not to be confused with others invented by Amici or with the roof angle prisms of other inventors, is designated as the Amici roof-angle prism. Complex as it appears a roof prism is simply the right-angle prism with its hypotenuse folded lengthwise to make a roof with slopes intersecting at right angles. The prism still turns the rays at a right angle and still reverts the image, but it adds to these an inversion, so that top comes out on bottom and left comes out on right. The prism's appearance is complicated by three small facets that have no optical function and are merely the stubs of edges and ends that lie beyond the useful field and so are ground off to decrease the prism's bulk.

"A workable substitute for a roof prism can be made from two first-surface mirrors," Hamilton points out. "Fasten them to .m adjustable mount with the reflecting planes in a position that corresponds to the roof planes of a roof prism. Then, while viewing with the completed telescope some distant wires placed diagonally in the field, adjust the roof angle until the wires in the upper and lower halves of the image coincide. The joint at the roof ridge should be as good as possible, but need not be perfect, since it is out of focus of the eyepiece. Another substitute arrangement is to keep the two mirrors parallel to the regular roof planes and place them so that all the rays strike the first mirror, then the second mirror, and then the eyepiece. This arrangement has the advantage of having no roof-ridge joint to degrade the image, and the roof angle is not critical, since any misalignment merely rotates the entire field. The disadvantage is that it uses up more focal length and so requires a larger Newtonian diagonal.

"Of course these rough mirror arrangements will not equal a roof prism which has a perfect ridge joint and a light transmission of some 90 per cent, but they permit an amateur amateur to make in one evening a satisfactory roof-prism substitute good enough for most terrestrial uses. A professional amateur can obtain a larger roof than is practical with a solid prism. It may well be that substitutes of this kind will largely supplant roof prisms if recent developments to increase the efficiency of reflective coatings prove successful.

"A little time spent in tracing reflections through the optical train shows that the image will not come out like the scrambled monster in the left-hand drawing but normal, as in the other."

Instead of tracing reflections through a complex optical system on paper, it is safer and less confusing to set up the optical elements in dummy form and test the result. Experiment with a concave mirror, a flat, a roof prism and an astronomical eyepiece has verified the correctness of the Hamilton telescope. Anyone who builds this telescope should avoid crowding on high magnification since this magnifies vibration and unsteadiness in proportion. Hamilton says that he learned to follow airplanes in flight, "after a fashion," with 28 power, but that 10 to 20 power is more practical for hand-held observation.

John M. Pierce of Springfield, Vt., has pointed out that there are but three positions in which a lens-type erector gives an erect and normal image with a Newtonian telescope: 1) with the eyepiece tube horizontal and on the right side of the telescope; 2) with the eyepiece tube horizontal and on the left side of the telescope and 3) with the eyepiece holder vertical and on top, and the top of the observer's head turned toward the object. However, in no possible position can a Newtonian with a simple lens-type erector give an erect and normal image with the observer facing the object. Thc Hamilton arrangement makes this possible and, in addition, eliminates the aberrations and extra length of a lens-type erector.

There is a considerable demand for terrestrial telescopes but relatively little literature about them, and that little is scattered and fragmentary. Among amateur telescope makers there must be at least one or two who have explored this subject widely, perhaps built and used a number of types of terrestrial telescopes and, in short, made u specialty of them. Such persons are urged to make available to their fellows what they have learned.

A QUARTER of a century ago Russell Porter set a fashion of answering the letters of fellow amateur telescope makers by scribbling replies and comments in the margins or on the back of the letters. This method, followed by this department when exchanging notes with workers, has the advantage not only of spontaneity and simplicity but of placing the answer where it belongs, so that the record is fully integrated. It is less easy to make the "gears" of a formal reply mesh with those of the original inquiry.

Thus today scores of papers with Porter's annotations, short and long, often accompanied with little sketches and personalia, are preserved as valued records and frequently turn up when piles of accumulated papers are sorted over. From 1925 on, this department weekly sent to Porter a fat batch of letters from amateurs, they were returned with interlineations in his clear round handwriting. No conventional trimmings were bothered with. Often these letters were forwarded to other workers, and from them gathered fresh marginalia-serious, humorous, cynical, even unprintable. Many of these round-robin "files" turned into slow-motion conversations, like a game of chess played by mail.

Not long ago, before Porter's death, one of these papers was reread and, in the belief that it would be interesting and useful to others, it was sent to Porter with a request that he make a drawing suitable for publication. The drawing appears below. The exchange of notations ran thus:

Porter: "S'pose you wanted to make a very deep convex paraboloid, how'd you s'pose you'd go about it?"

The reply: "Not knowing, I cannot say."

Porter: "Well, I do it thisaway. The glass rotates while the template slides over it, back and forth. The linkage keeps the template always vertical. Throw on the abrasive and both glass and template automatically go paraboloid, the only curve we know of that can be made that way. Starting, say, with a hemisphere, or any segment of a sphere, and a thin, circular template, you can keep going and get any focal length you want. Of course, the above is a convex paraboloid but it works equally well on a concave one.

"The reason why this gadget works this way lies in the equation of the paraboloid and the fact that every section cut from a paraboloid, like a, b, c, d, e, is a parabola and all are identical.

"If you slide the spherical tool A down the side of a sphere it will touch only at B and therefore will begin wearing faster at that point than anywhere else along its edge. Conversely, the sphere will begin to wear away too. It's easy to show just cut a piece of cardboard so it fits over the center of a tennis, croquet or billiard ball and slide it down, keeping it always vertical."

Later Porter wrote: "That convex paraboloid is to be used as a mold from which to cast concave plastic mirrors to be used for some kind of infra-red research and the experiment consumed 111 hours of my time. After Anderson [maker of the 200-inch mirror] tested it and said it was 20 times better than the tolerance laid down I felt greatly elated, 1t for he is not in the habit of exaggerating."

IDEALLY, when the amateur's telescope is not in use it should remain out of doors under an observatory dome, a runoff roof or at least a tarpaulin. But circumstances force what is probably a majority to keep their telescopes indoors and lug them outside for each use. Since even a six-inch reflector may weigh 100 pounds or more, this often results either in diminishing use or in such a reduction in weight that the instrument is shaky.

Edwin F. Bailey of the Astronomy Section of the Franklin Institute in Philadelphia hit on a happy solution of this dilemma. He used as his telescope base a 10-gallon expansion tank from a hotwater heating system. With the telescope unscrewed, this tank may easily be carried to the back-yard site. There it is filled with 88.8 pounds of water from the garden hose, which makes it quite heavy enough. After the observing period the water is let out of the tank. Tanks of this kind measure 11 by 18 inches and are tapped at the top for a 3/4-inch pipe. In place of this tapping Bailey welded a two-inch pipe flange, and inserted a two-inch-length nipple to serve as the telescope-post attachment.

RECENTLY a newly organized amateur astronomical society in Australia asked this department for guidance in planning a workshop that would encourage its members to make their own telescopes. They asked, "What is required in the way of working space, machine tools and, in short, the whole setup?"

Copies of their letter were transmitted by this department to several American amateur astronomical groups, with the suggestion that copies of the replies be made available in these pages to others who may be planning similar organizations. From replies that were made available the following excerpts are extracted.

From Leo N. Schoenig, shop superintendent of the Amateur Astronomical Association, Pittsburgh, Pa.: "Our set-up at Buhl Planetarium consists of a rectangular space 24 by 33 feet in dimensions, or an area of 726 square feet. This is divided into a general workshop 16 by 24 feet, rough- and fine-grinding and polishing rooms each 9 by 11 feet, and a testing tunnel 6 by 24 feet. The space was arranged for our needs when the PIanetarium building was erected in 1939.

"At the start we held a tool shower, and the members responded well. Thus we have in the general workshop a metal lathe, drill press, bench grinder, vises, an anvil, a gas furnace for making castings, patterns and scrap metal, a large layout table for general construction layouts, 10 lockers for individual members, and numerous hand tools. In the rough-grinding room: eight spindles for grinding, a workbench, electric plate and additional lockers. In the fine-grinding room: a grinding machine, four polishing spindles and a large bench for mirror storage. In the testing room, or tunnel: a table for supporting mirrors and a movable stand for the testing equipment."

Schoenig confirms the assumption that the true purpose of the anvil he cites is as a substitute for the famous hydrant mentioned in Amateur Telescope Making, page 287, with its use extended to other mirrors.

From H. L. Freeman, executive secretary of the Los Angeles Astronomical Society: "We have about 7S0 square feet of shop area in the Griffith Observatory, divided into a large general shop, a small polishing room, a small office and a hall for testing. Telescope-making activities are carried out in the main room, which contains a quick-change, nine-inch swing, 54-inch bed lathe and its accessories; two drill presses, of which one is a high-speed precision type; a power-driven vertical spindle carrying a horizontal steel lapping wheel, for surface grinding glass to rough plane surfaces; a power-driven, two-spindle grinding and polishing machine, with pans for submerged polishing, capable of working 12-inch mirrors, three pedestals for hand grinding; a heavy workbench with bench grinder; a sink, a two-burner gas plate, a compressed air outlet for spray painting, a mercury vapor lamp and flats for testing, storage cabinets and hand tools.

"In the polishing room is another polishing machine, a sink with hot and cold water, and a gas plate for melting pitch. No activities not connected with polishing are permitted in this room. Entry is forbidden to anyone coming from the grinding shop, and entry by others is discouraged.

"In a hall is the Foucault equipment, which is big enough for mirrors 18 inches in diameter with a 20-foot radius of curvature. The easel that supports the mirror being tested slides on a heavy metal track in a curtained tunnel. The design provides for direct reading of the radius of curvature of the mirror from the knife-edge at the observer's seat. The distance is shown by a spring-driven steel tape attached to the easel and to the knife-edge stand where the tape passes under. a reference mark. Fore-and-aft movement of the easel is provided by a hand wheel at the test stand.

"The knife-edge stand is independent of the easel-track arrangement and rigidly bolted to the concrete floor and wall. The knife-edge mechanism is solidly bolted to the stand. No tremor of the pinhole image is perceptible under any circumstances. The knife-edge may be adjusted 8 inches vertically, 10 inches longitudinally and 6 inches laterally, a wide range that facilitates locating the reflected pinhole image. Vertical motion is controlled by friction wheels running on upright columns, and lateral and longitudinal motion by micrometer screws with half-nuts. When the half-nuts are lifted, approximate adjustments may be quickly made. When they are re-engaged, fine adjustment is immediately available. Gross fore-and-aft travel is registered by a reference mark traversing a plate graduated with .025-inch divisions which represent one revolution of the screw. This in turn carries n disk marked off into .001-inch divisions. This screw has complete adjustment against backlash.

"The light source is a 32-candle-power six-volt bulb supplied from the power line through a transformer. Its light reaches the pinhole after passing through a tube containing two achromatic condensing lenses placed to focus on the pinhole to which the light is finally guided by a right-angle prism. This prism permits bringing the knife-edge to within less than one half inch of the pinhole, which is chosen by rotating a disk containing a selection ranging in diameter from one fourth to .0015 inch.

"The knife-edge is a 3/8-inch square opening, so that the returning light beam can be intercepted in two dimensions, laterally and vertically. For Ronchi testing a plate carrying a slit can be inserted in this opening. Our Foucault test device has proved most satisfactory and is absolutely free from vibration.

"Nearly all our members have telescopes or are making them. We sell telescope mirror-making kits, abrasives and polishing materials to members at a modest increase over costs. Since our Society is a nonprofit organization and pays no salaries, the profits from such sales are used in the purchase and upkeep of shop equipment, to defray the costs of awards and prizes and to add to our building fund, which ultimately will enable us to build our own quarters and observatory. Mr. Ingalls is correct in saying that most of the amateur groups built up their facilities by patience and hard work." The statement here verified by Freeman was made because it has sometimes been sup- h posed that amateur telescope-making organizations to which planetariums have furnished free quarters are otherwise supported by these institutions.

Los Angeles was the first community that organized a telescope-making group, after the pioneer Telescope Makers of Springfield, Vt.

From Allyn J. Thompson of the Optical Division of the Amateur Astronomers Association, New York: "The A.A.A. has its rooms in the American Museum of Natural History and its Optical Division has quarters in the adjacent Hayden Planetarium. Partly through our dues money, but chiefly through funds earned in annually conducted mirror-making classes for the public, the members of the Optical Division have purchased all their own equipment, tools and so on, except for some oil drums which are partly filled with water and used as pedestals for mirror making.

"The group of you, 10 or 20 persons, will pool funds and efforts to set up a society and a shop. Preparation should be made for the years to follow. Who will run the show} Be particularly careful about any investment of authority. What are the rights of the members? Will others, strangers to you now, be permitted to join later, and what will be their rights? What steps will you take to screen undesirables, and how will you recognize them as such? What about new members who are green about machine work? Among you now or destined to enter later will be some who have a natural aptitude for optical work, mechanical work, or both, and who will acquire exceptional skill or knowledge B in their favorite hobby. Use should be . made of this talent. The vain ambitions of others should not be allowed to hamper or fetter it. A program embracing the aims and purposes of the club should be prepared. It should not be irrevocable.

"Your group will need an electric plate for melting pitch. A spherometer for checking mirror radii is desirable, but accurately made templates are a good substitute. The Foucault testing device can be made by the members; use a slit if possible. An optical flat is desirable for checking plane surfaces and can be made by a member of your group. A fluorescent lamp is satisfactory for interference testing.

"It is best if all mirror-making materials are stocked by the club rather than obtained at random by the individual members. May I suggest for abrasives Carborundum No. 80, Aloxite Nos. 120, 220, 400, 600, emery No. 305 (finest) and, for polishing, Barnesite.

"Essential equipment in the machine shop may consist of a lathe, drill press, milling machine and vises. The lathe should not be less than nine-inch, with 36-inch bed. The more completely it is equipped the more useful it will be. Two or more lathes and two or three vises, one for heavy duty, will be better. Such auxiliary equipment as a band saw to handle wood or metal, a jig saw and a table saw is valuable. Measuring tools and essential bench tools should be provided. Additional tools, such as drills, taps and reamers, will suggest their need later. It is best for members to provide their own lathe tools.

"Omitted from the list because they are nonessentials are a spindle for small lens work, a grinding machine for mirrors and lenses, and a woodworking lathe. Storage space and receptacles for materials and equipment of the club and its members should be provided.

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-877-527-0382 voice/fax

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

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