RUSSELL PORTER, at last getting some earned rest at 75, has become a TN again and sends the fraternity the following, via this department.

"The accompanying drawings (Figure 1) show an attempt to erect the image of an objective by means of mirrors instead of the two Porro prisms almost universally used today in binoculars. The four mirrors a, b, c, and d, are flat and aluminized. The axial paths of the objectives, o, are shown dash-dotted in all four views, and the marginal lines of the cone from objective to its focus are full lines.

"Mirror a receives the incoming light and reflects it upward at 45 degrees (the normal to its surface being at 22-1/2 degrees to the axial ray). Mirrors b and c are really parts of surfaces at right angles to each other, their intersections being parallel to the incoming and outgoing axial rays. I have shown b and c as elliptical in outline only to show more clearly the areas cut by the narrowing cone of light. Finally, the cone goes from c to d and thence to the focus. The normal to d is also at 22-1/2 degrees to the axial ray.

"By referring to the plan view one may see that the light path from b to c is not at right angles to the main axial rays, but makes an angle of 45 degrees to them.

"I have made two or three of these telescopes-one a binocular-and find that they perform very well. The light loss through reflection is comparable to the reflecting and absorption losses in Porro prisms. Unquestionably, their manufacture and adjustments are more complicated than when Porros are used. But the light paths being entirely through air (they cross each other several times on their way to the focus) render the telescope extremely short. For example, in the one shown in the diagrams, where the focal length of the objective is about 8", the length of the housing comes out only 3", hence it is very compact.

"I do not know whether this method has been tried out before. I worked this out myself some ten years ago."

SOME who try to build their final telescope first put much hard work on instruments they often wish were different but the telescope shown in Figures 2 and 3, made by Fred Larsen, 638 West 110 St., Los Angeles, Calif., when closely studied, looks more than most like a third-job design on a first-job instrument. Good judgment with regard to various details appears to have been used. It is partly a machine tool job but is not over-elaborated. Let's look it over.

Pedestal: Much more than mechanically adequate but esthetically just right. However, if looks don't mean a thing, as some claim, then perch a big telescope atop a mechanically adequate 2" or 3" post where it will look as if "all top." Posts are pretty cheap. Why skimp?

Drive: If there is to be one at all and unless photography is to be done, why must a drive be highly precise? Larsen's will keep an object in the field of view as long as most observers care to look. It is a simple sewing machine motor with an electric governor of the type used in movie projectors. If the telescope moves too fast one of the switch knobs at the end of the wire in Figure 3 will start it moving slowly backward; or vice versa with the other. These switches are within easy reach. So is the second integral pair of knobs a little higher up. This is a slow motion in declination and is simply a cross- screw on the end of a bar which has a friction fit bearing on the outside of the declination bearing, as on a surveyor's transit.

Drive gear: An 185-tooth worm gear cut with a 1" tap ("A.T.M.A.," p. 365). It came out very well, Larsen says.

Bearings: Tapered and therefore without play and solid.

Setting circles: Laid out on a

lathe using some of the thread-cutting gears held with an improvised ratchet to divide them.

Tube: Metal, skeleton. Tube has a rotating head; the heavier ring near the top is actually two rings, the upper rotating on the lower. "This," Larsen writes, "is very useful but must be accurately and rigidly made, else it will not stay collimated well." Such rings wring no necks, a big comfort.

Eyepiece holder: Triple turret type. No truant eyepieces.

Finder: Simple peep type. A 1/8" hole at the eye end and a 1" ring at the outer end. No cross hairs. Center by judgement.

Polarizing diagonal: Three flat mirrors the size of the telescope diagonal set at 57 degrees critical angle. Rotate to vary sunlight. Removable.

Site: Flat-top residence roof. Pier shown rests on crossing of wooden partitions below. It has proved solid and walking below does not shake it.

"To have spent many hours of spare time building this telescope has been one of the most gratifying experiences I have ever had," Larsen writes. "It has given me a little of the feeling that must come to an artist after he has finished a painting; one feels satisfied when anyone shows interest in his workmanship, and the ego shows signs of inflation.

"Thirty years ago in the land of the great Tycko Brahe and Ole Roemer,"he continues, "I came upon a description of a homemade telescope, a 3" made in the usual manner except that it did not mention the all-important Foucault test, described in 'A.T.M.' Carbo and rouge did not grow on any of the plants my father raised, but yea later I became again infected by the Bacillus telescopia and have had the fever ever since."

In describing Larsen's telescope pedestal this department characterized it as esthetically just right. Subsequently, two photographs of even fatter pedestals were found among old files, and these, the first by W. W. McClure, Grosse Pointe, Mich., the second by Ralph Munn. Pittsfield, Mass., seems even more pleasing though individual tastes in legs differ. Not wishing to single out any example of the opposite kind-spindly-we forbear from illustrating this extreme, but examples are numerous.

HOW Dr. Anderson tests the 200" mirror is explained by Porter: "Over the mirror he places a diaphram with a series of 5" holes 6" apart across a horizontal diameter. The knife-edge and light source are at the c. of c. of the mirror about 110' away. Starting from zero in the center, each pair of holes is numbered-1, 2, 3, and so on, to right, same to left.

With the knife-edge at A (Figure 5) the two holes (windows) will appear as at C and D, half darkened. The knife-edge is moved along the mirror axis until this condition is found. If the knife-edge is not where it should be, at A, but is at B, you will get the shadows shown at E and F and the knife-edge is accordingly moved back to A.

The radius of curvature for each pair of holes has been computed and thus any departures of A can be determined.

It takes only a few minutes to run through all the pairs.

In addition to this, the mirror is rotated to detect any astigmatism.

RECENTLY several amateurs were asked how they would vote to use 16" blanks. Walter Houston, Cincinnati, voted for an f/7 Newtonian; H. E. Bussey, Atlanta voted for an f/8 or f/9 , Newtonian; G. P. Arnold, Chicago, favored an f/8 Newtonian. Your vote?

PERHAPS Vicksburg's express agent will never be a TN. Mrs. Nona Ledbetter of that community, who is, says a local TN wanted to ship a mirror to be alun1inized but, fearing breakage, spent half an hour explaining the whole art, to impress the gentleman. He listened politely, looked puzzled, hesitated, wrote on the waybill: "1 pc. glass, value $50."

NEW LIFE for planetary and lunar observation among amateurs in America is promised by the inception of a monthly periodical, "The Strolling Astronomer," edited by the generally recognized leader of this activity, Walter H. Hass of the Institute of Meteoritics at the University of New Mexico, Albuquerque, N. M. It starts off with 50 subscribers and in mimeographed form. Amateurs interested in lunar and planetary observation now will gather around Haas and through this organ work out ways to end the existing situation in which each separated individual goes it alone, which is less fun than going it together.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.