WHICH IS THE BEST TYPE of tube for a reflecting telescope? Old argument, never settled, because it depends too much on personal factors. (Which is the best type-blonde, brunette or red-head?)

Alfred Bryant, 516 Egleston Avenue, Kalamazoo, Michigan, settled one of these questions, the one about the telescopes, for himself by making one telescope for each type of tube (Figure 1). From left to right these are:

Hexagonal tube (9 1/4" mirror, 78" f.l.).

Octagonal tube (12 1/2" mirror, 81" f.l.).

Square tube (6" mirror, 70" f.l.-solar telescope).

Skeleton tube (7 1/4" mirror, 58 f.l.). But Bryant throws out the skeleton tube for city use-extraneous light makes too much trouble.

Round tube (7 1/4" mirror, 58" f.l.).

Round tube was all right, Bryant says, but "it is hard for the beginner to direct his tube to find objects."

Hex and octagonal are all right but hard to make.

He votes for the unromantic square type. Easy to make. Easy to direct. Doesn't need internal ribs. Easy to adjust optical parts. "I admit," he says, "the square type doesn't look so handsome, but looks do not a telescope make."

The R. A. circle on the 12 1/2" consists of a 72" steel tape around a pulley, giving 3" spacing per hour, which is good for visibility.

Incidentally, Bryant must be industrious.

SCHMIDT camera and mounting, made by Philip Knowlton, 83 LaSalle Street, New York, New York, is shown in Figure 2. It has an 8" Pyrex mirror stopped to 7", a 3 1/2" correcting lens and rates as f/1.9.

Correcting lens is a disk of Vitaglass 1/32" thick, figured flat on one side; on the other convexed, then concaved near the edge. It has an extra stop which works at f/4, also a 77mm haze filter. Tube is 1/2" five-ply plywood reinforced with 16-gage iron.

"Fussiest job," Knowlton writes, "was design of film holder." Figure 3 shows the one made. "By experience I found that the focus is so short that the back of my hand contacts the mirror while loading film.

"Plate glass would have been easier than Pyrex to grind so deep for the mirror if no machine had been available. A machine should be used-for if anyone thinks there's a lot of rub-rub-rub to a common 6" f/8 mirror, let him try a Schmidt by hand-mine took the spare time of 10 months. But it was not as difficult as at first it seemed-mainly tedious.

"The fork of the mounting is of 3" angle iron, welded and bolted to a heavy stub-shaft resting in a roller bearing. The fork is long enough to permit the camera to be pointed north. Main supports are of 2" x 4" material, each leg having an adjustment.

"The clock drive consists of a lever (Figure 4) on the polar axis shaft, its broad flat end faced with hard rubber. This bears against the knurled knob of the alarm clock. The clock may be shifted sidewise and the lever lengthened or shortened to obtain adjustment of drive speed."

AS EACH new crop of amateur glass workers comes along, a few individuals discover coal tar pitch and are led to think it is something new. Following from an old letter from J. W. Fecker, Pittsburgh professional, gives answer:

"Both Lassell and Herschel used coal tar pitch and described the use of it in some of their earlier memoirs. We

have used it here in the shop ever since the business started and it is used on all large jobs.

"Coal tar pitch works very well, particularly on glass such as Pyrex and the ordinary crowns and flints. Some of the special glasses, like barium crown, fluor crown, and the extremely dense flints, do not polish so well on coal tar pitch due to some of the oils in the tar causing a staining action on the glass. Particularly is this noticeable on the extremely dense flint.

"We generally mix the coal tar pitch with asphalt, resin, bee's wax, pine pitch, or whatever is necessary to give us the requisite body. For small lens work where the spindles run at high speed, we use quite a bit of pine tar pitch. We also temper it by boiling it for a long time to boil out the oils and make the tar harder.

"The hardness and consistency of the polisher depends to a large extent on the type of work you are doing, and no one mixture is a universal polishing agent.

"For the amateur, I presume pine tar pitch is a little better, because coal tar pitch has a tendency to soften and give edge error."

NEAT housing for a telescope is shown in Figure 5. E. D. Tarbell, Hunter Avenue and Leeds Road, Kansas City, Nebraska, is the designer and maker. He writes "The housing itself is made of 20-gage galvanized iron braced by 1" angle iron ribs, though the one at the open end is 2". The whole housing slides on a two-by-four track set on posts. The sills of the cover are two-by-eights. The rollers are rear wheel car bearings from a junk yard. The rear pair are not placed near the end but about two thirds way back, enabling me to use a shorter track extension back of the telescope.

"The inside of the housing is painted black, and at the rear I have a tray in which I can change films for photographic work, even in full moon, without causing fogging."

ANOTHER amateur with a unique way of making a pitch lap is Robert E. Smith, D.D.S., Medico-Dental Bldg., Sacramento, California. He says he first obtains the "Pittsport Doormat," a rubber device prepared by the H. O. Canfield Co., Bridgeport, Connecticut, and used by numerous amateurs for casting tailor-made laps of pitch. One of these is shown figure 31, page 37, of "Amateur Telescope Making-Advanced." Usually when they come there is a web between the partitions and the user must chisel this out, but Dr. Smith leaves it in-he even asks the makers to select a mat having a good, flat web with no undulations or imperfections. He describes the job thus: "Cut the mat to fit the mirror, with its squares off-center, of course. Place mat on mirror. Wrap the customary collar around the edge: I use rubber for this. Paint the whole thing with glycerine. Pour melted pitch into all the squares, making sure there is more than enough. Place the warmed tool on this and the lap is practically made. Its neat channels also run clear down to the glass as is desirable.

"Let it cool somewhat and then remove mat and collar, chamfer the lap and cold press, and there you have it. Such a lap is pretty to look at and easy to make."

Maybe just at present Japan may have something to say about making a lap that way-rubber. But that should not last forever.

PRISMS. Cyril G. Wates, 7718 Jasper Ave., Edmonton, Alberta, Canada, offers simple tests for telescope prisms for diagonals, which anyone can apply. He writes:

"There is no way of testing the flatness of prism faces except by observing the interference fringes when the prism is brought into contact with a standard flat. This note deals instead with methods of testing the correctness of the angles. The right angle between the square faces may be tested with great accuracy by the method described by Russell W. Porter on page 54, 'A.T.M.,' and improved by John M. Pierce. The prism is placed with the large face toward the observer. A white card having an accurately circular hole about 1/8" in diameter is held close to the eye. The hole appears as a black spot in the center of the prism, the spot being bisected by a line which is the edge formed by the two square faces. If these faces are at right angles the spot will appear circular, as in A Figure 6. If the angle is more than 90°, the spot will be drawn together as in B, if less than 90°, the spot will appear elongated as in C. The corresponding shapes of prism are shown in the lower part of the figure. This type of defect is called axial-angle error.

"Assuming that the square faces form a true right angle, there remain two ways in which the prism may be

incorrect. In the first, the two acute angles, which should both be exactly 45°, may be unequal, as shown greatly exaggerated in D. Now, referring to E, imagine the prism cut by a plane surface exactly perpendicular to the face x. as shown by the dotted lines. In the second, the other two faces, y and z, should also be perpendicular to the same plane. If they are not? the prism is actually a section of a pyramid as shown in F, and this is called pyramidal, or sometimes side-angle, error.

"The two errors last described may be tested very simply by use of the principle of triple reflection. The set-up is shown in G. The prism is placed on a table in a dark room, with one of its square faces toward the observer, who should he at a distance of about 10'. The card with the hole is held in front of the eye, the back of the card being illuminated by means of a small, shaded light. Two images of the hole are formed, one by direct reflection from x; the other by triple reflection from z, y, and again from z. If the two acute angles of the prism are equal and there is no lateral distortion, as in F, the singly and triply reflected rays will coincide and one image only will be seen.

"If, however, the acute angles are equal, as in D, the triply reflected rays follow the course shown by the dotted line in H. Two images will be seen, displaced vertically, as in J, or, if the error is slight, the spot will appear elongated.

"In the case of lateral distortion due to pyramidal error, as in F, the triply reflected ray will be displaced in a plane perpendicular to the paper, and the images will be displaced horizontally, as in K. If both errors are present, the images will be displaced or elongated more or less diagonally.

"It is important to note that the triple reflection test is not a proof that the acute angles are 45°; only that they are equal. An isosceles or equilateral prism will also give a single image; therefore the Porter-Pierce test must first be applied to determine the angle between the square faces. Note also that the condition shown in F is perfectly consistent with all three angles being correct.

"As to the sensitiveness of the test, it should be easy to observe an error of 5' of arc, which is the generally accepted standard for a satisfactory telescope prism. I have a 2" prism by a noted maker which shows no trace of elongation I have another prism–a "pick-up" job from a pair of field glasses, which shows two images separated by the whole width of the face, at a distance of 4'.

"It is not suggested that the above test should replace the still more precise standard shop methods used by prism makers, but for checking at home prisms, of doubtful origin and quality its simplicity should recommend it to amateurs."

The still more precise method of testing prisms, alluded to by Wates, consists essentially of a refinement on the method he describes. Light from an artificial source, a narrow slit mounted in the optical axis of a telescope, passes through its objective and into the prism, which is placed at a distance of a few inches. It is there reflected internally, and returned by way of the same objective to the eye. If, under magnification of about 25 diameters, the returned image of the slit coincides with the slit itself, the prism shows no error greater than approximately two seconds of arc. This is the test by autocollimation.

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