OF SPRINGFIELD type mountings for reflecting telescopes, in contradistinction to the real McKay, which is the straight Springfield originated by R. W. Porter, described in the instruction book "Amateur Telescope Making" and revised by him in its sequel 'Amateur Telescope Making-Advanced," there are no end, each different and each ingenious. The one in Figure 1 was made by Robert Phillips, 3448 Greenview Ave., Chicago, Ill. He says the mounting was made from two Buick front wheel assemblies welded together at right angles with struts and cemented into a 5" pipe. Setting circles are marked on the rims of the brake drums and, these being 36" in circumference, it was easy to mark off the divisions, 1/4" equalling 2-1/2 degrees in dec. and 10 minutes in R. A. The tube is made of 1/2" maple dowel stock on rings of plywood-too light according to standards, Phillips says, but he adds that he had no trouble with it. Total cost of telescope was $25.

Another ingenious version of the Springfield type is that in Figure 2 an 8" with Micarta tube and an external framework of structural aluminum angle-a kind of exoskeleton. Carl Oman, 433 Springdale Avenue, East Orange, N. J., is the maker.

When, once before, as above, we called something the real McKay, somebody wrote in to say that the correct expression was "the real McCoy," referring to the late boxing man, Kid McCoy. That, however, is a corruption-one that, in fact, is not the real McKay-of the correct expression. That expression had reference to a fancy whisky once made by McKay in Scotland, pronounced McEye. Anyway, the virtual McCoy-McEye in Springfield mountings, except for its modified counter-weight support, is shown in Figure 3. You don't see many of them because it's a real job of work to make all the necessary patterns, castings, and machined parts. Yet the end-product looks sweet enough to justify the pains if you have the equipment to do the making. Charles F. Pope, 621 Courtland Ave., Park Ridge, Ill., sends in the photograph and says the telescope was made by Richard Traub of Park Ridge. Figure 4 shows Pope's observatory dome, of 20-gage sheet steel over wooden frames.

FLATS are preferred by many, instead of prisms, as diagonals in reflecting telescopes. In the following note Cyril G. Wates, 7718 Jasper Ave., Edmonton, Alta., Canada, discusses the subject pointedly.

"When the amateur has completed a really fine parabolic mirror, and has finished gloating over the beautiful optical 'doughnut' which the Foucault test reveals, he finds himself faced with the problem of diverting the reflected rays through the side of his telescope tube into the eyepiece. Shall he use a right-angled prism, or shall he use an aluminized diagonal? I fail to find that sufficient emphasis has been laid upon the relative merits of prism and diagonal, either in these columns or in the pages of 'ATM' and 'ATMA.' Perhaps there is a tendency among TNs to regard the two as optically equivalent, but this is far from being the case.

"Taking the diagonal first, it has one outstanding advantage. Any diagonal mirror, no matter how much its surface may deviate from a plane, is absolutely achromatic. Suitable diagonals with surfaces correct to one quarter wavelength are not too difficult to make, or may be purchased quite reasonably in the smaller sizes. This degree of accuracy is sufficient to insure realization of the fine qualities of a good objective, according to H. H. Selby and others ('ATMA,' p. 131). In the days of silvered surfaces, amateurs were justified in fighting shy of diagonals, but modern methods of aluminizing remove all objections to these little mirrors. Those who make their own diagonals may bear in mind, as I have pointed out in this department (September, 1938), that for long-focus mirrors the difference between a circular diagonal and a 'correct' ellipse, is purely academic. The light cut off by the unused part of a circular diagonal is infinitesimal.

"Turning to prisms there is one bit of advice which should be burned into the brain of every aspiring amateur-avoid inferior prisms as you would the plague. A diagonal mirror has only one place at which it can display defects; the surface. The prism has six: three angles and three surfaces. While it is as easy (and as difficult) to figure the diagonal surface of a prism to the required standard, this perfection will be largely nullified if any of the angles are incorrect, or either of the other surfaces imperfect. A right-angled prism with an accurate diagonal face, but with the other surfaces irregular and the angles incorrect, is exactly equivalent to a bit of cheap plate glass interposed between the eyepiece and the objective. Lack of parallelism between the faces of such a piece of glass would cause color fringes, and irregular surface would result in distortion and definition.

Even a perfect prism or a perfect 'plane-parallel' (the two are optical equivalent) causes color dispersion an effect which, according to Pierce is quite visible even with a focal ratio of f/8. This dispersion, which appears as a faint fringe around such objects as the Moon and Venus, is unavoidable, but is so unobjectionable that the amateur need have no hesitancy in using a prism, provided it is a good one. Reputable dealers will, of course, supply just what is ordered, and it is unreasonable to expect a Grade A prism for the price of a discard. A fine prism is necessarily more expensive than an equally good diagonal, for the reasons already stated. For short-focus mirrors, such as the RFT, it might be wise to decide upon a diagonal and avoid any possibility of color fringes, which are much more objectionable with a wide cone of rays.

"What is the standard by which a prism should be judged? First, all surfaces should be correct to a quarter wavelength. Second, the angles should be accurate within not more than 3 minutes of arc (Martin). The familiar 'split image' test ('ATM,' p. 54) may be made extremely accurate, especially if a circular diaphragm or washer is used between the eye and the prism (Pierce); but this guarantees the correctness of the right-angle only; it is perfectly compatible with gross errors in the other angles. If you are in doubt, go to the physics department of the nearest university and get them to measure the angles for you.

"A word with regard to collimation of a prism. The prism mounting must be adjustable in all directions. Push-pull screws are hard to beat, combined, of course, with a rod sliding in the boss of the spider. Eliason's method of reflection from the eyepiece face of the prism is excellent ('ATMA,' p. 272), but does not go far enough The eyepiece face may be at right angles to the optical axis of the: adapter tube, yet the mirror face (of the prism) may be hopelessly out of adjustment.

"It is advisable, therefore, to extend Eliason's adjustment by some such method as that shown in Figure 5, which has worked out well in practice. A disk of cardboard is attached to the mirror end of the tube. In the exact center of the disk is a hole (at least to avoid diffraction), with two heavy black lines drawn diagonally through the hole, as shown. The disk is illuminated by means of a flashlight, the end of the tube being covered by a black cloth.

"The prism is first adjusted to the eyepiece adapter by Eliason's method. It is then further adjusted by watching the reflection of the diagonal lines and eyehole, in the mirror face of the prism. When this adjustment is correct, the push-pull screws must not be altered again. If the prism no longer responds to Eliason's test, it must be corrected (1) by sliding the prism mounting in its collar, (2) by rotating the prism, (3) by altering the alinement of the eyepiece adapter. If this method is pursued, both square faces of the prism will be exactly at right angles to the axes of both tubes.

"If, after the prism is squared up as described, the mirror is not centered as seen through the adapter tube, the prism is probably bad, and Porter's method for warped mirrors should be tried ('ATM,' p. 287).

"Finally, by all means use a prism if you can afford a really good one, but remember that bad collimation may cancel all the perfection for which you have parted with your hard-earned dollars. If funds are somewhat limited, use a diagonal, especially with short-focus mirrors. For absolute freedom from color effects, the diagonal is certainly the choice, but even a hard aluminum film is delicate and subject to scratches. A prism is free from this objection. So there you are!"

Wates calls for prisms fiat within a quarter wavelength and with angles within better than 3 minutes of arc from perfect ("pyramidal error 3'," the professional would term this). Just how good is this, compared with the prisms amateurs have been buying? Definite data are not available on the quality of all the prisms sold to amateurs in the past but here are some criteria. The Perkin-Elmer Corporation, professionals, rate prisms as follows: A-quality-precision, 1/8 wave or better and 5" pyr. error, or better. A-quality, 1/8 to 14 wave and 1' pyr. error or better. B-quality, 1/4 to 1/2 wave and 5' or better. C-quality, 1 wave and 10' or better. Thus, the prism Wates asks for is, after all, nothing especially superlative, falling in Class B and costing, for a prism with one-inch-square face, between $3 and $5. How many, if any, Class D, E, F, G . . . X, Y, Z prisms have amateurs purchased in past years?

Your scribe recalls his first telescope, which had a prism picked up free because it was a reject. You could have thrown a cat through the gap between its face and a try-square, yet the telescope provided a big thrill, at least for a little while, even if the prism did make of it almost a spectroscope. At that time, 1926, the amateur telescope making hobby was a cub with burrs in its hair. Porter's statement, in "ATM," page 54, written also at about that time, that "the right angle of the prism need not be exactly 90 degrees for our purpose," didn't seem and then wasn't much out of place. It is literally true today-within the tolerance quoted by Wates from Martin but the language will be altered in future reprintings. Also by this test a prism could have a perfect right angle yet the other angles might be, say, 44 and 46 degrees respectively, without the tester being the wiser. Pierce claims that, with the prism held at 30", the split image test is good to 34 seconds of arc. If made with a small telescope this accuracy would be correspondingly multiplied, he points out.

WATES next continues with the description of a diagonal support of his design:

"The 'classic' method of adjusting the diagonal by means of one or more slots in the tube is not only poor mechanical construction, but also has the serious objection that, when the screws are moved in the slots, the diagonal is swung away from the optical center of the tube. My earlier design, which appeared in the October, 1935, number, is satisfactory for a prism, but is too bulky for an elliptical diagonal mirror. The design shown in Figure 6 has been used in an RFT and can be adapted for any size telescope. Because of the spherical surface between A and B, the diagonal moves as though pivoted on the optical center, thus greatly simplifying adjustment.

"The diagonal is mounted in a brass tube, as described by Hindle in 'ATMA,' except that the space between the glass and the plate A is packed loosely with coarse steel wool.

The top surface of A is turned to a spherical surface with radius r, which could be measured with a caliper, but may be more accurately determined by the use of the equation

"The method of attaching the four vanes E to part B is shown in Detail 1. The slots in B are cut carefully with a fine hacksaw. Note that the slots are slightly off-center. Half an inch of the ends of each vane is softened and tinned. One eighth inch is folded over, to fit into the holes in B, which is then heated and the holes filled with solder. The tube of my RFT being sheet aluminum, the wrought iron ring F was used for stiffening.

"D is a spring steel washer, shown in Detail 2. The diagonal and. its mounting can be moved in any direction without changing the position of its optical center and, when correct, the screw is driven home.

"In turning the three disks, A, B, and C, templates, both concave and convex, should be cut; one pair to radius r and the other to radius r plus the thickness of B. After turning to correct curve, the various surfaces may be ground together with fine Carborundum.

"To forestall the criticism that there is no provision for longitudinal adjustment I will say that, although such adjustment can be provided, it is quite unnecessary. If the spider ring is accurately mounted on the tube, the center of the diagonal will be in line with the center of the eyepiece tube. If not, the ring can be moved until it is correct, and then permanently fixed."

In the May number of The Journal of the British Astronomical Association, F. J. Hargreaves describes his pet polishing lap-wax on pitch, as follows: "The wax-on-pitch polisher consists of an under-layer of pitch supporting wax facets on a netting foundation. The wax does not yield or flow under pressure, and therefore the grooves between the facets do not fill up in use, as with polishers of pitch only. The pitch under-layer, however, can yield or flow to allow the wax facets to make contact uniformly with the surface being polished. This under-layer can therefore be made as soft or as hard as desired, according to circumstances, leaving the wax-polishing layer unaffected." Grim are the circumstances under which this is reported in summer, from a meeting on the banks of the Thames.

FROM S. L. Walkden, father of the RFT or Richest-Field Telescope, who lives in London, came, in the midst of the liveliest bombardment, a long letter about telescoptics and not a syllable about the war except to say that, London being permanently blacked out, he found with rejoicing that he could see fully three magnitudes of stars that previously were invisible, and with his own RFT he had found some thrilling Milky Way views.

Your scribe lives in a New Jersey suburb on a corner surrounded by eight street lamps and bathed by three flood-lamps from a filling station, and the only time he ever even sees the Milky Way is when out of town.

IN CASE you are planning to use the King test ("ATMA," p. 269) and have intended to use the sugar and water solution suggested, don't; it won't work. Other liquids-oils-have been substituted. Write in and ask, if especially interested.

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