C. B. Moore, Jr., Tech Box 2742, Atlanta, Ga., a chemical engineering student, does this, as shown in Figure 1. "The RFT has a 6" f/3.5 Pyrex mirror," he states, "with full 100 percent correction. The eyepiece is a Ramsden of 1.14" f.l., giving a field of 2.50. Tube and mirror cell are of aluminum. The telescope weighs only 12 pounds, hence it can be carried on the sling (an old rifle sling) without discomfort. In other words, please add my name to the list of satisfied RFT owners."

The left-hand side of the illustration shows a typical mounting for the RFT -simply held in the arms like a baby. That, in fact, is one of its strong points- simplicity and lightness.

SECOND story of a two-car garage is the spacious observatory (Figure 2) of Charles A. Morrison, 39 Radcliffe Road, Rochester, N. Y. The supporting pier of concrete blocks rises from an earth foundation and passes through the second floor without contact, to prevent transmission of vibration to the telescope. The dome is a frustrum of an octagonal pyramid.

The tube is made of wooden strips, beveled and glued.

The mounting.(Figure 3), was made from castings purchased from a dealer,

and these required a rather complete shop to machine them.

The diagonal is an elliptical plate, the second diagonal being the outside surface of a prism.

The 8" Pyrex mirror deviates not more than 0.07 wavelength of sodium light from a true paraboloid, Morrison states, and a lunar photograph submitted but not shown here justifies that claim.

All three reflecting surfaces are aluminized.

The telescope has a Telechron drive.

BEGINNERS who are learning the conventional stroke used in grinding and polishing a mirror, and who have noted Ellison's description of the three motions ("A.T.M.," page 77), often inquire whether the mirror should be rotated in the same direction as the worker walking around the barrel, or in the opposite direction. In ordinary work this makes little noticeable difference. The purpose of this second of the three motions will be seen best if we stop to analyze and separate the purposes of all three.

First, the "travel to and fro" of the upper disk across the lower has the purpose of wearing away the two differentially, so that the one will become concave by an effect explained in "A.T.M." page 2-that is to say: "the pressure per unit area, and therefore the amount of abrasion, is increased on the central portions of the upper disk and outer portions of the lower one when the upper disk overhangs" during the stroke.

Second, if the mirror were not rotated in the hands, it would soon tend to develop into a trough, or cylinder, instead of a sphere. It would have astigmatism.

Third, if the worker did not "walk slowly round the barrel" the lower disk would after a time tend to become worn more on one side than on the other, though the mirror itself, because it is rotated, would not be similarly affected. (Thus No. 3 is the least important of the three elements, and this explains why the work may be done at a bench, as in the frontpiece of "A.T.M.," provided the lower disk is turned around once in a blue moon; it also explains why there is no real need, even when a barrel is used, to take a step with each stroke, as some have supposed necessary, and why it is entirely in order instead to stand in one place and work for some time, though in polishing and especially in figuring this should perhaps be taken less literally.)

Now it will be evident that, with regard to elements 2 and 3, if the upper disk is rotated in the same direction as the worker it will rotate faster with regard to the lower disk than it will when it is rotated in the opposite direction; the difference between the two being possibly quite marked. Even then it should, however, make little or no appreciable difference, and thus the question is largely academic. In other words, the worker should suit his own whims.

But when a machine is used to drive the mirror, some have observed a difference. Years ago Leo J. Scanlon of Pittsburgh observed that rotation in the same direction deepens the mirror's center, while reversing it brings the wear upon the edges, and the late S. H. Sheib of Richmond verified this by speeding up the tool still faster on a machine. He, too, found that at low speed the abrasive collected in the center but traveled to the edge with higher speed. "Apparently," he wrote, "this movement of the abrasive is due to centrifugal force, and hand work is too slow for this effect to be felt."

C. G. Wates of Edmonton analyzes the factors thus: First, centrifugal force, and second, relative speed between tool and mirror. ( Centrifugal force increases as the square of velocity and directly as the radius.)

On a machine, then, and perhaps in some workers' hands, this phenomenon may be used to control the curve of

the mirror. As Wates states it: Relative speed slow-center deepened; relative speed fast, edge deepened. But the beginner is likely to find these latter considerations too subtle to concern himself about.

IN SEPTEMBER 1941, in The Sky, Carl Hellman of Washington, D. C., described a peculiar telescope invented by Ludwig Schupmann, and gave basic data. He also gave details of a 4" and a 10" Schupmann which he had made or was making. The Sky added an analysis of the optics of this type of telescope, and in April 1942 gave an account of a Schupmann which had been made, though it had given multiple images.

Since the dates of Schupmann's patent was not stated in The Sky, further than to say that it was "some time ago," it looked as though the Schupmann might be reasonably recent. On digging out the patent (Figure 4) its date was seen to be 1899. However, this, at that, was "some time ago!" (What is the elastic limit of a word?)

Its age is not, however, a reflection on the Schupmann. In fact, suspecting that other long-forgotten patents may be better appreciated by the present generation of constructors than in earlier times, your scribe hopes some day to find time to explore all optical patents systematically back to say, 1893, and re-present some of them here. If this is done, the dates will be stated.

Since you can obtain a copy of the Schupmann patent, giving full details, simply by sending a thin dime (Government will not accept stamps) to the U. S. Commission of Patents, Washington, D. C. and asking for U. S. Pat. 620,978, only the basic details will be given here. In The Sky, Hellman pointed out that, by using, in combination with the Schupmann's one-element objective, a concave-convex correcting lens of the same glass as the objective, and with its nearer surface curved to produce a chromatic aberration equal in magnitude but opposite in sign to that of the objective, and with its farther (lower) surface silvered as a concave mirror curved to produce a convergence equal in magnitude to the divergence of the nearer surface, Schupmann had found a way to correct the chromatic aberration of the simple objective without at the same time losing its convergence. This gave a telescope having many advantages: low weight of glass; low light absorption; corrector easier to work than that of a Schmidt; silvering permanently protected by paint; all surfaces spherical; tube closed; eyepiece conveniently placed.

Hellman used the arrangement in Fig 7 (note: of the patent in Figure 4) except the corrector, which was like that of Fig. 1 (of the patent).

In 1942, Joseph Dwight, Hyannis, Massachusetts, advised this magazine that he had been experimenting with an unfinished Schupmann (Figure 5). He modified the design used by Hellman, substituting for the corrector used by him one which, it later proved, had been anticipated by Schupmann (Fig. 2 of the patent, which Dwight had never seen).

"The Schupmann" he writes, "is a good type for an amateur since the mirror and convex face of the objective can be ground together, and the bottom of the mirror and flat faces of the corrector and objective can also be ground together. Then the outside plane of the objective can be silvered and painted for protection while the figuring of the telescope is done on the convex face of the objective. Then paint and silver are removed.

"I used 8" disks of rolled optical crown (N = 1.523, V = 58.83 from the Pittsburgh Plate Glass Co. The objective and mirror radius is 58.5" and that of the plano-concave lens is 26.25D. The objective has a f.l. of something under 120". From objective to concave lens is 65" but this should be decreased in order to give more correction. The objective was stopped down to 6" and the e.f.l. was about 150".

"Though the lenses were not even well polished, and the mirror was only partly figured, a 1" monocentric eyepiece (15OX) gave pretty lunar views at sunset but later the Moon was too bright, with prismatic effects on some of the sunlit crater walls, due perhaps to under-correction or to bad alinement. An orange-tinted 'Mars' glass concealed this trouble and removed the glare. Since the paper tube, sheathed with galvanized iron, tilted unsteadily on a rickety wooden 'mounting,' I could not count the belts on Jupiter though at least one was visible.

"This compact, long-focus type of telescope, with a lasting silver film and few air-currents, seems well worth further trial, especially as it may be relatively, easy for the unskillful to make-I writ' as one of them. Of course, care must 1;. taken to have the lens edges of uniform thickness. And any change in glass o' proportions requires the tracing of red and blue paraxial rays. To make a good telescope is impossible for me under present conditions. I can only show that it is feasible."

The photograph in Figure 5 was taken at this department's request, to show the readers that tangible work had been done and Dwight apologizes for the telescope's general lack of finish and sex appeal. I was only a sort of test set up.

PROBABLY few persons not instructed in astronomy sense the stupendous difference in volume between the Sun and Earth-those so instructed do not always find it easy to sense it. Even the difference in diameter is difficult to realize. Simple little stunt, in case you happen own a hemispherical observatory dome is to let the dome represent the Sun's diameter and then hang up inside it somewhere a ball representing the Earth's diameter, also a bead representing Moon's diameter. In the average case Earth ball will not be far from an inch in diameter, the Moon's a quarter inch. This was done at the Municipal Observatory at Preston, Lancashire, England, described in The Journal of the Royal Astronomical Association. When visitors come, this makes an interesting demonstration. There is no attempt at spatial relationship-just size. Ratio of 864,390: 7914:2160 (miles) is close enough. I fact, for a thing like this, 100:1:1/4 is probably good enough.

MAKERS of objective lenses must try to keep the edges the same thickness all around. To facilitate keeping tract of the edge thickness when grinding, C. L. Barton of Hollywood does as follows"

Divide the lens into eight equal sectors, or for finer grinding, 16.

Measure the edge thickness of each sector and mark the thinnest zero, thicker sectors 1 or 2 or 3, or whatever it is, these figures corresponding to thousandth of an inch.

Mount the lens on the pedestal, fin laying a piece of paper over the latter.

Extend to this paper the radii that bound the sectors.

Take grinding strokes in numbers proportioned to the numbers on the paper.

Taper off with a couple of rounds of ordinary strokes.

Repeat the above process till the diminishing errors have been resolved.

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/

At Surplus Shed, you'll find optical components such as lenses, prisms, mirrors, beamsplitters, achromats, optical flats, lens and mirror blanks, and unique optical pieces. In addition, there are borescopes, boresights, microscopes, telescopes, aerial cameras, filters, electronic test equipment, and other optical and electronic stuff. All available at a fraction of the original cost.

SURPLUS SHED
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Phone/fax : 610-926-9226
Phone/fax toll free: 877-7SURPLUS (877-778-7758)
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