SETTING CIRCLES ARE GRADUATED rings that enable the astronomer to set his telescope exactly on desired celestial objects, from coordinates given in published lists, without even looking at the sky. Since these circles are a luxury, they are not usually fitted to the beginner's telescope. This is good, because it compels him to learn the skies. The larger instruments that amateurs usually build after a year or two are generally equipped with setting circles, one attached to the polar axis to indicate celestial longitude, the other on the declination axis to show celestial latitude.

Ralph Ingels Fenn of Kokomo, Ind. has placed his circles on the inside wall of his observatory instead of on his telescope, and he has connected them with his telescope by means of Selsyn electric position-indicating equipment. The illustrations below show the Selsyn transmitters, the concrete pedestal of the telescope, and the circles and indicators, on the wall.

All this may seem to be unnecessary complication, but Fenn correctly states that "we have found this much more satisfactory for a small observatory and telescope than the standard circles mounted directly on the respective axes of the telescope. They are out of the way, and the figures may be made much larger, more legible and better illuminated without interfering with the telescope. "I hope," Fenn concludes, "that others will enjoy using Selsyns as much as we do."

Through their use on aircraft and elsewhere as position indicators, Selsyns have-become widely known. Each transmitter is connected electrically to its rotor and indicating pointer. The rotor mimics with high accuracy any and all motions of the transmitter, whether fast or imperceptibly slow, forward or backward, regular or irregular, so that the two function as if mechanically geared together. This is true whether the three-wire circuit that connects them is an inch or miles in length. So surprising is this mimicry that the novice usually tries to fool the indicator by turning the transmitter in irregular ways. The party fooled will always be that of the first part, however, for the transmitter and rotor pairs are inexorably "geared" together by electrical forces, and they work as one.

Selsyns, made by the General Electric Company, are induction motors with three-coil, three-phase-wound rotors. Their popular designation as motors is correct, but misleading, since they are seldom called on to spin rapidly as most familiar motors do.

"I attached one indicator to the concrete pier of my telescope," Fenn writes, "so that its shaft would be precisely parallel with the polar axis, and another indicator parallel to the declination axis. Cords running on these axes transmit the angular rotation of the telescope to the Selsyn transmitters through pulleys of exactly equal diameter, arranged so that transmitter and indicator will turn

in the same direction. Small weights at the ends of the cords keep the slack taken up. I did not have space to use gear trains instead of cords.

"My Selsyns were surplus-stock railroad-signal equipment designed to work on 45-volt direct current, but were easily converted for 110-volt 60-cycle alternating current by the insertion of an electrolytic condenser, following instructions included with them.

"The impulses from the transmitters are carried by triple wires to the indicators. These are equipped with nine-inch dials of heavy paper, with the numerals for the hour angles and degrees printed on them in India ink and lacquered over. They are recessed in a frame with concealed lights (red, very dim) at either side.

"When I first go out at night I focus my telescope on some bright star that I am familiar with, center it in the field look up its right ascension and declination in the star atlas, and then, after raising the drive cord off the transmitter pulley, I turn it manually until the dial reading is correct. The same procedure is followed for the declination. From then on I can forget local time. To find an object I need only point the telescope to its coordinates listed in the atlas. Once the basic time for a known star is taken, all other stars maintain their respective locations for that night. This is essentially the principle of the slip ring that is described by Russell W. Porter in Amateur Telescope Making, page 145.

"I once heard Mr. Porter say that he hoped all amateurs' telescopes that are on fixed mountings would some day be equipped with Selsyns, and that one need only operate a telescope thus equipped to appreciate the convenience and accuracy they offer. From experience I confirm this enthusiastically. Mine are a great improvement over the circles on the telescope, as I formerly used them."

DISCOVERY of a star within only six light-years of the sun has been announced by Dr. Willem J. Luyten, director of the astronomical observatory at the University of Minnesota. It has been named L 726-8, the letter L being in honor of the discoverer, following an established custom in the astronomical world. Only two stars, Proxima and Alpha Centauri, both 4.3 light-years distant, are nearer the solar system than L 726-8. The newly discovered star is a double of the visual type. Both components are extremely dim red objects, visible only in very large telescopes. A short time ago one of them flared up in an atomic explosion.

A few of the textbooks of astronomy contain lists of the very nearest stars to our solar system. Though these have no special significance, they are of some interest as our nearest neighbors in outer space. An uncommonly full list of these prepared by the astronomer G. P. Kuiper appears in William T. Skilling and Robert S. Richardson's Astronomy. It names 29 stars within a radius of 12.7 light-years of the sun. Even this short distance relative to known space, only 75 trillion miles, is virtually meaningless, since the human mind cannot conceive a greater distance than a human being can traverse and then look back across. If we try to imagine or conceive a greater distance than this, such as the 3,000 miles we may have traveled from ocean to ocean, we succeed only in a kind of additive or synthetic fashion, by at tempting to combine the separate looks." Only by some kind of abstraction or miniature of the reality, such as a map or globe, can we imagine or think w e imagine even 3,000 miles. Large as is the 75-trillion-mile radius of the nearby sphere of space containing 29 (now 30) known stars, it represents approximately one hundred-millionth the radius of known space, the billion light-years which has already been reached by the 200-inch telescope on preliminary trials.

The very nearby stars do not form a system; they. are simply the stars that are near us or, more pointedly, that we are near. Within this tiny, spherical bit of the universe there- are two stars, Sirius and Procyon, which are respectively 21 and 5.8 times brighter than the sun; Alpha Centauri, which just equals the sun in brightness; and 27 others much dimmer than the sun. Most of these are "cool" bodies of type M, with a temperature of 3,000 degrees C. or lower.

L 726-8 is a binary star with a period of revolution of 20 to 25 years; its exact period cannot be determined for many years to come. The two components, one of which is 40,000 and the other 60,000 times less bright than the sun, are something less than 300 million miles apart. Together they are moving away from the solar system at 26 miles a second. Thirty thousand years ago they were very much nearer than they are at the present time.

Both components of the double star are surrounded by huge clouds of incandescent hydrogen and calcium gas. On December 7, 1948, the fainter component was seen to flare up suddenly to 12 times its normal brightness, and subside within 20 minutes. Here, Dr. Luyten said, is a phenomenon that is thus far unique among the stars. "In this very faint star the atomic explosion-for such it must have been-amounted to the equivalent of a billion atomic bombs of the Hiroshima type."

The discovery of L 726-8 was not made visually, but by comparing photographic plates made by Dr. Luyten in 1930 at the Harvard University Observatory in South Africa with plates that were taken by him at the same observatory in 1944, and with other plates that were made at the University of Arizona and at the Union Observatory in Johannesburg, South Africa.

Dr. Luyten was the discoverer of another dim star only 9.9 light-years from the sun. This bears the designation L 789-6. He is best known for his discovery of 70 white-dwarf stars of the type having exceedingly high density. Born in the Netherlands East Indies, he came to this country in 1921 and was connected with the Harvard College Observatory from 1923 to 1930.

IF every amateur telescope maker were given an optical flat perhaps one foot in diameter, it no doubt would quickly become customary to test paraboloidal mirrors for reflecting telescopes at the focus, instead of at the center of curvature. (The latter is now the usual method because few own the necessary flats.) Testing would then be simpler and, some assert, twice as precise. The worker would not seek to obtain the conventional mirror shadows, but instead would alter his mirror so as to eliminate all shadows. This test at the focus is explained in Amateur Telescope Making, page 16.

As a secondary result of the universal possession of flats, refractors might become as plentiful as reflectors, which may be made without a flat as an accessory; they might become more plentiful, in fact, since many consider them superior.

Waland's first drawing shows how a mirror may be tested with an oil flat, an arrangement somewhat comparable with the one in Amateur Telescope Making, page 15, except that a Ronchi test grating is used instead of a knife-edge. The secondary mirror shown may be aluminized, or a prism used, to gain-added and usually needed reflectivity. By using a slit and a 35-watt headlight bulb shielded from the tester's eye, Waland obtains enough reflected light to make the test.

This test is not recommended for mirrors larger than about 10 inches, because of the sag in their necessary edge-supported, horizontal position. A poor secondary mirror also will impair the test, just as it will impair the performance of the telescope-often causing the eyepiece or primary mirror to be wrongly blamed.

The second Waland drawing shows the oil flat as used in the autocollimation test of an objective lens. The lens lies in its cell, which rests on a circular wooden frame ( shown also in elevation) with three equidistant leveling screws.

Now some precautions Waland gives on the basis of his experience:

1. The oil, a medium "lube," should be placed in a container at least two inches greater in diameter than the lens or mirror, to avoid capillary effects at the edge.

2. The container should rest on a rigid foundation, not on a wooden floor.

3. The oil should be clean and free from surface dust. If dust disturbs the surface a strip of metal should be trailed over it to draw the particles to one side. If a few particles persist, no harm will result, since the worker can easily recognize and ignore them. However, Waland explains, "I have not used the knife-edge test with oil; I use the Ronchi test. With the grating very near the focal plane, so that about two lines 11 traverse the surface, it is very easy to differentiate between optical errors and local disturbances of the surface due to dust and so on. But I should imagine that the oil surface would have to be particularly clean for the knife-edge test, otherwise it would be confusing."

4. Calm days are best; wind outdoors disturbs the oil surface. Slamming doors, footsteps, heavy traffic in the neighborhood or other human disturbances may limit use of the oil flat to the late hours of the night.

5. Even on calm days a draft excluder should be placed round the container, as shown in the drawings. This should reach three or four inches above the oil, to reduce convection currents in the air which so easily cause disturbances of the surface.

6. To avoid temperature differences when such exist, the test apparatus should be left for an hour before being used.

These are not difficult precautions, yet it is safe to say that a few will omit some of them and report that the oil flat is worthless.

"Use mercury instead of oil?" In reply to this question, Waland wrote, "Don't!" He tried it. Though he was able to watch the Irish boat train 15 miles away, he says, "The mercury registers every tremor." The same effect with a water flat was described by Porter in Amateur Telescope Making, page 60. Neither water nor mercury has much viscosity.

A hint for an experiment comes from Walter G. Thompson of Minneapolis, Minn. "Five per cent of sodium carboxylmethyl-cellulose, obtainable from E. I. du Pont de Nemours, Inc., when added to water, will damp its surface waves by increasing the viscosity several thousand times. Proper technique is to stir the powder and water together and let the mixture set overnight. On standing, the air bubbles in the mixture will rise and burst. Warm or hot water increases the rate of solution."

DESPITE the prevalence of aluminizing for telescope mirrors, silvering is by no means extinct and probably .never will be. "It is often a problem," H. Lynn Bloxom of Fort Dodge, Iowa, states, "to dry freshly silvered mirrors without leaving some trace of the dissolved matter contained in the fluid last used for rinsing. These impurities often fasten themselves to the mirror with great tenacity. When the last water rinse is followed with grain alcohol, as is often done, the results in nearly all cases approach perfection. But generous use of grain alcohol is a privilege not enjoyed by all. The drying can be done quickly by flowing over the mirror a very dilute solution of soap in distilled water. When this solution is rinsed off with pure water, the water rolls off as from a duck's back. But no hard water should be used to rinse off the soap, since this would leave a greasy precipitate."

Micrometric control of silvering is obtained by Clifford E. Lloyd of Thompson Ridge, N. Y., by the following simple method.

Instead of mixing the silvering solution and the reducing solution, the two are put in separate dishes and the mirror is dipped first in one, then in the other, and so on. By this method silvering takes about 10 minutes, but can be closely watched. A larger proportion of the silver goes on the mirror and Lloyd finds the coating thicker than any he has obtained by the regular method.

Continuing on another subject, Lloyd says: "I made a mirror-grinding tool of Wood's metal, which has a melting point of about 147 degrees Fahrenheit. Thus a tool of this metal may be made by pouring it on a piece of glass of the desired curvature. However, the glass should first be warmed. A paper tape suffices to hold the metal on the glass until it has solidified."

Suppliers and Organizations

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