MANY READERS OF THIS magazine nibble a year or two at this monthly department before they decide to join the legion of the lost and make a telescope. Their careful approach resembles that of a black bass that follows a trolling line for half a mile wondering, "Is this my lunch or does it hide a hook?" In the four accompanying drawings the illustrator of the department shows what happens when the victim bites. The beginner has made the simplest telescope, had a season's fun with it and learned facts about telescopes that books cannot impart. Now he has deserted his first love and has built and is avidly using his second telescope, a larger one. Even the second is not the last, however. At the end of the second season it has been stripped of useful adjuncts and joined the cobwebbed company of attic antiques. Now the mirror for the third telescope, a foot in diameter, is under way, and the builder is becoming an advanced amateur and belongs to astronomical organizations.

A few try for the last telescope first. They usually embody mistakes in it, miss the fun the others have and wind up wiser.

DURING THE past four years a group of amateur and professional. opticians and scientists have exchanged about 50 letters with one another and with this department on the possibility of using the natural glass called obsidian for telescope mirror disks. A less expensive material than Pyrex would be very welcome to amateurs planning 18- or 20-inch telescopes and to professionals planning larger ones.

The proposal to use obsidian was first made before 1926. (See page 816 of Amateur Telescope Making.) By now a fairly large number of small optical surfaces have been made on it and proved satisfactory, but questions remain about the uniformity, reliability, availability and cost of obsidian disks. G. Dallas Hanna of the California Academy of Sciences describes his experience with obsidian as follows:

"During the late war the California Academy of Sciences was engaged in repairing for the U. S. Navy certain instruments used in navigation, such as azimuth circles, which have a small rectangular mirror made of black unsilvered glass. Spare mirrors were not obtainable. It occurred to some of us that obsidian might be a suitable substitute. C. C. Church obtained the first piece from Glass Mountain, two miles northeast of St. Helena in Napa County, California. It took a brilliant polish. Some of the pieces he brought were made into mirrors and they met all the requirements of the experienced inspectors. Allyn G. Smith and Edwin Over then collected a large supply and more than 100 reflectors were made and used.

"It was soon discovered that the raw material varied considerably in opacity. Some chunks had tiny bands of light gray and black. The St. Helena material was found to be completely free of visible bubbles and crystalline inclusions. It had the appearance of a well-mixed glass. It was harder than any glass we had ground and polished except fused quartz, and we soon noticed that heat effects were practically nonexistent. We could pass at once from polishing lap to Foucault tester.

"A few optical flats from two to six inches in diameter were made by J. E. Steinbeck, D. O. McLaren, Allyn G. Smith, C. C. Church, L. A. Parsons and Edwin Over, all experienced optical workers. They agreed that from the heat standpoint the obsidian had most desirable properties. We tested these flats from time to time to discover whether they contained strains, and found through the months that they did not change shape.

"One mirror made from St. Helena obsidian was 5.5 inches in diameter and very accurately spherical. The image of the sun from the uncoated obsidian was occasionally reflected upon the ceiling of the workshop to show the employees changes in sunspots. We also brought the moon into the field. Steinbeck, an experienced observer of lunar topography, pronounced the image exquisite in fine detail.

"This little group of workers was not the first to consider obsidian as a possible substitute for glass in some optical uses. In 1926 Russell W. Porter polished a piece flat and gave it to A. G. Ingalls. W. P. Bush of Berkeley, Calif., used obsidian of unknown source for a six-inch telescope mirror which Donald Jenkins of the Tinsley Laboratories tested and considered to have an excellent figure.

"Obsidian is very difficult to define accurately because it varies greatly in chemical composition and physical properties from deposit to deposit. Usually it would be called black, but often thin slivers are greenish or brownish with transmitted light. All true obsidians are presumed to be volcanic in origin. The chemical composition is often very close to that of the volcanic rock rhyolite. However, there are gradations into other rocks, so that it is very difficult to define its boundaries. It often contains bubbles, evidently caused by expansion of gaseous material, probably water. Sometimes there are cavities filled with minerals such as calcite or cristobalite. In some deposits there is a sprinkling of crystals which are softer than the obsidian, making it very difficult to work to an optical surface. A study of obsidian, chemically, physically, optically, geologically and mineralogically, is overdue."

The Mineral Information Service of the Division of Mines, State of California, says that another Glass Mountain in eastern Siskayou County, Calif., contains one of the largest deposits of obsidian in California, if not in the U. S. measuring several miles in length. The side slopes of the flow are composed mainly of talus blocks broken off from the flow. In the Warner Mountains near Davis Creek are several deposits of obsidian. There are also deposits in Yellowstone Park, Wyoming, and in Utah, Oregon and New Mexico.

Mirror makers seeking light on obsidian often look into the dictionary and learn that it is volcanic glass with the same composition as granite or rhyolite. At this point many become confused. If volcanic glass has the composition of granite, how can it be suitable for optical surfaces? The trouble is that the dictionary must be brief. For a more complete understanding of obsidian we must look inside the earth.

Within the earth's crust form molten solutions of mineral molecules, which are atomic groups or associations of oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium. These elements constitute more than 9S per cent of the earth's outer zone. This molten matter is charged with gases, including water, carbon dioxide, sulfur fumes and others. When such an igneous magma rises from the depths, is obstructed and cannot reach the surface, it is insulated by the overlying rocks and cools extremely slowly. This allows time for atomic groups to separate from the magma as crystals. The first to crystallize out of a granite-forming magma and to form the mineral hornblende are complex molecules containing oxygen and silicon with calcium, magnesium, aluminum and iron. As the magma cools further, molecules of oxygen with silicon, potassium, aluminum and iron separate as crystals of black mica. Later still come the feldspars, composed of oxygen, silicon, aluminum and potassium or sodium, and last, at lower temperatures, are quartz crystals of oxygen and silicon, to fill the spaces between the other crystals.

The size of all these constituent mineral crystals depends on the length of time of cooling. If it is great, the crystals will be large and the rock described would be a granite. If the cooling time is much shorter, they would be very small and the rock would be rhyolite. Since the quartz has hardness 7, the feldspars 6, the hornblende sometimes 5 and the mica 8 or less, and all have different crystal forms and different physical properties such as coefficient of expansion along each axis, and since most of the crystals have cleavage, it is obvious that granite and other crystalline rocks would make poor optical surfaces. The, softer minerals would polish to lower levels than the harder. Yet the proposal to use granite has been made to this department about twice a year for a quarter of a century.

Now obsidian most commonly has the composition of granite. Is it not therefore discredited for mirrors? This argument arises nearly every time obsidian is discussed, usually from those who have consulted the dictionary definition which stresses its composition. The answer is that the decisive property is not chemical composition-in fact, the: chemical elements in granite are the same ones used in manufactured glass- but the physical condition of the material. If the magma described above, which cooled to granite or rhyolite, had escaped from a volcano as lava and been chilled too rapidly to crystallize, the rock formed would be obsidian. The important fact for the mirror maker is that this rock is a glass.

What is a glass? Thousands who work with glass do not know what it is. Again, for reasons of necessary brevity, the dictionary definition stressing the chemical content is superficial from the scientist's point of view. A less superficial definition that stresses the physical condition, though it might leave the average dictionary user dizzy, is that glass is a liquid so viscous that it is rigid.

Viscosity is resistance to flow in a liquid or plastic. At 68 degrees F., pitch is 10,950 times as viscous as water, and glass is almost infinitely more so. But, however rigid glass is, it is still a liquid, because as it cools there is no freezing point, no change to the crystalline state with its orderly repetition of atomic structure. Instead, there is continuity from the fluid to the rigid condition. Because glass has not crystallized at temperatures at which it is rigid, it is classed as an undercooled liquid. (Sometimes it does crystallize, or devitrify, in cooling, but then it is no longer glass, and the manufacturer is no longer happy.) This does not mean, however, that glass has all the properties of a normal liquid. In the liquid state the atoms of matter are in a random arrangement, and are continually changing neighbors. X-ray diffraction analysis shows that in glass each atom has permanent neighbors. But these are not at definite distances; hence glass is not crystalline.

The nature of glass is discussed by C. J. Phillips of the Corning Glass Works in Glass the Miracle Maker, and by George W. Morey of the Geophysical Laboratory of the Carnegie Institution of Washington in The Properties of Glass. Morey's definition of glass is more precise than that given in the paragraph :above and requires 51 words, with 2,200 .words of further explanation-so elusive are the concepts involved, and so imprecise some of the terms we must use. The approximate definition given above is a compromise between Morey's 2,251 words and the dictionary's capsule version.

Since obsidian is in the glassy, uncrystallized condition, it is a full-fledged glass, not an imitation. Not all specimens in the field are

entirely free from crystals, however; Nature is less interested in standard production control than the Corning Glass Works with its Pyrex. (Nature, too, has bubble troubles.) The user must therefore either study the subject and know exactly what he is about or live dangerously and gamble on his obsidian. An expedition to secure a few two- or three-ton blocks of obsidian for 18- or 20-inch mirrors might easily cost more than Pyrex if the worker had the evil cost-accounting habit, but if time, gasoline and backaches are ignored the adventure might pay well in satisfaction. Much of the Siskayou County obsidian deposit is in the public domain under supervision of the U. S. Forest Service. There can be no objection to collecting minerals on it.

The next problem, of course, would be cutting the blocks into usable slabs. Some workers, not amateur, have trucked obsidian to monument works and had it sawed into slabs on granite saws of the type used in the monument industry. These are manufactured by the Patch-Wegner Company of Rutland, Vt., and shown in the Monumental News-Review of Buffalo, N. Y., a publication which gives some interesting insights into modern methods of working granite. Gangs of six long, heavy parallel strips of metal half an inch thick are reciprocated over the granite blocks by 40- to 100-horsepower motors. With chilled steel-shot abrasive fed abundantly by centrifugal pumps, they saw downward at eight inches an hour. An alternative method uses endless wires and abrasive grains to saw 16 inches an hour. The amateur telescope maker, who may have used a similar method on a one-arm-power scale for sawing glass, could devise his own obsidian saw; the boulders are too large for circular saws. A one-man industry–quarrier, sawyer, dealer–might result, and telescope makers might thereby obtain disks in times of Pyrex shortage like the present.

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

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.

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