Popular Science Monthly/Volume 24/November 1883/A Home-Made Telescope
|A HOME-MADE TELESCOPE.|
By Dr. GEORGE PYBURN.
TO render easier of attainment instruments which assist in the investigation or contemplation of natural phenomena, and which supplement man's sense-organs, is to forward by so much the diffusion of real knowledge, and to aid the work of human enlightenment and progress. Indeed, it is not to be doubted that the popularizing of instrumental aids for experimentally verifying the teachings of scientific discoverers will form a notable part of the work of the future schoolmaster.
A few years ago I derived great pleasure from successfully constructing a home-made microscope, guided by directions contained in "The Popular Science Monthly," at a time when my means did not enable me to purchase a good instrument from the optician. I now lay before my fellow-readers the following directions which, step by step, I myself have put in practice, in making a really serviceable achromatic telescope, which will exhibit the moon's surface magnificently, and show very satisfactorily the spots on the sun's disk, the satellites of Jupiter, and other celestial phenomena.
Some people conclude that, if they can not possess a first-class instrument of this or that kind, they are better off without any; but a moment's consideration will show the fallacy of this conclusion, and that, on the contrary, even a very poor instrument of observation or precision, or generally of research, in aid of the senses—be it telescope, microscope, spectroscope, balance, thermometer, chronometer, or chemical reagent—is vastly better than none. We have but to remember the great strides made in the acquisition of knowledge by the aid of the very imperfect first-forms of every instrument which has been invented, to be assured of this. Moreover, reflect!—so far as vision is concerned, men, on an average, without instrumental assistance, are inexorably kept at a distance from "things" of ten inches, and must view them under the angle thence subtended. But the use of a simple lens of two and a half inches focus annihilates three fourths of this distance, quadruples the angle of vision, and enables us to see objects only one sixteenth as large as the least we can see with the naked eye. And for some purposes a poor instrument is as good as the best: an egg or a potato gives the housewife all the advantages, in measuring the density of her brine, which she would derive from the most skillfully-constructed hydrometer, or the most accurate balance and specific-gravity bottle. Galileo, with his simple-lens telescope, saw what, perhaps, never man before saw—viz., the moons of Jupiter; and by exhibiting the partial illumination of Venus, with the same imperfect instrument, he removed one of the strongest objections raised against the heliocentric theory of Copernicus. A word to the wise is enough. To my fellow-students I say: Whatever may be your several lines of study, get real knowledge, where possible, by seeing and handling things for yourselves; and, if you can not possess or have the use of a good instrument, do not therefore refuse the assistance of a poor one; but in all cases get and use the best you can. Rembrandt made pictures with a burned stick before ever he possessed pigment or pencil.
The lenses requisite for such a telescope as I have constructed, and shall describe, can be purchased of an optician by those who live in large cities; those who reside at a distance may have them sent by mail at a trifling additional cost. They are: 1. An achromatic object-glass, one and a half inch diameter, with a focus of thirty inches. 2. Two plano-convex lenses of the respective foci of two inches and three fourths of an inch. The object-glass will cost about two dollars, and the other two lenses about seventy-five cents each.
Now procure a straight cylindrical roller of pine, two and five eighths inches in diameter, and thirty inches long; procure also a roller seven eighths of an inch in diameter, and fifteen or sixteen inches long. These are for forming the tubes on. Take stout brown wrapping-paper, and, with book-binder's paste, form a tube, twenty-nine inches long, on the large roller. Spread the paste on evenly, and rub the several layers of paper down smoothly with a cloth. Nine or ten thicknesses of paper will form a tube sufficiently thick and firm for our purpose; but only three or four layers should be laid at one time, and, when these are dry, three or four more may be added, and so on, until the requisite thickness is attained. When thoroughly dry, which will be in three or four days, you will have a stiff, straight, and light tube, the ends of which must be neatly and squarely cut off with a sharp knife, so as to leave it, when finished, exactly twenty-eight inches long. With a bit of sponge tied on the end of a stick, and some common or India ink, black the whole inside of the tube, and set it aside, on end, until the other parts are ready.
Next form a tube on the smaller roller, with only four or five thicknesses of paper, fifteen inches in length. When this is dry, proceed to form a third tube, over this second one as a roller, using six or seven thicknesses of paper in its formation. This last is to be used as a draw-tube for focusing with, and must be cut neatly and squarely off at the ends to a length of fifteen inches. A portion of the inner tube on which this was formed will be required for the eye-piece, directions for making which I shall give further on. Blacken the insides of both tubes, and set them aside, on end.
One more tube is required, viz., that in which the draw-tube shall slide. It needs to be only six inches long, but, in order to smooth working, should be lined inside with fine cloth or cotton-velvet. Procure, therefore, a piece of black broadcloth, six inches long, and of sufficient width to fit easily and accurately around the draw-tube. Then, using the latter as a roller, first neatly fit the cloth thereon as a first layer; next paste or gum the back of the cloth, and, with this for the innermost layer, form a short tube, six inches long, with paper and paste, as before directed, using here not more than six thicknesses. The draw-tube will now be found to move easily and smoothly back and forth in this cloth-lined sheath; but, for fear that the gum or paste should have penetrated the cloth lining, and should stick the tube and its sheath together, it will be safer to draw them apart before drying, and thus save needless trouble and annoyance.
On comparing the external diameter of this sheath with the interior diameter of the large tube first made, it will be found that some packing is required, to hold the former steadily and concentrically within the latter. Take, therefore, some three-quarters inch strips of brown paper, and, having pasted them, wind around the sheath at each end, to form rings or collars of equal thickness, and large enough to fit snugly within the main tube. The appearance of the sheath when completed will be as shown in Fig. 1, where a a' are the collars just described.
Now take the compound object-glass, consisting of a double-convex crown-glass lens, A (Fig. 2), and a plano-convex flint-glass lens, B. They will come from the optician's shop separate, but loosely fitted into each other. Be careful to see that their several surfaces are bright and free from specks, and, in handling them, touch only their edges. Remember, also, that the double-convex lens must be outside when the telescope is fitted up. Have ready a strip of tissue-paper, just the width of the thickness of the lenses at the edges: gum this on one side, and, holding the two lenses together with the fingers of the left
|Fig. 1.||Fig. 2.|
hand, wind the strip around the edges, so as to fix them together, and thus make a single piece which can be easily handled. When this is dry, take a strip of brown paper one and a quarter inch wide, and with paste form a short tube or cell, C, around the object-glass, using (say) five thicknesses. Fig. 2 shows the object-glass and cell in section.
To form the eye-piece: cut off a portion of the smallest tube—that on which the draw-tube was rolled—one and three-eighths inch in length, and make the ends even and square. Make, now, two disks of blackened cardboard, of the diameters respectively of seven-eighths inch and one inch. Punch or cut out exactly in the center of each disk an aperture one quarter inch in diameter. Gum the edges of the smaller disk, and fit it into the tube, exactly three quarters of an inch from one end, and, of course, five eighths of an inch from the other end. Then take the two-inch plano-convex lens, and, having made it perfectly clean, cement it on to the end of the tube nearest the perforated disk, with the plane surface inward. Use shellac varnish, or gold-size, for cementing the lens on to the edge of the tube. Cement the three-quarters inch plano-convex on to the one-inch perforated disk, centrally over the aperture, and with the plane surface next the card. When the cement on both lenses is dry, which will be in a day or two, fasten this one-inch disk to the open end of the tube, keeping the lens inside. A single layer of tissue-paper, gummed on to the out-side of the tube, and turned down about one sixteenth of an inch all around the edge of the two-inch lens, and around the disk at the other end, will now serve as a sort of fastener to both, and will complete the eye-piece, which is shown in full size in section, Fig. 3. The smaller lens a must be next the eye when the telescope is fitted up; the larger lens b, called the field-glass, will be inside and facing the object-glass.
For fitting together the various parts now completed few directions are needed. The cell containing the object-glass must first be slid into one end of the large tube, and made to fit neatly, by even-wrapping with tissue-paper or other soft material. The sheath (Fig. 1) must now be slid into the other end of the large tube, and fitted in a similar manner. Now push the draw-tube into the sheath, and slide the eye-piece about half-way into the end of the draw-tube, and the telescope is completed. Those who are aesthetically inclined may give an extra finish to the main tube, and also to the draw-tube and eye-piece, by using for the outermost layers gilt-paper, or other smooth and colored
material. A sun-shade, consisting of a wide tube, three inches long, may also be made to slide over the object-end of the telescope; and a cap may be added to this to keep out dust. A kind of cap, perforated with an aperture one quarter of an inch in diameter, may also be constructed for slipping over the eye-piece, so as to preserve the proper distance between the eye and the eye-lens when making observations; and a second similar cap should be made, and furnished with a disk of black or red glass, for protecting the eye when viewing the sun. For myself, I use a disk of thin microscopic glass, smoked and fastened in a cap which slips over the eye-piece.
But a telescope, even such as I have described, and which has a power of only twenty-five or twenty-six diameters, needs a stand, and this can be constructed easily and cheaply of one-inch pine and a few nails and screws, something after the pattern shown in Fig. 4. By laying the telescope on the two end-supports, Y Y', greater steadiness is secured than by using a single support in the center; and the rods y y' are easily raised or lowered, and may be fixed in their positions by the little wedges w w'. The stand is thirty inches high, sixteen inches broad, and twenty-five inches long. The rods y y' are forty inches and sixty inches long respectively. The blocks B B' are built up of pieces of one-inch board, nailed together; then an auger-hole is bored through the whole, so as to form a sheath or tube in which the rods may slide easily, but without so much lateral motion, or "wiggle," as they would have if they only passed through one thickness of board.
By following these directions you will have a really useful achromatic telescope; small, indeed, and insignificant when compared with the six-foot reflector of Lord Rosse, or with one of Clark's twenty-six inch refractors; but, nevertheless, a veritable Jacob's ladder, by which you can ascend—if not into—at least twenty-five twenty-sixths of the way toward heaven; a perpetual source of pleasure, to a family of intelligent children, on moonlight nights and on occasions of eclipses; worth a whole year's "schooling" as an incentive and help to the study of the universe, and a practical realization of an answer to the oft-mouthed prayer—
"Nearer, my God, to thee!"