Preface - In 1934, a small group of young men, imbued with a mutual interest in astronomy and a curiosity about the tools of the astronomer, organized themselves into the New York Telescope Makers Association. With the erection of the Hayden Planetarium in 1935, a new focal point of astronomical interest was created for the New York metropolitan area. Shortly thereafter the telescope making group became a part of the Amateur Astronomers Association, an organization now of about 500 members, sponsored since 1927 by the American Museum of Natural History.
1. Story of the Telescope - Prior of the time of the telescope, man's view of the celestial universe was woefully restricted when compared with what now can be enjoyed on any clear evening with ordinary binoculars. There were visible to him then only the naked-eye objects, the sun and the moon, five of the planets, and on a clear night stars down to about the 6th magnitude, some 2,000 in all. A few hazy spots could also be seen, and there would be an occasional comet. Completely unknown were the outer planets, satellites of the planets, Saturn's rings, and infinite numbers of stars and galaxies.
2. Story of the Telescope
#2 - Despite the attendant difficulties, a number of very large specula were made, some of the best by William Herschel. Born in Hanover, Germany, Herschel settled in England in 1757, where he became interested in astronomy and later (1776) turned his attention to telescopes. Working entirely by hand, at first as an amateur, he practiced and developed his technique on a great number of Newtonian telescopes, and learned how to figure the mirrors far better than had any of his predecessors.
3. Materials and Equipment - The materials and equipment which are needed to produce the mirror are:
A 6-inch pyrex mirror blank, from Corning Glass Co., Corning, N. Y. The sides of a pyrex blank are tapered, giving to one surface a slightly larger diameter than the other. The larger surface is the one that is to be ground to curve for the mirror.
A 6-inch plate-glass disk, of a thickness at least 1/8the diameter, from any plate-glass manufacturer. This is to serve as the tool.
4. Mirror Grinding - It is assumed that a cellar or other room in which the temperature is fairly constant is available. Grinding, even polishing, might be done almost anywhere, but testing and figuring can be carried on only under conditions of uniform temperature.
There are three motions that the optician must employ in order to preserve a surface of revolution on his mirror. First, the back-and-forth grinding stroke produces the curve. Second, the mirror must be rotated in order to produce this curve on all diameters.
5. The Pitch Lap - We are now ready for the polishing lap. First, cut a strip of newspaper at least 20" long, and 14" wider than the thickness of the tool; run it through some melted paraffin and set it aside. This is to serve as a collar, to be wrapped around the tool to retain the hot pitch while it sets. Now melt the pitch slowly on the stove, in a pot or can. It is highly inflammable, so keep the flame low, and have on hand a board or other cover that can be immediately placed over the pot to smother any flames. Use a wide, thin stick for stirring. If a can is being used, it may be gripped with a pair of pliers while pouring.
6. Polishing–Testing–Correcting - We know that grinding, as performed by the mirror maker, is a fragmentation of the surface of the glass. The older theory of polishing was that it consisted of a sort of continuation of this process on a fine scale; that is. that the rouge particles, measurable in diameter in units of a wave length of light, were partially embedded in the pitch, the protruding edges having a planing action on the pitted surface of the glass. (This is what occurs when the surface of a pitch lap is charged with a fine grade of carborundum. A fairly good semi-polish can be obtained by-charging the lap with fine emery.)
7. The Paraboloid - As mentioned in the first chapter, the mirror must have a paraboloidal figure in order that all rays entering the telescope parallel to its axis will converge to meet in a single point in the focal plane (Fig. 10b). It was also stated that a spherical mirror might be altered into that figure in any one of three ways. But not the same size of paraboloid would be derived in each case, although, like spheres, the shapes of all paraboloids are the same.
8. The Paraboloid #2 - The center of curvature thus found is not that of the extreme edge zone, but of a zone of 2.8" radius, or the mean radius of the zonal opening. A calculation will show that the center of curvature of the extreme edge zone, which is what is ultimately sought, lies about 0.01" farther from the mirror. Consequently, when it is deemed that the shadow appearances in the edge openings conform to prescription, the knife-edge should be withdrawn from the mirror by that amount.
9. The Diagonal - Rays of light from a distant star, upon reaching the earth, are sensibly parallel, and the rays that strike the surface of our paraboloidal mirror are contained in a cylinder 6" in diameter. They are then converged by reflection to form a point image of the star at the focus of the mirror. If a strip of screen is stretched across the focus, and if other stars are present in the field, their images will be observed on the screen extending a number of degrees on either side of the optical axis.
10. The Diagonal #2 - Reasonably monochromatic light is required for testing selected pieces of plate glass as well as the figured diagonal. This light may be obtained from any of the now rather common gas-tube lights; an inexpensive neon lamp is excellent, and argon is also good, but requires a darkened room. Either type may be purchased at a radio supply store. The bulb may be screwed into an ordinary light socket; an adjustable desk lamp is convenient. A thin piece of absorbent tissue wrapped around the bulb will provide satisfactory diffusion.
11. Tube Parts–Alignment–The
Finder - The wooden cell described here is easily made and permits of ready adjustment. The materials required are: two well-seasoned hardwood boards, one 6" square and: ¾" thick, and the other about 8" square and 1" thick; three WSO round-head stove bolts, 3" long, and nine washers to fit; three very stiff compression springs, ¼"to ½"long and large enough in diameter to fit freely over the stove bolts; three ¼-20 wing nuts; three 10-24 round-head machine screws %" long: three 10-21's 1" long. Clips for securing the mirror to the cell can be cut from brass angle shapes obtainable from the 5 & 10.
12. Tube Parts–Alignment–The
Finder #2 - At this point a full-scale sectional drawing of the eyepiece end of the tube should be made, similar to Fig. 59, showing in detail the diagonal, holder, spider supports, and so forth. The block on which the diagonal rests, B, is a prism-shaped piece of hardwood cut from a square stick of the same width as the diagonal. A ⅜" square-sectioned strip of wood is glued across the base of the hypotenuse face of B, and later one half of it is planed away, making a neat-fitting seat, C, for the diagonal A.
13. Eyepieces and Related
Problems - Nearly every optical principle as applied to a telescope requires consideration of the function of the eyepiece when the instrument is used visually. Most first telescopes are built for visual observing, so a knowledge of eyepieces is essential if the best possible results are to be obtained from the instrument as a whole. As will be seen in this chapter, there are good and poor eyepieces, and some good ones are not applicable to every type and size of telescope; be very sure, therefore, to equip your instrument with suitable eyepieces of good quality.
14. The Mounting - Everyone is more-or-less familiar with the surveyor's transit, in which two axes at right angles to each other enable the telescope to be aimed at any point in the celestial dome. Its motions are one in azimuth, about the perpendicular axis, and one in altitude, about the horizontal axis. The names of these motions have been combined in the term altazimuth, which is used to describe this type of mounting. When the telescope is swung through 360° in azimuth, the field of view traverses an orbit about the zenith, whereas the stars, in their daily or diurnal motion, describe orbits around the celestial pole.
15. The Mounting #2 - Table I lists sizes and descriptions of parts for both permanent and portable mounts. A 45° elbow with a tee, used as shown in Fig. 71, rather than the wye of Fig. 72, is preferred for the portable mount, as it dispenses with the counterweight on the polar axis. The wye is more rigid, of course, and so is chosen for the fixed mounting. In thus dispensing with the counter-weight, the radial thrust load on the polar bearing is in-creased and is in counter directions at the opposite ends.
16. Aluminizing and Cleaning - As mentioned in Chapter 7, it is best to defer the aluminizing until the mirror cell and other tube parts have been completed; then the mirror and diagonal can be treated at the same time. It seems almost superfluous to add that it is the polished and figured surfaces that are aluminized. The lustrous metallic coating, of the order of a quarter of a wave length of light in thickness, increases their reflectivity some 22 times.
17. Setting Circles–Equatorial
Adjustment - In order that the maximum benefit may be derived from its use, the permanently mounted telescope should be in accurate equatorial adjustment and equipped with setting circles. When the circles are finally adjusted and fixed, the telescope can then be set to the known declination and hour angle of a celestial object, and it will be found to be in the field of view. The hour angle is the difference in time between the hour circle of the object and the observer's meridian; in other words, it is the difference between the right ascensions of the object and the observer's meridian. The right ascension of the meridian is always equal to the local sidereal time.
18. Optical Principles–Atmosphere–Magnitudes - We found in chapter 13 how magnification in the telescope is brought about. But this is not the only function of a telescope when used astronomically; depending on the uses to which an instrument is put, resolving power and light-gathering power may be of considerably more importance. A knowledge of some of the optical principles associated with these functions will enable the amateur to operate his own telescope to best advantage. The discussion here is necessarily brief, and may be supplemented by reference to various books listed in the bibliography.
19. Optical Principles–Atmosphere–Magnitudes
#2 - By moving a high-power eye-piece inside or outside of focus, a bright star image will be seen to expand into a luminous disk which can be resolved into a number of concentric rings, atmosphere permitting. To the initiated eye, these extra-focal rings tell the story of the optics of the telescope. Half a dozen or more may be visible, the outer one being the widest and brightest, and the inner rings diminishing in brightness inwardly (somewhat like a reversal of Fig. 53b). With a perfect mirror, the appearance of the rings will be identical at equal distances either side of focus. (Visual tests will be upset if spherical aberration is present in the eyepiece.)
20. A Second Telescope - Often, before his first telescope finished, the enthusiastic amateur is planning on a second one. This is the usual manifestation of a healthy interest in an exacting and fascinating hobby, but it should be subjected to a modicum of restraint. The tyro should be wary of any impulse to make a telescope of great aperture and focal length, and should carefully consider the practicability of such a project, and the probability of its successful conclusion. He must also consider the ways, if any, in which a second instrument can supplement the observing program planned for the first telescope.
21. A Second Telescope #2 - Almost everything that has been said of the 6-inch can be applied to a 4¼- inch, asize of reflector which seems to have been considerably underrated. It is the regrettable truth that vast numbers of large and excellent telescopes are collecting cobwebs in attics, garages, and cellars, simply because they are too massive lo be readily moved outdoors and in. No doubt their owners were once enthusiastic amateur astronomers, but their ardor has been dampened, and they have been denied many anticipated pleasant evenings of observation, because a too-ambitious program was undertaken as an initial venture. The 4¼-inch reflector is a powerful instrument in its own right, far more telescope than Galileo ever had. Including a tripod, its total weight is trifling, between 20 and 30 pounds.
Appendix A - It sometimes happens that the tool and mirror blanks may not be of exactly equal diameters. A difference of about 1/8" does not matter, as the working surface of either disk can be beveled by whatever amount is needed to make them both equal.
Occasionally, before completion of the mirror, either it or the tool may be broken. If this happens to the tool in the polishing stage, a flat iron disk or another glass disk can be obtained, and a new lap made on one of its flat surfaces. The greater thickness of pitch at the center will introduce no difficulties, and polishing and figuring can be successfully concluded.
Appendix B - The method described in chapter 8 for grinding and polishing the diagonal is not the one usually pursued in the making of an optical flat, although it is capable of a high order of accuracy. Of course, any means of grinding that will result in an apparently perfectly flat surface, when tested against a high-quality straight-edge, should be sufficiently reliable to warrant polishing, in the expectation of producing a perfect plane. A master flat of known precision is essential, however, if the operation on a single surface is to be successfully concluded.
Appendix C - (Dates refer to the latest editions of these books, although in many cases earlier editions will prove satisfactory.)
Amateur Telescope Making, edited by Albert G. Ingalls. Scientific American Publishing Co., New York, 1935.
Amateur Telescope Making Advanced, edited by Albert G. Ingalls. Scientific American Publishing Co., New York, 1944.
Applied Optics and Optical Design, Alexander Eugen Conrady. Oxford University Press, New York, 1929.