Read an Excerpt
Exploring the Moon through Binoculars and Small Telescopes
By Ernest H. Cherrington Jr.
Dover Publications, Inc.Copyright © 1984 Ernest H. Cherrington, Jr.
All rights reserved.
An invitation to explore the moon through binoculars tonight may seem as absurd as an invitation to cross the Pacific Ocean in an outboard motorboat. But wait! It does make sense. True, we live in a world of huge, powerful, and expensive instruments –giant optical telescopes up to 200 inches in aperture and vast radio telescopes up to 1000 feet in diameter. Yet, of the 4 billion people who inhabit the small planet Earth, only a negligible percentage even have seen such a telescope, and of them only a negligible percentage have had an opportunity to observe with it. Few, indeed, have gazed outward into the universe through a telescope the diameter of the 200-inch Hale telescope. Even if we go down to its diameter, the number of people who have access to such a telescope remains very small in spite of the fact that amateur telescope making has been a popular hobby for the past 60 years.
With binoculars the situation is entirely different. Fifty years ago they constituted a luxury item sold by opticians and pawnbrokers. Today they are sold in large volume by thousands of department stores, discount houses, mail-order firms, drugstores, and other retail establishments. Since the informed and critical buyer can obtain a good "glass" for as little as twenty to forty dollars, no home need be without one in this prosperous country of ours.
But are ordinary binoculars good for anything more than watching birds, horses, and the neighbors across the street? The answer is definitely "Yes!" Carefully selected 7 × 50 binoculars (magnification, 7 diameters; aperture, 50 millimeters or 2 inches) constitute a far better astronomical instrument than any professional astronomer possessed during the first half-century that followed the invention of the telescope. In fact, even a casual glance at the first quarter moon through 7 × 50 or 6 × 30 binoculars may show you more detail of the lunar world than was seen by the great Galileo who invented an astronomical telescope three and a half centuries ago and with it inaugurated the modern age of science through a series of unparalleled discoveries. The largest telescope Galileo ever used had a lens only two inches in diameter. It was mounted in a tube about 4 feet long, and its magnifying power was 33 diameters. However, the lens was of poor quality by modern optical standards, and it evidently resolved only the larger lunar surface features, judging from the drawings of the moon which Galileo made at the telescope and published in his book Sideereus Nuncius (1610).
If, therefore, you do not happen to own a private observatory, do not conclude that the thrill of personal observation of the moon, planets, and stars is an experience which you never can enjoy. You need not be restricted to vicarious exploration of outer space through the writings and the photographs of those who have observed. You can make direct personal contact with the moon and other worlds beyond. You can launch your own Space Program right from your roof or back yard. You can become not only well informed about the moon but intimately acquainted with its surface features. You will come to know it as a world which you have explored personally. Then the landings of unmanned space probes followed by the Apollo modules carrying astronauts will be as meaningful to you as the daily news of events around our terrestrial world. The moon is today's frontier—the New World of Tomorrow.
Perhaps you are still skeptical about the effectiveness of binoculars as an astronomical instrument. If so, let me give you an observational fact. In the monumental work Photographic Lunar Atlas, edited by Gerard Kuiper (1960), there are listed a total of 670 named lunar features. By actual observation check I have found that 605 of those features can be seen with ordinary 7 × 50 binoculars. Don't expect to resolve all the fine details shown in the illustrations in this book, however, because many of our pictures are enlargements from negatives taken with one of the world's largest telescopes. The smaller named craters can be made out as tiny dark or bright specks, but the larger lunar craters and mountain ranges show well through binoculars and with considerable detail.
Either you already possess the principal piece of equipment needed for your personal exploration of the universe, or you can obtain it easily and at little expense. You are, therefore, virtually ready to begin.
The moon is a good starter for several reasons. Not only is it easier to find and identify than any other inhabitant of the night skies, but it is the most satisfying object that the beginner can observe. It looks the biggest and exhibits lots of surface features. It shows well even in the brilliantly lighted city. It has intrigued the observer and confounded the theory maker for thousands of years. Recently it has experienced a renaissance of professional interest, and the tremendous drive to put a man on its remote surface during the 1960s attracted the attention and stirred the imagination of every thoughtful person.
Just as binoculars serve well in the exploration of the moon, they also reveal to advantage much of the stellar universe beyond our solar system. There are hundreds of double stars, variable stars, star clusters, gaseous nebulae, and stellar galaxies which can be located and examined by the armchair astronomer who holds his instrument in his hands and who knows where to direct it.CHAPTER 2
An ocular is an eyepiece—that system of small lenses at the bottom of a telescope (or top of a microscope) with which the observer examines the image formed by the main lens (or mirror) of the instrument. Consequently, a binocular is simply a double eyepiece arranged so the observer can use both eyes at the instrument instead of one.
Such a device is used frequently in connection with microscopic studies but rarely is found attached to an astronomical telescope. The plural form "binoculars" is used to designate any optical device in which two identical telescopes are fastened together side by side for simultaneous visual use by the observer. Generally when we employ the term today we have in mind the modern "prism binoculars" which are characterized by stubby, offset tubes with main lenses about twice as far apart as are the eyepieces.
In principle, a telescope is a simple optical device, as you probably will agree once you have demonstrated its operation to your satisfaction. The basic kit for the laboratory exercise consists of a magnifying glass and a plain white card. The de-luxe kit contains two magnifying glasses of different sizes. On a sunny day pick a spot indoors near the wall opposite a window where the illumination is as poor as possible and from which you can see objects at various distances out-of-doors. Hold the card vertically near the wall and bold the magnifying glass vertically a few inches from the card in the direction of the window. Move the glass slowly toward or away from the window until a small image of the window frame appears on the card. Slight adjustment will bring a sharp picture of the window and the plants or other knicknacks on the sill–all in full color and all inverted. The tree in the yard appears slightly fuzzy on the card, but it can be brought into good focus by reducing the separation between glass and card by a tiny amount. Additional small reductions bring into sharp focus the images of objects located successively farther beyond the window.
The magnifying glass is doing exactly the same thing that the main lens or mirror of a telescope does. It is gathering light from distant objects and using that light to form images of those objects in the focal plane of the lens, which is the plane occupied by the card in your other hand. If you replace the card with a sheet of ground glass or translucent plastic, you can view the image from the opposite side by looking through the translucent sheet as you adjust it between your eyes and the glass while gazing in the direction of the window. Actually, you are looking through a telescope, but it lacks an eyepiece. The smaller magnifying glass in the de-luxe kit will serve that purpose, but you need a third hand to place the second glass between your eye and the sheet that holds the image. Even if you are merely ambidextrous you can complete the exercise, because as soon as you place the eyepiece properly you no longer need the translucent sheet to hold the image. Place the small glass directly in front of your eye, hold the larger glass just beyond it, and move the larger glass slowly toward the window until suddenly you glimpse a distant tree or building inverted. You now have a telescope in your hands. It is not a good one, but it is a telescope, and its fundamental secrets have been revealed to you.
How do we get that inverted image turned right side up again? Each binoculars tube has a bulge in it. Inside that bulge are two double prisms that force the entering light to make four right-angle turns. That maneuver erects the image, and it also makes the instrument more compact since the tube can be cut several inches shorter than would be required if no prisms were used.
In selecting binoculars or telescopes do not yield to the common urge to get the highest possible magnifying power for the investment you plan to make. That is like buying an automobile solely on the basis of the maximum speed which the dealer claims it can attain. Maximum speed is a powerful bragging point for an automobile, but there are various other criteria of much more importance to most drivers. So it is with magnifying power. The novice may announce proudly that he is the owner of a "50-power telescope." Probably it does magnify about 50 times, but if the main lens is only 1½ inches in diameter and the tube is two feet long, it produces only a very dim, fuzzy image. Moreover, the field of view is probably so narrow that it is very difficult to set the telescope on a given star and even harder to hold it there. Usually a question or two addressed to the owner about his observing experience brings out the admission that his telescope "doesn't work very well, but it cost only $9.79." Beware of such bargain instruments! Binoculars offer you the best optical quality and the most utility and convenience per dollar invested of anything available in the broad category of telescopic devices.
Magnification is important, but of even greater importance to the astronomer are light-gathering power and resolving power, both of which increase with increasing size of the objective (main lens or mirror). Light-gathering power, in connection with the focal length of the objective (camera F value) , determines the brightness of the image. It also sets the limit of star faintness beyond which the telescope will not go. Resolving power indicates the ability of the telescope to separate close double stars and to reveal surface markings on the moon and planets. Magnification of 100 power, for example, makes the diameter of the moon appear 100 times as large as it does to the naked eye, and it also increases the apparent sizes of lunar markings in the same ratio. However, it likewise magnifies 100 times all atmospheric disturbances in the line of sight, and it amplifies a hundred-fold the effect of slight movements of the telescope. If the atmosphere is unsteady and if the telescope is not rigidly mounted, the highly magnified image of the moon will "boil" and shimmer and jump about like a cork on a choppy lake. Under such conditions far better results can be attained with much lower magnification.
Binoculars are more or less available in a wide variety of sizes with magnifications ranging up to 40 power and lens diameters up to 125 millimeters (five inches). The larger ones are heavy instruments that must be supported on a tripod or pier like a telescope. The most popular sizes are 6 × 30, 7 × 35, and 7 × 50. During World War II the U. S. Navy subjected binoculars of all feasible magnification and aperture combinations to an exhaustive series of performance tests. The conclusion was that the best all-purpose hand glass for night use is the 7 × 50 size. For similar reasons the 7 × 50 is recommended first for general astronomical viewing, but the 6 × 30 is a very close second in performance. In fact, the smaller glass actually is preferred by some observers since it is lighter and easier to hold steady.
One may not be able to measure precisely the degree of excellence of binoculars at the store counter, but with a few simple tests he can spot the poor ones to be rejected. First, note that the two telescopes or oculars are fastened together by a pair of hinges on an axle running between the two and parallel to the line of sight. By grasping the instrument with both hands you can turn it about these hinges, thus changing the distance between the two eyepieces until the separation matches that of your eyes. Looking at a distant wall, make this adjustment until you get a single circular field of view of maximum brightness and of uniform brightness with no flitting shadows near the center or periphery. You will find a graduated scale and index mark at the end of the axle near the eyepieces. Through experimentation at home on the night sky you will discover the best setting for your particular eye separation, and thereafter you can set the binoculars at that reading each time you remove them from the carrying case. If you wear glasses, there is a good chance that you will not need them when using binoculars. Deviations in focus of your eyes can be compensated by focusing the instrument unless you are very nearsighted or have severe astigmatism. Glasses and binoculars can be used together if necessary, but from experience I recommend that you use them separately if at all possible.
If you are looking at center-focus binoculars you will observe that near the base of one of the eyepieces (usually the right one) there is a graduated scale and an index mark. Place the large lens cap over the objective lens of this particular ocular, and direct the instrument toward a printed card or sign as far away as possible down the store aisle. Look through with both eyes open and relaxed as if you were gazing at an object miles away. Now turn the central focusing knob until the card, or other detailed object selected, comes into sharp focus. Don't focus your eyes. Keep them relaxed, and let the ocular do the adjusting until you can see the object sharply. Next remove the lens cap from the right ocular and place it over the left ocular lens. This time look at the same object, but do not touch the central focusing knob. Do all the focusing by turning the focusing eyepiece of the right ocular until you get a good, sharp image. Now remove the lens cap and look with both oculars at the same object. You should get an excellent image. Moreover, the image should be bright—almost as bright as it appears to the naked eye. Check this carefully. If the picture through the binoculars is conspicuously weaker than it is without them, the glass is a poor one. Don't buy a dim glass. It might do for a bright object such as the moon, but for star study it would be inadequate.
Individual focus binoculars have no central focusing knob, but a graduated scale and index mark is found at the base of each eyepiece barrel. To focus an instrument of this type place the lens cap over the objective of the right ocular, look at a distant object, and rotate the eyepiece of the left ocular until a good image is obtained. Then move the lens cap to the left ocular, look at the same object, and rotate the eyepiece of the right ocular until you get a good image. The binoculars should then be in focus. After some practice at home, focusing on stars to get the smallest possible image, you can note down the setting for each eyepiece and thereafter check the proper focus for your eyes before going out to observe.
When a telescope or any other ocular is focused properly, the rays of light emerging from the eyepiece toward the eye are parallel rays as far as any given point in the picture is concerned. This is the same condition that holds for the light rays that come to you directly from an object a few miles or more away when you look at it without optical aid. That is why you should relax your eyes completely when focusing binoculars or telescope.
With the two oculars properly separated for your eyes, and with each correctly focused, you are now ready for a most important test. Take the binoculars to a location in the store or outside where you can look at an object at least 300 feet away. Check the focus again. Since the light leaving the eyepieces is in the form of parallel rays, you should be able to move the binoculars slowly forward toward the object at which you are looking without losing the picture. Of course the field of view will narrow rapidly as you move the instrument away from your eyes, but the central part of the picture should remain in view undiminished in size. When you have advanced the binoculars five or six inches from your eyes you still should have a clear view of what appeared at the center of the field when the instrument was in the normal position at your eyes. Hold steady and close one eye, leaving the other open. Then open the first eye and close the second. Alternate rapidly from eye to eye and note what happens to the picture. If nothing changes, the binoculars have been aligned properly by the manufacturer, and the instrument is acceptable. If the picture jumps up and down or from side to side as you switch from one eye to the other, or if you see a double overlapping image with both eyes open, the optic axes of the two oculars are not parallel, and use of the instrument will produce eyestrain. It must be rejected even though the individual oculars may be of excellent quality.
Excerpted from Exploring the Moon through Binoculars and Small Telescopes by Ernest H. Cherrington Jr.. Copyright © 1984 Ernest H. Cherrington, Jr.. Excerpted by permission of Dover Publications, Inc..
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Table of ContentsInvitation
How to Find the Moon
The Man in the Moon and Other Features
How the Moon Behaves
A Month with the Moon
The New World of Tomorrow
Lunar Photographs Supplement