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 fast ened together side by side for simultaneous visual use by the observer. Generally when we employ the term today we have in mind the modem "prism binoculars" which are characterized by stubby, offset tubes with main lenses about twice as far apart as arc 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 deluxe 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 hold 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 tip again? Each binoculars tube has a bulge in it. Inside that bulge arc 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 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 59.79." 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 X 30. 7 X 35, and 7 X 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 X 50 size. For similar reasons the 7 X 50 is recommended first for general astronomical viewing, but the 6 X 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 arc 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 yon 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 skv you will discover the best setting for your particular eye separation, and thereafter you can set the binoculars at that reading each time you remov e 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 eyepicce 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 instalment will produce eyestrain. It must be rejected even though the individual oculars may be of excellent quality.
A few words about the care of your astronomical binoculars may be in order here. Of course I do not have to caution you never to let vour fingers touch any of the lenses, but watch your friends. Some people, when in the presence of an optical instrument, seem to be seized with an uncontrollable urge to touch a finger to the lens if it is within reach. Others are just downright clumsy and can't pick up a magnifying glass without grabbing it in the middle of the lens instead of by the handle. Still others apparently wonder whether or not their finger will leave a print. It always does, and it always will, and every fingerprint reduces somewhat the efficiency and quality of the instrument. Like testing the law of gravity by jumping from the roof, this is an experiment that hardly needs to be performed again.
When not in use. keep the lens caps on both ends of your binoculars, and store them in the carrying case. That procedure will keep the lenses clean for a long time. When dust eventually does accumulate on the eyepiece and objective lenses, get a one- or two-ounce syringe at the drugstore. Use it to expel quick air blasts onto the lens, taking care that the nozzle does not touch the glass. If that does not suffice, the lens must be washed. Never touch a dusty lens with a dry cloth no matter how soft and clean the cloth may be. Dip a piece of clean cotton in water and touch it lightly to the lens turning the cotton over in your fingers until the entire lens is wet. Then dip a fresh piece of cotton and swab lightly, holding the binoculars in such a way that water cannot run inside. Finally blot dry by light application of a dry piece of cotton or a freshly laundered handkerchief. Any lint that remains may be blown off with the syringe.
On cold nights with high humidity (when you can see your breath) it may be difficult to keep condensed moisture off the lenses. Hold and move your binoculars so that you do not breathe toward them. Even with that precaution, vapor from the atmosphere may condense on the outer lenses and vapor from your moist eyeballs may condense on the eye lenses. Do not wipe it off. Use the syringe to blow it dry. If such treatment becomes necessary every few minutes, the best remedy is to close down operations and wait for a better night.
When observing or carrying binoculars out of the case always use the neck strap. This practice saves observing time and insures against damage should the glass slip from your grasp.
Was this article helpful?