A very large amount of information can be gleaned from some very basic preliminary testing. This will eliminate binoculars that are grossly unsuitable. Remember that with very few exceptions, aberrations, faults, or features that are merely irritating during initial testing will become infuriating under the stars. However, while you should not expect to find a binocular that is entirely free of all aberrations or faults, you should expect to find that they exist in lesser number and severity in more expensive instruments. What you can expect to have to accept depends to a large extent on your budget, and you may find that you are more sensitive to some aberrations or faults than to others. Your final choice must inevitably be a subjective one, but will ideally be one that is guided by a measure of objectivity. With all tests that use touch or hearing, remember that closing your eyes tends to make these senses more acute for many people.
Do not even consider fixed focus, zoom, or "quick focus" binoculars; they are unsuitable for astronomy. Here is a list of things to look for in a quality pair of binoculars:
• Visual Overview. Reject any binoculars that have "ruby" optical coatings, loose screws or screws with damaged heads, covering material that is not in complete close contact with the binocular housing, scuff marks anywhere, any evidence of dust or other foreign matter on the inside of the optics, or internal baffling that is not uniformly matte black.
• Mechanical Overview. Give the binoculars a good shake. Reject any in which you feel or hear any movement of components.
• Focus Mechanism. Run the focus mechanism through its full range. The feel should be uniform throughout the range and should not be too stiff or too loose. If it is stiff, it is very difficult to find a precise focus. If it is loose, it is difficult to maintain a good focus. Feel for any stiff regions or any "sloppy" regions. A different feel in different regions is indicative of poor mechanical tolerances in manufacture. Feel for any difference between dynamic friction and static friction ("stiction") by stopping the focus at various points throughout the focal range and feeling for a slight jerk or "catch" when you start to refocus. This is usually due to poor-quality lubricant and makes precise focusing difficult. On binoculars that have individual eyepiece focusing, test each eyepiece focus individually. On center-focus eyepieces, remember to test the focus ring of the right-hand eyepiece.
• The Bridge. The bridge is the pair of arms that connects the eyepieces to the center-post focus mechanism in Porro prism binoculars. All bridges will rock to some extent and, as they do so, they change the focus of either or both sides of the binocular. On well-made binoculars the rocking is minimal and requires considerable pressure, more than will be put on the eyepiece housing in normal use. On budget-quality binoculars there can be considerable rocking with minimal pressure. This rocking gets worse with age. To test the bridge for rocking, merely hold the binoculars by the prism housings with the eyepieces downward, then press down alternately on the eyepiece housings with the tips of the forefingers. The severity of any rocking becomes immediately apparent. If you are unsure how it will affect you, hold the binoculars to your eyes, focus on something and, by rocking the binoculars from side to side, put slight pressure alternately on each eyepiece housing with your eye socket. If the focus changes, reject the binoculars.
• Interpupillary Distance (IPD) Adjustment. In hand-held binoculars, the IPD adjustment is usually the central hinge. For larger binoculars, it can be either a hinge or eccentrically rotating prism housings or eyepiece turrets. If it is loose, it is difficult to maintain any given IPD. If it is very stiff, or if it is jerky, it is difficult to set the IPD. If only one person is to use the binoculars at any one time, this need not be a significant problem. Be aware that several center-hinge binoculars need to be folded to near the minimum IPD in order to fit into the case; test if this is necessary with your IPD. Check that the IPD range accommodates all intended users. Most binoculars do not cover the entire range of IPDs for adults; this range is usually considered to be about 43 to 80 mm with a mean at around 65 mm, and about 90 percent of people have IPDs within 8 mm of the mean. Most binoculars do not deviate more than about 10 mm from the mean. If you know your IPD, the binocular IPD adjustment is easy to check merely by using a piece of card with your IPD marked on it and holding it over the binocular eyepieces. If not, you can check this when you test the binoculars for optical quality (below). Check that the eyepieces go comfortably to your eyes when the IPD is set for you. If you have narrow-set or deep-set eyes, or if your nose has a wide bridge, this may not be possible for you especially with wide-angle eyepieces, which are typically larger in diameter than "normal" ones (Figure 4.1).
• Tripod Bush. You will be able to see significantly more with mounted binoculars than with unmounted ones, even if you do intend to use them primarily as a hand-held instrument. Most medium-sized modern binoculars have a quarter-inch UNC (20 t.p.i.) bush in the distal end of the center-post. This bush is covered by a cap, usually made of plastic, but sometimes of metal, which unscrews. Remove it and check the quality of the bush. If possible, try an L-bracket and ensure that you can easily connect the bracket to the binocular, with the thread of the L-bracket bolt easily meshing with the thread in the bush without danger of cross-threading. Remember that you will most likely be wanting to do this in the dark, possibly with cold or gloved hands.
• Prisms. Hold the binoculars away from you, pointing toward something relatively bright (e.g., the sky or a light-colored wall or ceiling—not the Sun!), and look at
the bright circle of light in the eyepieces. If it is actually a circle, all well and good. If it is tending toward lozenge shaped, this is an indication of undersized prisms; undersized prisms are themselves indicative of cost-cutting. If there are blue-gray segments of the circle with a brighter lozenge inside, this is indicative of cheaper BK7 glass in the prisms (Figure 2.8). In both cases, the prisms will cause some vignetting of the image. With roof prism binoculars, bring them up to the eyes and carefully examine the image of the bright surface. Do you see a faint line if you defocus your eyes? If so, you are seeing the "ridge" of the roof prism. If the line is obtrusive, it will result in flaring of bright astronomical objects. This is easier to test for using a bright point of light against a dark background, but this is usually not available in the store.
• Eye Relief. When you look through the binoculars, the image in the eyepiece should be surrounded by the crisp dark edge of a field stop at the mutual focus of the eyepiece and objective. If there is insufficient eye relief, you cannot get your eye sufficiently close to enable this. Most modern binoculars have fold-down rubber eyecups around the eyepieces to enable their use by spectacle wearers (Figure 3.1). However, not all rubber eyecups fold down, and not all permit a bespectacled observer to get his eyes sufficiently close. Even if you do not wear spectacles, do check that the eyecups fold down. On cold or dewy night, warm moisture evaporating from your eye can condense on the eyepiece, causing it to fog. If you fold the eyecup down, air can circulate between your eye and the eyepiece, reducing the likelihood of fogging.
• Comfort. Comfort is particularly important if you intend to use hand-held binoculars for long periods of time. In general, lighter binoculars are less tiring to hold, but this is not always the case. A heavier binocular that is well designed from an ergonomic perspective can be less tiring than a lighter instrument that is poorly designed. There is no substitute for experiment when it comes to determining how a particular binocular suits you as an individual. When you perform the optical tests below, take your time and perform them consecutively without taking the binoculars from your eyes. Be conscious of how tired your arms become.
• Focus. (See also "Spherical Aberration" below.) To focus a binocular, do one optical tube at a time, with the other side covered with a lens cap on the objective side. First, set the interpupillary distance. Check by alternately shutting eyes or alternately covering objective lenses, so you have a complete field of view, surrounded by the field stop, on both sides. If the eyepieces focus independently, it does not matter in which order you do them. With center-focus binoculars, cap the right-hand objective and focus the left-hand optical tube on a distant object with the focus wheel (called "focus band" in some binocular instruction sheets). Critically examine the image to determine if it "snaps" to a good focus or if there is a range where it looks less "mushy." When you have the best focus, swap the lens cap to the left objective and, without adjusting the focus wheel, focus the right-hand tube with the focus ring on the right-hand eyepiece housing. Again, critically examine the focus. Remove the lens cap and again examine the focus. Then focus on a nearer object and, by alternately covering the objective lenses, verify that both sides are focused, not merely the one used by your dominant eye. Do not be tempted to use a hand instead of a lens cap or, worse, to focus individual tubes by closing one eye; this is rarely satisfactory. The hand almost always changes the mutual orientation of eyes and binoculars from the usual observing position. This is true also when the binocular is mounted, merely because of the act of stretching the arm to a non-observing position. Closing one eye can cause the other to squint. When you do visual optical tests, you want your eyes and body to be relaxed.
• Focal Range. When binoculars are used for astronomy, it is not immediately apparent why one would need any focal distance other than infinity. There is, of course, the trivial case of the applicability of binoculars to terrestrial use, which often requires that you can focus them closer than infinity. The other case is that of eyeglass wearers who wish to observe without eyeglasses. If your eyes are hypermetropic (farsighted), there is usually no problem because the binocular adjustment is to what would be focus on a nearer object for a person with normal vision. However, if your eyes are myopic (nearsighted), to focus on an object at infinity the binocular must be adjusted to focus on what,to a person would normal vision, would be beyond infinity. Whereas most binoculars have some facility for this, the amount of extra focus travel varies enormously. If your eyes are myopic and you wish to observe without your eyeglasses, you should verify that the binoculars have sufficient focal range to accommodate your eyesight. You should try to focus on an object at as great a distance as possible—at least a Kilometer (about half a mile), but preferably more—away from you. Additionally, because the depth of focus of your eyes is reduced when your pupils are dilated, as they will be when the binoculars are used for astronomy, you should try to focus on a dark object without a bright background. Dark vegetation on the horizon is ideal. Be aware that you will almost certainly need a bit more "beyond infinity" travel at night time than you need during the day, so allow for this.
• Internal Reflections. Internal reflections, be they reflections off the interior walls or components of the binoculars, or "ghost" reflections off poorly designed or inadequately coated optical components, are both distracting and detrimental to astronomical observation. They tend to be most obtrusive when a small bright object is observed, off-axis, against a dark background (i.e., exactly the conditions that are often found in astronomical observation!). To test for this, use a bright light source (not the Sun), such as a recessed halogen lamp in the store ceiling or a Mini-Maglite® with the lens assembly removed, slightly off axis.
• Chromatic Aberration. All binoculars will exhibit some degree of chromatic aberration. In very good binoculars it may only be noticed off-axis and then only just perceptible. The purpose of this test is to compare binoculars, not to find one that is perfectly achromatic. Chromatic aberration is most obtrusive with high-contrast objects, such as many astronomical targets. Although it may not be noticeable on fainter or lower-contrast objects, if it is present it will degrade the image by reducing contrast. The simplest daytime test is usually to view a distant television antenna or electricity post or similar object against a bright sky. It is important to ensure that the IPD is properly set, since chromatic aberration can often be induced by moving the eye off axis. Focus the target object at the center of the field and slowly pan the binoculars so that the object moves toward the edge. Chromatic aberration will be visible as colored fringes at the interface of light and dark, usually magenta on one side and cyan on the other.
• Spherical Aberration. (See also "Focus" above.) Spherical aberration occurs when light from different regions of the lens is focused at different distances from the lens. It is usually well corrected for in modern binoculars, but does exist in budget ones. It is visible as a "mushy" focus (i.e., an object at the center of the field of view does not "snap" to a good focus).
• Field Curvature. Lenses do not focus images on a plane, but on a curved surface that is concave toward the lens. The result is that if the eyepiece is focused on an object at the center of the field of view, objects at the edge will be out of focus. This is field curvature and it is potentially present in all binoculars. In excellent binoculars it may not be noticeable at all; in budget binoculars it can be obtrusive less than halfway to the edge of the field. Unless it is severe, it is not a major problem for daytime use, where the attention is on objects at the center of the field of view, but astronomers prefer pinpoint star images right to the edge. It can be ameliorated by addition of extra lens elements, and the field of view can be limited by field stops so that it is not apparent. The extra lens elements will absorb some light and reduce contrast, an important consideration for astronomical use. In ultrawide-angle binoculars, unless they are extremely well designed, it can render most of the field of view unusable for astronomy, thus negating the perceived advantage of a wide apparent field of view. To test for it, merely focus on an object at the center of the field of view and move the binoculars so that the object moves toward the edge. If it goes out of focus but can be refocused, this is field curvature. (If it cannot be refocused, it is probably coma.)
• Coma. Coma is a form of off-axis spherical aberration. It is very difficult to test for during the day as it requires a point source of light. This is sometimes possible by viewing a glint of sunlight reflected off a very curved shiny object such as a metal car radio antenna. Focus the glint on the center of the field and move the binoculars so that the object moves toward the edge. If coma is present, the image of the glint will flare toward the edge of the field, giving it the appearance of a comet (hence the name coma) with its head toward the center. Coma is often present in binoculars that are not specifically designed for astronomy. This is because it is not normally visible or particularly degrading of daylight images, largely because birders, for example, use binoculars to examine birds at the center of the field.
• Astigmatism. Astigmatism is an aberration that, like coma, is very difficult to test for during the day. It manifests itself as a point object, such as a glint of sunlight, being seen as a short line when just out of best focus, that changes orientation through 90 degrees from one side of focus to the other.
• Vignetting. Vignetting results from the outer parts of the field of view not being illuminated by the whole of the objective lens. It manifests itself as a darkening toward the edge of the image. It is present in all terrestrial binoculars, where it is not obtrusive when they are used to examine objects at the center of the field of view, and most astronomical binoculars. Mild vignetting can be difficult to test for during daylight because the human visual system readily adapts to a very large range of illumination,but flicking the gaze back and forth between the edge and center of the field of view will usually reveal it.
• Kidney-bean Effect. The kidney-bean effect, also known as flying shadows, is an affliction associated with some widefield eyepieces and is a result of spherical aberration of the exit pupil. Instead of being a flat disc, the exit pupil is curved and it is therefore impossible to focus the entire field of view at once so you have to hold the binocular slightly further from the eye to focus one zone than another. There is a position at which your iris will cut off the light from a zone between the center and the periphery and, if your eye is not perfectly aligned with the optical axis of the eyepiece, the result is these flying shadows that have the shape of a kidney bean. The effect is more pronounced in daylight, or when viewing a bright Moon, than at night, and is therefore best tested for during daylight.
• Distortion. Almost all binoculars will exhibit some distortion toward the edge of the field. To test for it, focus on a straight object, such as a telephone pole or a roof ridge that extends across the diameter of the field of view. Move the binoculars so that the edge of the object forms a chord near the periphery of the field. If the object curves inward toward the middle, you have pincushion distortion; if it curves outward, you have barrel distortion. A small amount of pincushion distortion can be desirable for terrestrial use, but it has no advantages—or significant disadvantages—for astronomy.
It is a common phenomenon, even among experienced binocular users, that when they optically evaluate a binocular, they notice pincushion distortion and comment adversely on it. It is far less common (I hope that this book will go some way toward remedying this) that they know the whole reason. It is indeed true that pincushion distortion,which manifests itself as straight lines at the edge of the field of view appearing to curve in toward the middle of the field, results from increasing angular magnification away from the center of the field, but this difference in magnification is not an error, it is intended. If there is equal angular magnification, the linear magnification at the edge of the field is less than in the center, and an optical phenomenon called rolling ball effect occurs when the binocular is panned. This may not be noticeable when the binocular is used astronomically, but when it occurs in terrestrial use, it can be distinctly unpleasant and even cause nausea. A small measure of pincushion distortion eliminates this rolling effect. Different manufacturers choose different compromises between rolling ball and pincushion; the choice of which particular compromise is best is entirely subjective.
• Collimation. Unless miscollimation is severe, this is usually considered to be the most difficult condition for which to test. Unfortunately, it is present in a large number of lower-priced binoculars. Severe cases manifest as a double image that cannot be eliminated. Our eyes can compensate for mild miscollimation, but the price is eyestrain, which can lead to fatigue and headaches if it is prolonged. The acceptable limits for miscollimation are given in Chapter 2, but these limits are not ones that can be measured during a preliminary test. A daylight test for convergence and divergence would be to support the binoculars and focus on a distant vertical target such as the edge of the wall of a building or a telephone pole, and close one eye and place the target at the edge of the field of view. Alternately close your eyes and see if the image in one eye is laterally displaced from that in the other. To test for step (i.e., vertical shift), use a horizontal target such as the ridge of a distant roof. We are far less tolerant of step than we are of convergence or divergence, so if you detect any step at all, you should reject the binoculars.
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