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Binoculars for Astronomy
by Peter Abrahams

   Binoculars are the best 'first instrument' for astronomy, and retain their usefulness to the advanced stargazer and of course to observers who venture out in the daylight hours. There is a vast array of binoculars on the market, for although the number of manufacturers is small, they make many different models. Some of the major brand names sell models that are offered under a different name by other retailers. Specific models and brands will not be the subject of this article, but instead the characteristics of all binoculars will be described, hopefully allowing an informed decision concerning a purchase. An excellent article in the April 1997 Sky & Telescope gives additional information, including a descriptive list of some of the best available models. These cost between $300 and $1000, and are worth the money. However, do not disdain inexpensive binoculars. They all show noticeable defects, but most give a sharp view in the center of the field. A $50 binocular will increase your unaided ability to observe more than a $1000 model will improve the view of the lesser one. It is a marvel that the limited quality of the inexpensive models is achieved, for all binoculars require components made of costly optical glass, assembled to a very high degree of precision. The availability of cheap instruments can be of great benefit to astronomy, both in attracting newcomers without a telescope, and in allowing the observer to forget about the painstaking care of expensive & fragile instruments. However, cheap binoculars are much less rugged, and will not be repairable (or will cost more to repair than replace). Since astronomers place a very high value on their views, it is wise to invest in the best instruments you can afford, for there are easily noticeable differences in quality between the price ranges.

   Do not buy a binocular without trying it out, even if you have the specifications for field of view, weight, etc. Some have dimensions that don't fit your eyes and are uncomfortable to use. For example, oversize oculars can give wide field views but if your eyes are close set, you will not be able to center your pupils in the ocular. Some smaller models have shallow oculars that do not sufficiently project above the bridge that joins them and let the bridge hit your nose before you can see the entire field (Westerners have deep set eyes compared to Orientals, where most instruments are made). Extreme eye relief for eyeglass wearers forces you to hold the binocular suspended in front of your eyes. If children are going to use them, remember their small interpupillary distance means that they will not be able to see through both sides of most binoculars. There are also the physical dimensions of weight and size that are hard to judge without a trial. Light weight is always more comfortable for extended use but usually means a less rugged glass. Thus, mail order is hazardous unless you have tried the binocular you are ordering. Several local stores have selections of binoculars. Some mail order firms have lower prices, but will not be there to assist you or advise on repairs; and the local purveyors of equipment are an important resource to us and will not survive without customers. There are camera stores in New York with extremely low prices, for the manufacturers sell to them at huge discounts, allowing them to sell for less than the local businesses pay at wholesale. Some of these firms are honest, but none have well-informed personnel; and many sell 'grey market' models without U.S. warranties, or without straps & case, or using a variety of disreputable sales tactics.

   The first important choice to be made is the size of the objective lenses, and for astronomical use they range between 35mm and 100mm. 35mm is a vast improvement over unaided vision, 50mm transmits twice as much light as 35, 80mm are difficult for almost all people to hand hold, and 100mm are very expensive and heavy. Each increase in size gives a significant increase in light gathering, price, weight, and size. A light weight 50mm binocular can be comfortably worn around the neck, and is a good choice. The 80mm binoculars sold by many astronomy suppliers are a mixed blessing. Quality control is variable, and some give very defective views. They are sold in 11, 15, and 20 power (there was a 30x model which was a disaster,) and are typically identical bodies with different oculars. The mechanical requirements for maintaining alignment in these high power glasses are very strict, and most models are inadequate to the task. The oculars are carried as a pair on a shaft, and there is usually a fair amount of play or wobble at that point. Most 20x models show significant color error, usually only the 11x and 15x models are acceptable, and these should be very critically examined before purchase. A good 80mm binocular is an excellent astronomical instrument, with an ability to reveal dim objects comparable to a low power 4 inch refractor.

   Magnification is the second choice to be made, and is co-determined by the field of view of the binocular. True field of view is the extent of sky viewed by the user, typically 6 to 8 degrees. Apparent field is the true field multiplied by the magnification, and is the viewing angle seen through the eyepiece. Modern wide field telescope eyepieces have up to 82 degrees of apparent field, but 60 degrees is also wide field and is found in some binoculars. Magnification ranges between 6 power, giving wide field views of up to 10 degrees (the size of the bowl of the Big Dipper); and 20 power, which must be tripod mounted and is frequently accompanied by unacceptable aberrations. There are excellent binoculars in the 15 power range, which allow views of open & galactic clusters, Jupiter's moons, and some double stars. The Fujinon 16 x 70 is a phenomenal binocular that allows the distinction between galaxies & stars (and is very bright and high contrast, but I find the huge oculars to be unusable, my eyes are too closely spaced and deep set.) High power is no more useful than low power, it is a choice to be made by the user. The main advantage of higher power to me is the wider apparent field it allows. Almost all lower power binoculars have oculars with much smaller apparent fields than higher power binoculars, to get a wide apparent field with a low magnification requires a very large ocular. It is similar to the limited apparent field of old 40mm eyepieces, where it looks like you're looking down a tube. I much prefer 7x to 10x for astronomy, they are noticably steadier and it is certainly easier to find objects in a wide true field.

   It can be useful to distinguish between the defects of an optical system. A birder can accept coma or curvature of the field, since the center of the field is sharp; but needs excellent rendition of colors to distinguish the hues that identify a bird (they also buy many more binoculars than do astronomers). An astronomer likes to have minimal coma and curvature of the field, giving pinpoint stars to the edge of the field, but can accept some inaccurate hues; and distortion is usually imperceptible at night. There are no binoculars that are totally free of aberrations, optical design is a balance between them, for example curvature of the field can be minimized by introducing distortion. Consumer testing for aberrations requires time & concentration. Above all the user needs to look through a variety of binoculars under different conditions, another reason to support local retailers and RCA functions.

   Curvature of the field is present in all binoculars. A lens forms a sharp image on a surface that is curved, concave side to the lens. When the ocular is set to meet the focused center of the image, it must be racked in to meet the sharp image of the edges. When the center of the image is in focus, the edges are out of focus, to varying degrees and over a varying area. A quality instrument shows this only at the very edge of the field. An inexpensive one can be out of focus half way to the edge, and this means that most of the area of the image is soft. Since we tend to use only the center part of our field of view, many users discount this aberration. However, a truly flat binocular, that is sharp across almost the entire field, is most impressive to use, giving views that are quite unlike unaided vision. A binocular with a wide apparent field that is flat across most of the field is very difficult to design. I very much like a wide apparent field, like TeleVue eyepieces, but the extra lens elements needed to give a wide apparent field can reduce contrast and overall sharpness. A manufacturer can give any instrument a flat field by limiting the field of view with field stops. All binoculars have their FOV limited by a stop, and any maker can increase the field by enlarging the stop, which just gives wide field that is mostly fuzzy. Cheap binoculars in particular do this to sell them, and the advertised field of a binocular is not a sufficient basis for a purchase.

   Spherical aberration occurs because a spherical lens surface focuses the light rays from the edge of the lens to a closer focal point than the light rays from the center of the lens. This causes the entire image to be soft. Correcting this fault was a priority of historical lens design, and the two element lens was designed since the 1700s to correct spherical aberration as well as color error. It is usually well corrected in any modern optical system, but fast, short focal length lenses are difficult to fully correct. Binoculars use very fast lenses, perhaps f4, and passing such a steep light cone through a prism also effects spherical aberration. No doubt some inexpensive models suffer this problem.

   Coma is spherical aberration of light rays through the edge of the lens, where it similarly smears the image of extended objects, but stars at the edge of the field show a V shape, with a faint tail somewhat like a comet. This is a common defect that is difficult to detect during the day. A glint of sunlight off a shiny surface can provide a point source, if there is no handy star. Focus on it at the center of the field, and move the binocular to place it at the edge. It will flare out into a V if the binocular is comatic, and will become fuzzy if the field is curved, although this distinction is academic to most users.

   Astigmatism occurs when light rays in a radial plane (that passes through the center of the lens) have a different focal length than rays in a plane that is at right angles to a radial plane. Astigmatism is also difficult to distinguish during the day, for an astigmatic image is blurred at the edge, similar to the effect of coma and field curvature. However, a star or other point source is imaged as a short line, vertical on one side of focus, and horizontal as you rack through sharpest focus to out of focus on the other side. Astigmatism can sometimes be seen by viewing fine, perpendicular lines at the edge of the field, where one line will be sharp and the other fuzzy.

   Distortion causes straight lines to appear curved at the edge of the field. pix at the edge of the field are magnified less than central pix (barrel distortion), or magnified more at the edge (pincushion distortion). It can be seen when the edge of a building is viewed at the top and bottom of the field, or a pole is viewed at the right and left edges of the field. The lines seem to curve or flex as the image is moved across the field. Barrel distortion caused the lines to curve outward, and pincushion distortion causes them to curve inward. This is a relatively acceptable aberration that can cause discomfort during daytime use when the binoculars are used to sweep across landscapes that contain lines. At night, when sweeping across the sky with a binocular with distortion, the image expands & contracts as it moves across the field, which is annoying but not usually too objectionable.

   Color error can consist of longitudinal color, where the lens is acting like a prism and focusing the blue rays closer than the red. This error covers the entire image, and stars will show blue and red as you focus in and out. Lateral color occurs when a lens creates an image with the red rays that is a different size than the image made by blue rays. This error increases towards the edge of the field, and it is questionable whether it is noticeable on stars. Planets will lose their detail and show color fringing, increasing as they are moved to the edge of the field. Color error has been corrected since the first doublet lenses of the 1700s, but at high powers most modern refractors show color fringes on the planets. Binoculars use very short focus lenses that are prone to color error, and the 15x and 20x models all show color to varying degrees. Low power 6x to 10x models are usually free of color. A related issue is the color rendition of the binocular, which is the accuracy with which the object's colors appear in the image. Most optical systems will have some effect on color balance, perhaps filtering some of the red, or giving the greens a bluish tint. This is not critical to astronomers, but birders (hard core types who will travel the globe to add a new species to their list), need accurate color rendition to distinguish between species, and will pay for that characteristic.

   Resolution is the overall sharpness, usually measured at the center of the field. There is a difference in the resolution of different models, but it can be hard to see, even when viewing a suitable test object (a printed page at a distance will do.) It can be seen by placing an auxiliary scope behind the ocular, and viewing a magnified image of the binocular field. A binocular with superior resolution may not give you a noticeably sharper image than a typical quality glass, but there will be subtle effects on the image, giving higher contrast and seeming to snap into focus.

   Collimation is the alignment of the pix from the two objective lenses. New, inexpensive, binoculars can arrive out of collimation, and all cheap models are easily misaligned if they suffer an impact. To test for this, focus on a distant, very small object. Gradually draw your eyes back from the oculars until you are holding them about 6 inches from your eyes, meanwhile retaining the image of the object in the oculars. If the binoculars are misaligned, the pix from the right and left eyes will separate and appear as two pix. If you can fuse the two pix into one while the oculars are held away from the eyes, they are aligned. Any binocular will be knocked out of alignment if dropped or misused, and an occasional repair bill is inevitable for heavy users of binoculars. Collimation and cleaning will typically cost about $100. at a repair shop. This could motivate you to get a very good binocular, which is worth repairing, or a disposable model. A small misalignment is not apparent in use, but will cause eyestrain and headaches after prolonged use, and will appear in the above test.

   Binoculars are complex instruments, and there are many more decisions and tests that a purchaser can make.

   Prisms are made of two types of glass, Bak4 prisms allow all light rays from the edge of the field to be fully reflected and give an exit pupil that appears as a bright, white circle. They are brighter and superior to Bk7 prisms, which show a bright square in the exit pupil, with blue edges. However, this distinction is not critical and some quality binoculars use Bk7 prisms. Porro prisms are generally less expensive than roof prisms, and the very best binoculars use Porro prisms, but they are easier to knock out of alignment than roof prisms.

  Lens coatings are very important to the astronomer, for there are up to 12 glass-air surfaces in each side of a binocular, and a very small light loss at each surface will reduce light throughput and decrease the visibility of dim objects. The very best coatings (possibly Fujinon's) are noticeably better than the average, and cheap binoculars sometimes have just a few coated surfaces. Look through the objective, and find the reflections from the different glass surfaces. All of these should be colored, for a white reflection indicates an uncoated surface. 'Ruby coatings' have no use at night, if they have any use at all. Good coatings also increase contrast, which greatly aids visibility of dim objects. Light reflected off an interior lens surface bounces around inside the binocular and can exit out the ocular, washing out a dark area. A very important, and much neglected, contrast enhancer is baffling within the binocular, which is not used in some otherwise good glasses. Look into the ocular from a short distance, like it was the opening into a box, and you will see the exit pupil as a white dot, and if there is inadequate baffling you can see additional light from the edges or corners.

   If you are using the binoculars for astronomy only, then individual focus eyepieces are much more rugged and weatherproof. 80mm binoculars have very few terrestrial uses, and the typical center focus giant binocular is not an appropriate hybrid. Eye relief is very important to eyeglass wearers who have astigmatism, but for those who don't observe with spectacles, the extra long eye relief means the binocular must be held a distance from the eyes. This is a good example of the need to try out a glass before purchase. While you are at the store, check out the premium models. There is a difference, and who knows--maybe Santa will be good to you this year (or maybe not).

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