Binoculars Buyer's Guide: Selecting the Right Pair for YOU

-Phil Harrington-

Even in this age of advanced amateur telescopes, one of the most useful and versatile tools for viewing the universe remains a simple pair of binoculars. Not only are they portable and easy to use, binoculars also let us enjoy the night sky as it was meant to be viewed with both eyes. Indeed, studies show that some people enjoy up to a 40% increase in their perception of diffuse objects just by using two eyes instead of one, as with a telescope.

Today, with hundreds of binoculars flooding the market, trying to decide which will be best for you is growing increasingly difficult. Not all are designed for studying the universe. Some binoculars, for instance, are better used for daytime viewing, such as bird watching or viewing distant terrestrial sights; others are meant to fit into a purse or jacket pocket, to be taken to the theater or a sporting event. The hints offered here will help you decide whether a particular pair is suitable for viewing the night sky.

Choosing Binoculars

Let's begin with the basics. Binoculars may be thought of as two refracting telescopes mounted in parallel to one another. Like refractors, light enters each side of the binoculars through a large lens called the objective and exits into the observer's eye through an eyepiece. Unlike astronomical telescopes, however, which produce upside down or left to right images of whatever they are aimed at, modern binoculars use internally mounted prisms to bend and reflect the light until the image exiting the eyepieces is right side up, proper for terrestrial viewing.

A, internal optics of generic binocular; B, schematic of optics of roof prism; C, shematic of Porro Prosm optics. (Adapted from wikipedia.com)

There are two types of prism assemblies used in today's binoculars: porro prisms and roof prisms. Is one better than the other? Roof prism binoculars are immediately recognizable by their sleek, straight barrels, which are made possible thanks to how closely roof prism assemblies fit together. And while their tiny size make them ideal for carrying to a ball game or the opera, roof prism binoculars are not as ideally suited for astronomical use as porro prism models. One disadvantage is that one internal surface of each roof prism must be aluminized in order to bounce light through the binoculars. As the light strikes the aluminized surface, a bit of it is lost, causing a dimmed image. A second disadvantage to roof prism glasses is their high cost, which is caused in part by the precision required for their assembly. Unless constructed with great care, the image quality of roof prism binoculars will be unacceptable.

The second, and more common, binocular design uses porro prisms to reflect light to the observer's eyes. Porro prism binoculars are immediately recognizable by their familiar zigzag tube design, a result of the path that light follows while traveling through the prism assemblies. When constructed of a high quality glass, poor prisms have "total internal reflection"; that is, all the light going in passes through and exits without the need for an aluminized surface. The net result is brighter images, an important consideration for astronomical viewing. More on this later.

Porro prism binoculars come in either German or American barrel designs. The German style consists of a two piece body, with the objective lenses held in separate barrels from the prisms and eyepieces; the American style of a one piece body. Personally, I prefer the one piece design, as it offers a tighter seal against dust and moisture infiltration.

Regardless of body style or prism design, all binoculars are rated by two numbers separated by an "x," such as 7x35, 10x50, and so on. The first specifies the magnification, while the second refers to the aperture, or the diameter of the binoculars' front objective lenses in millimeters. Magnification is one of the most important considerations when selecting a pair of binoculars. Low power binoculars (that is, 7x to 10x) are great for wide field views of the Milky Way, but higher power glasses (11x and above) are preferred for the Moon, Sun and planets. Keep in mind that as magnification increases, so does the binoculars' weight. This is especially important if you plan on hand holding the glasses. Most people can support 10x binoculars with minimal strain and fatigue. If the glasses are increased to 10x, the extra magnification will also magnify shaking, which could make hand support impractical.

Selecting the proper aperture is just as important as choosing the right magnification. For optimum performance, the objective diameters should be matched to both your eye and your observing site. The amount of light entering the eye is controlled by the central opening in the iris called the pupil. For most people, the pupil contracts to about 2.5 millimeters in bright daylight and expands, or dilates, to about 7 millimeters in total darkness. For optimal nighttime viewing, the diameter of the beam of light exiting the binoculars' eyepieces, known as the exit pupil, should be the same as our pupil's diameter. If the exit pupil is too small, then some of the eye's light gathering capability is wasted; if too large, the view will suffer from low image contrast, making it more difficult to discern objects than through smaller, properly sized binoculars. So, exactly how large is the exit pupil of a particular pair of binoculars? Simply divide the objective's diameter by the power to find out. Therefore, a 7x50 pair has a 7.1 millimeter exit pupil, while 10x50 glasses have a 5 millimeter exit pupil.

If you are below the age of about 30 and spend most of your observing time under dark rural skies, then your eyes' pupils should dilate fully to about 7 mm. Therefore, a diameter/power combination producing a matching 7 mm exit pupil should be chosen, such as 7x50, 10x70 or 11x80. But, if your location suffers from light pollution, or you are over the age of 30, then your pupils may never dilate beyond 5 mm or 6 mm. Urban and suburban observers will likely do better by selecting smaller exit pupil binoculars, such as 7x35's or 10x50's.

One of the nicest features of binoculars is their ability to grasp a wide slice of sky into a single field of view, offering incomparable views of Milky Way star clouds, wide open star clusters, and large nebulae. A binoculars' field of view is usually found stamped on the barrels' tail stock, and may be expressed in two different ways. Typically, manufacturers will either note the area of view in degrees, or express it as a separation in feet at 1000 yards (e.g., 325 feet at 1000 yards). This second method simply means that if a 325 foot long ruler were viewed through these glasses from a thousand yards away, it would just squeeze into the eyepieces' field of view. If your binoculars use this second method of specifying the field, you may determine their true angular field by dividing the number of feet by 52.5. Therefore, the view through the "325 feet" binoculars actually covers a little over 6 degrees of sky.

Let's Go Shopping!

Once magnification and aperture are decided upon, it is time to look over what is available that will meet both your needs and your budget. Take a look at the binocular market today and decide which brands and models interest you. Then, it's time to go shopping. I always recommend comparing several different brands and models side by side, if at all possible. Mail order certainly has its advantages, but unless you know exactly which binocular you want to buy, it is always preferable to check each prospect out individually and in person.

Begin with the optics. Does the manufacturer state that the lenses are coated? Optical coatings reduce lens flare and improve light transmission, two desirable characteristics for astronomical binoculars. A plain, uncoated lens reflects about 4% of the light striking it, causing low contrast images against a muddy background. By applying an anti reflection coating to both sides, light reflection is reduced to about 1.5%. Most optical instruments available today, including camera lenses and binoculars, are coated with magnesium fluoride (abbreviated MgFl), which is applied by evaporation to the glass surface in a vacuum chamber.

The precise thickness of the applied coating is critical to its effectiveness. A lens properly coated with magnesium fluoride will reflect a purplish hue when held at a narrow angle toward a light. If the coating is too thin, it will appear pinkish; if it is too thick, then a greenish tint will be cast. Uncoated lenses have a whitish glint.

Top of the line binoculars receive multiple antireflection coatings. These reduce reflection to less than 0.5% to produce even finer views. Multicoated lenses will show a greenish tint when held toward a light. Be careful here, however; you do not want to mistake improperly single coated optics for multicoated optics. If a lens looks greenish, make sure that the manufacturer states "multicoated optics"; otherwise, be suspicious!

A further word of caution here: there are coated optics and then there are coated optics. Some lesser binoculars that state "coated optics" should be interpreted as "only the exposed faces of the objectives and eye lenses are coated"; the internal optics are probably not. Others may state that all air to glass optics are coated, which implies all internal surfaces as well. However, the manufacturer may "forget" to inform the prospective consumer that some of the internal lenses are made of plastic. Chances are those non glass elements are uncoated.

One reason I recommend buying binoculars from a local store rather than by mail order is that this allows you to check and test the specific unit that you are purchasing beforehand. Always check the binoculars' optical alignment. Our brains are used to getting two nearly identical images from our eyes. Any misalignment between the binoculars will result in two slightly off axis images. In an effort to correct this apparent malfunction, the brain will struggle to bring the images together, causing undue eye strain and headaches when used for any length of time.

In his book "Binoculars and All Purpose Telescopes," the late Dr. Henry Paul recommends a simple way to quickly check the alignment of binoculars. While looking at a distant terrestrial object, cover one side of the binoculars with your palm, but keep both eyes open. Then quickly remove your hand to uncover the lens. If any misalignment exists, two images will be seen for an instant before the brain tries to snap the images together.

After the alignment check, hold the binoculars at arm's length and look at the circle of light coming out of each eyepiece. Are they both clear circles, or do you notice a diamond shape within? Clear circles indicate that the pair's prisms are made of BaK 4, a superior quality glass. The index of refraction of this type of glass allows total internal reflection of all light entering the prisms (e.g., all the light entering the prisms, exits). The diamond effect is caused by the light fall off from using cheaper BK 7, or Crown glass, prisms. Crown glass does not permit total internal reflection, forcing manufacturers to aluminize one face of the prisms in order to reflect light. The result: dimmer images.

Besides noting optical quality, take a long, hard look at the mechanical assembly, starting with the focusing mechanism. Most of today's binoculars have a knurled focusing wheel between the barrels, while others require each eyepiece to be focused separately. Regardless of the method used, the focusing device should move smoothly. Incidentally, some less expensive binoculars use a "fast focus" thumb lever instead of a wheel. This latter approach permits rapid, coarse focusing, which may be terrific for moving objects but it is not accurate enough for the fine focusing required when star gazing.

Next, hold the binoculars up to a light and look into the front end of the barrels through the objectives. Are the lenses free of scratches and stains? Look through the objective lenses and into the front of the barrels. The inside walls should be free of any foreign matter such as dust, adhesive, and metal shavings. If not, the binoculars are unacceptable.

Another item to check is the eyepieces' interpupillary distance adjustment. Binoculars must be adjustable to fit a wide range of observers. To achieve this versatility, glasses have a central hinge to allow the distance between the eyepieces to be comfortably adjusted for your eyes. Also check if the particular model you are interested in has a built in standard photographic tripod socket. In most cases, the front end of the barrel hinge is threaded for this purpose, which makes a right angle adaptor necessary for mounting the glasses onto a tripod. Personally, I prefer to mount binoculars onto some sort of tripod, regardless of size and magnification. Not only does this practice make it easier to go between a star chart and binoculars, but it also provides a far steadier view.

A Shopper's Guide

Once you decide on the size binoculars that you want, here is a shopping list highlighting some of the most important considerations for the prospective purchaser.

1. Check the lenses to see if they are coated, fully coated, multicoated, or fully multicoated. Compare what the manufacturer states to what you find by inspecting the binoculars.
   
2. Check the type of glass used for the prisms. Is it BK 7 or BaK 4 (preferred)? The answer should be printed somewhere on the binoculars themselves, but double check by holding the binoculars at arm's length and examining the exit pupil's appearance.
   
3. Check the binoculars' alignment by following the simple test outlined above. Reject any that are even the slightest bit off.
   
4. How do the binoculars focus, center or individual? If center, is there a thumb wheel (preferred) or fast focus lever? How does the focusing mechanism feel? Is it smooth or does it bind?
   
5. Look into and through the objective lenses. Do both sides appear free of scratches and smudges? Are there any metal shavings trapped inside the barrels?
   
6. Check the interpupillary distance to ensure that you will be able to adjust them to match the width of your face and eyes.
   
7. Do the binoculars have a center tripod socket for attaching to a camera tripod? While many people hold binoculars by hand, I strongly recommend that you place them onto a substantial camera tripod, to make going from a star chart to eyepiece easier.

The Bottom Line

As with nearly everything we buy, your budget will be the final dictating factor. Perhaps you cannot afford the best, but after performing these simple tests, you will be able to select the best binoculars within your price range.

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Montauk Observatory member, Phil Harrington, is a contributing editor for Astronomy magazine. Each month, he writes a column called Binocular Universe.

Many of us started our apprenticeship of the night sky by looking at single stars, and then used the skills developed to study and observe objects which not only presented a wonderful array of colors, but also allow precise measurements to be made - multiple star systems. The study of multiple stars is one that has a great pedigree. It was, and in fact still is in some respects, an area of astronomy where the observer can make useful detailed observations. Fortunately, many of these systems not only present a delight to the eye, but also have a glittering array of colors. The winter is a good time to begin seeking out these celestial jewels. So without further ado, let’s begin.

Let’s start with something really spectacular – The Trapezium, in Orion [this group is also known as q 1 and q 2]. This is probably the most famous multiple star system in the sky, and always a test for small telescopes. The four stars that make up the famous quadrilateral are set among the wispy embrace of the Orion Nebula, M42, one of the most magnificent vistas in any telescope. They are very young stars recently formed from the material in the nebula, and so should all appear bright white, but the nebula itself probably affects the light that is observed, so the stars often appear as off-white, delicately tinted yellowish and bluish. Many amateurs have reported the colours as pale white, faint lilac, garnet and reddish! The current astrophysical theories predict hat that the four stars of the Trapezium contribute nearly all the radiation that makes the Orion Nebula glow. This is one of these unique and special objects that are well worth spending an entire evening observing. It is, without a doubt, a glorious sight!

Fortunately for us there are several other lovely multiple systems in Orion, and the first of these is s Orionis . This is an easy multiple star system of white and bluish stars. Then there is t Orionis, which is a nice triple system of blue and white and very pale red stars. And located among and near several bright and dark nebulae. A good test for small telescopes is i Orionis, a nice colour-contrasted triple system. The stars are coloured white with delicately tinted blue and red companions. Now for a system that can prove difficult to resolve - h Orionis.. Also known as Dawes 5, this is a wonderful system. Even under high magnification, the two brighter members will appear as two white discs in contact. h Orionis is also a spectroscopic binary. Or final look at an Orion star system concerns. l Orionis. This is a nice quadruple star system of white and blue stars. Various observers have reported them as yellowish and purple and pale white and violet.

Still staying in December we can see the lovely triple system of pale yellow stars along with a very faintly tinted blue companion known as e Monocerotis. This nice system is set against the star fields of the Milky Way. Also in the constellation Monoceros is b Monocerotis, a magnificent triple, first discovered in 1781 by Herschel. All the stars are a lovely steely blue–white in colour. What makes this system so unique is that all the stars are very nearly equal in brightness.

Located in the constellation Auriga is OS 147, this is a wonderful triple star system forming a triangle of yellow and blue stars. We now come across a fascinating but rare triple system, with all its members forming a nearly perfect equilateral triangle. Struve 939

Our final group of stars takes us into January. First there is 17 Canis Majoris A nice triple system, with wonderful colour contrasts of white and orange–red stars. This is followed by Burnham 324, a nice multiple star system in the constellation of Canis Major, consisting or many white and blue stars. Our penultimate star group is Struve 1245. This is a lovely multiple star system in the constellation Cancer. The triple aspect is seen in small telescopes consisting of yellow, pale yellow and white stars. Depending on the telescope used, you will see a double, a triple or a multiple star system! Ending on a high point is the magnificent system t Canis Majoris . This is a wonderful triple star system set within the open cluster NGC 2362, and so its yellow and blue components are set against a glorious backdrop of faint stars.

There are of course many more multiple stars in the winter sky, enough to keep you busy many observing sessions, but I hope these few will have wetted your appetite. Good observing!

Brookhaven Adopts new Lighting Law Amendments

-Tom Madigan-

1). A sunset clause that will compel those who operate non-compliant lighting to bring it into spec within 10 years;

2). A tough-as-nails streetlighting provision that compels the town to install replacement streetlights and any new streetlights with full-cutoff luminaires. Existing streetlights will be replaced over time with full-cutoff luminaires and are subject to the 10-year sunset provision;

3). All due diligence to be performed when evaluating roadway lighting applications and a determination made as to whether the purpose of the lighting could be achieved through passive means with an eye towards energy conservation. This is essentially the concept of a lighting warrant;

4). Language that clearly addresses light trespass and glare with very specific remedies;

5). 11:00 PM lighting "curfew" (lights out after 11:00 PM unless you're open all night – this includes shopping malls and convenience stores, shopping centers and all those who think that dusk-to-dawn is the way to go);

6). Gives authority to code enforcement to determine if a lighting fixture or installation is a public safety hazard and that lighting fixture or installation be removed or brought into compliance immediately.

Full details will be available shortly on the Town Of Brookhaven public website at http://www.brookhaven.org.

These amendments, once filed with the NY State Secretary of State, become effective and enforceable immediately.

The Town of Brookhaven Town Council is to be commended for this landmark legislation and congratulations to all involved on a brilliant job well done! Special recognition is to be given to Councilman Kevin McCarrick who originally sponsored the legislation, his office staff, John Kreutz, his legislative aide, Councilman Steve Fiore-Rosenfeld for his enthusiastic support and to the Town of Brookhaven Ad Hoc Lighting Committee (Susan Harder, Phil Harrington, David Cohn, Tom Madigan and Alan Stadler) who worked closely with Councilman McCarrick's office and with Councilman Steve Fiore-Rosenfeld to enact these amendments thereby completing a lighting law that we can all be proud of, that will preserve and protect the night sky and our environment for generations to come and that will be a standard and a model for others to aspire to.

• January. 18th- Myths of the Winter Sky
  Observing session at Theodore Roosevelt County Park following the lecture
  
• February- Mystery at the Center of the Milkyway (date to be announced)
  Observing session at Theodore Roosevelt County Park following the lecture
  

Naked-Eye Planets for January- February 2008

Diagram of Lunar Eclipse, February 20, 2008

Key Dates January - February:

January

• 4 - Quadrantids meteors.
• 15 - Moon - first quarter.
• 20 - Moon is 1° north of Mars.
• 22 - Mercury is at greatest elongation east (19°).
• 22 - Full Moon
• 25 - Moon is 3° south of Saturn.
• 30 - Moon - last quarter.

February

• 07 - New Moon.
• 14 - Moon - first quarter.
• 16 - Moon is 2° north of Mars.
•  21 - Full Moon.
• 21 - Total lunar eclipse.
• 24 - Saturn is at opposition.
• 26 - Mercury is 1° north of Venus.
• 29 - Moon - last quarter.

If you would like to make a contribution to this newsletter please send your article or essay to montaukobserver@montaukobservatory.com.

Montauk Observatory Board of Directors

David Larkin, President
Terry Bienstock
Susan Harder
Sean Tvelia
John Imperatore
Robert Young, Architect of Record
Eva Growney, Architectural Consultant

Board of Advisors

Dava Sobel, Author
Dr. Mike Inglis, Astronomer, SCCC
Dr. Dean Peterson, Astronomer, SUNY Stony Brook
Joe Malave, Science Teacher, Montauk Public School
David Cohn, Director, Educational Projects Network
Donna McCormick, President, Custer Institute
Tom Madigan, Custer Institute
Robert Deluca, President of Group for the South Fork