You may already own some. Or for no more than £100 you can buy a pair that will reveal galaxies millions of light years away, rich star-packed fields along the Milky Way, bright star clusters and nebulae, moons orbiting Jupiter, and craters on the surface of the Moon. Binoculars are the one instrument everyone with an interest in astronomy owns, or should own. Even avid amateurs with a houseful of telescopes will have one or more pairs of binoculars at the ready. They know that binoculars provide something no telescope can - a wonderful wide field of view in a package that requires no more effort to use than simply lifting them to the eyes. In fact binoculars provide unexcelled views of objects such as the Andromeda Galaxy, the Pleiades star cluster, the nebula-rich regions of Sagittarius, earthshine on the Moon, groupings of planets, lunar and solar eclipses -- the list is long enough that you never outgrow the need for binoculars.
Yet despite years of having amateurs extol the virtues of binoculars, few beginners realize the merit of starting with these affordable and handy instruments. The best binoculars for astronomy may be the pair you already own, stuffed away in your closet and pulled out only for ball games.
But if you are shopping for a new pair, here's my suggestion of what to look for.
Pick any size of binocular and there's a wide range in prices in that class. As you go up the price ladder, you get better prisms, better coatings, longer eye relief, and slightly sharper images, especially in images at the edge of the field. You also get more rugged construction and completely sealed, waterproof optics. But you needn't spend a great deal to get a good deal. You can find binoculars in the £ 100 to £ 150 range that will provide many features and enough value to last you many years of use.
For astronomy, everyone should own a pair from the 40- to 50mm-aperture class. These are the lightweight, all-purpose binoculars you'll use most often, even after you acquire a telescope. Those wanting a true 7mm exit pupil or the longest eye relief should select a 7x50 or 8x56. Older viewers or those wanting slightly higher power for greater resolution of small deep-sky objects should select a 10x50. Those wanting a slightly smaller, lighter binocular for serving double duty as a birding or daytime instrument should select a 7x42 or 8x42 model.
After you own a small binocular that can be held comfortably in your hands, you might consider buying a giant 70mm- or 8Omm-class instrument. Essentially twin telescopes, these can provide stunning views of comets, deep-sky objects, and star-fields. For a 7mm exit pupil and for the widest field select a 10x or l1x model. For the higher power required to resolve many star clusters and to pick out small galaxies, select a 15x or 16x model. Big 2Ox80 binoculars are praised by some amateurs, but their narrow 3.5-degree field of view make them more difficult to use for beginners. You also sacrifice eye relief when you move to higher power models.
Binoculars need very little in the way of accessories. The priority item is a tripod adapter. These inexpensive L-shaped brackets are available from many telescope dealers. Binoculars without a tripod socket can be fastened to a tripod with flexible wrap-around holders.
Although a conventional camera tripod works fine for small binoculars, the best mounting solution for big 70 to 10Omm binoculars is one of the binocular stands now offered by several manufacturers. These cantilevered devices hold the binoculars away from the tripod so you can get underneath them when you want to aim high. An even more luxurious accessory is some type of binocular observing chair. These suspend the binoculars in front of your eyes, while you remain comfortably seated scanning the skies.
Binocular Optical Features
As with telescopes, the most important specification of binoculars is the diameter of the front lenses. For binoculars intended for daytime use, when lots of light is available, aperture isn't so critical. Binoculars with 35mrn or even 2Omrn-aperture lenses can work well in the daytime. But for astronomy, as for any night time use, binoculars with at least 40mrn to 50mrn lenses are best.
To determine the aperture, look for a number such as 7x50. The last number, the "50" in this case, is the aperture in millimetres. The larger this number, in general, the brighter the images will appear. The trade off is that the bigger the aperture, the bigger, heavier, and more expensive the binocular.
In a designation such as 7x50, the first number is the magnification. Binoculars come in models with magnifications from 6x to about 20x. But again, as with telescopes, high power is not the advantage you might think it is. As you increase the power, you decrease the very trait you are seeking in a pair of binoculars, wide field of view. You sometimes sacrifice optical quality as well: High-powered binoculars are difficult to hold steady, so the greater detail you were hoping to gain by going to high power is lost because the image is shaking all over. The most popular magnifications for astronomical binoculars are 7x to 11x.
Aperture and magnification combine to produce another key specification for an astronomical binocular, the exit pupil. Divide the aperture by the magnification and you get the binoculars exit pupil. This is the diameter of the beam of light the binocular projects into your eye. For example, a 7x50 binocular has an exit pupil of 7mrn (5Omrn divided by 7). A 10x50 binocular produces an exit pupil of 5mrn. A giant I6x80 binocular also gives a 5mrn exit pupil.
The best binoculars for astronomy are models with exit pupils from 5mrn to 7mrn. The diameter of the light cone projected from these binoculars nicely matches the diameter of the pupil of the human eye, assuming the eyes are dark adapted to night time conditions. These binoculars are providing as much light as your eyes can accept and, as a result, are producing the brightest images for their aperture.
Most human pupils can open no wider than 7mrn, so a binocular with an exit pupil wider than 7mrn would waste light -- only the centre of the light cone could enter the eyes. In fact, there are no binoculars with exit pupils greater than 7mrn. Those with exit pupils equal to 7mrn (7x50, 8x56, 9x63, and llx80 models for example) are often called night glasses. These are some of the best models for astronomy.
As we get older, however, our pupils lose their ability to open wide at night. For those of us over 40, our pupils commonly open no wider than 5mm to 6mm, even when our eyes are fully dark adapted. Under these conditions, binoculars with a 5mm to 6mm exit pupil are best. This includes 7x42, 8x42, lOx50, 15x80, and 16x80 models.Actual field of view
Two binoculars with the same magnification and aperture, two lOx50s for example, can provide quite different fields of view. A wide-angle IOx50 will show more of the sky than a standard-field 10x50 will. The difference lies in the type of eyepiece used.
Field of view is often given in "feet at 1,000 yards" or "meters at 1,000 meters." These numbers tell you how wide an object would completely fill the field if it were 1,000 yards or meters away. But in astronomy we are looking at objects across the universe. For us, a field of view measured in degrees makes more sense. A field of 5 degrees, for example, encompasses 10 Full Moon diameters or the distance between the Pointer Stars of the bowl of the Big Dipper.
To convert "feet @ 1,000 yards" into degrees, divide by 52.5. To convert from "meters @ 1,000 meters," divide by 17. A binocular said to give a field of367 feet at 1,000 yards, a common number, will produce an angular field of 7 degrees. A binocular with a field of 120 meters at 1,000 meters will also yield a 7 -degree field.
A field of 5 to 7 degrees is normal for most 7x binoculars. Wide-angle models may show 8 to 10 degrees of sky.
Apparent field of view
Another way to compare binoculars is by their apparent field. To calculate apparent field, multiply the actual field in degrees times the magnification. For example, a 7x binocular that shows 7 degrees of sky has an apparent field of 49 degrees. This number tells you how wide a circle of light you will see when you look through the binoculars. Apparent fields of 50 degrees are normal for most models. Wide-angle binoculars can have apparent fields of up to 70 degrees. Binoculars with narrow apparent fields of 40 degrees will give you the effect of looking through a tunnel.
Getting as wide a field as possible might seem best, but there are tradeoffs. Wide-angle binoculars usually exhibit distorted or out-of-focus star images at the edges of the field. And with wide-angle binoculars you usually have to sacrifice what is known as eye relief.Eye relief
Eye relief is the distance your eye needs to be from the top of the eyepiece in order to see the entire field at once. As a general rule, wide-angle binoculars have reduced eye relief; your eyes need to be quite close to the eyepiece to see anything.
An eye relief of 10mm to 15mm is normal for most eyepieces. Less than that and you have to press your eyes uncomfortably close to the eyepiece lenses.
If you wear glasses and wish to keep them on while viewing through your binoculars, you'll need binoculars with at least 15mm of eye relief. Models with eye relief of 2Omm or more are ideal for eyeglass wearers. This requirement may rule out wide-angle binoculars (those with apparent fields of 60 to 70 degrees) for eyeglass wearers, as few of these models have eye relief greater than 15mm.
Optical coatings placed on the surfaces of lenses and prisms increase light transmission (giving brighter images) and cut down on internal flare and reflections (increasing contrast). Most binoculars' feature at least single-layer coatings on some air-to-glass surfaces. Better binoculars have all optical surfaces with one layer of coating and some surfaces treated with multiple coatings for even greater light transmission and contrast. In the best binoculars all optical surfaces are multicoated. Quite often the difference in clarity between otherwise similar models of binoculars stems from the coatings.
When inspecting a pair of binoculars, look down onto the main lenses. Do you see lots of white reflections from lights or windows? If so, it is a sign the optics are not multicoated or not coated at all. In a fully multicoated binocular the reflections will be dim and will look deep green, blue, or purple.
Type of prisms
Binoculars are compact because they use prisms to fold the light path back and forth within the body of the binocular. Porro prism binoculars are the most common and most affordable. These models have the familiar N-shaped bodies. Roof prism binoculars have straight tubes, making them more compact. But roof prism binoculars are more costly to make well, putting the best of these models in the premium price class.
For astronomy, all but the finest roof prism binoculars are outperformed by porro-prism models. Roof prisms split the image into two and then recombine it, a process that can lead to spikes of light off bright stars on lower cost models. Porro prism binoculars of any price tag don't have this quirk.
However, not all porro prism binoculars are equal. Ones with prisms made of BK7 glass are economical but suffer from some dimming of the edge of the field. Better binoculars use BaK4 glass prisms for the brightest field of view.
Binocular Mechanical Features
Type of focusing mechanism
Most binoculars focus with a central thumbwheel that moves both eyepieces back and forth in tandem. These "centre-focus" models also have a separate adjustment on one of the eyepieces (usually the right eyepiece) to compensate for any difference in focus between your two eyes. Because focusing requires only one quick adjustment, centre-focus binoculars are the easiest to use.
Some models use a focus adjustment on each eyepiece. Focusing these "individual-focus" models means always having to adjust two eyepieces. This is inconvenient and slow. The reason for going this route is that it is difficult to make centre-focus binoculars waterproof. Individual focus is the method of choice for marine binoculars. For applications such as astronomy, where the subject is always at the same distance, constant refocusing is not necessary and the inconvenience of individual focus eyepieces is not a problem and may be a fair trade off for other features.
Some binoculars are built to survive being plunged into water. Others, known as splash-proof or fog-proof, are only water resistant and won't survive immersion. While neither class of water-proofing might seem useful for astronomy, in practice waterproof binoculars are a good choice. After a constant routine of exposure to damp night air and condensation from warm rooms, binoculars can develop hazy images if mould or mildew form inside the optics. Cleaning the optics can be expensive.
Waterproof binoculars intended for rough use usually feature some type of rubberized covering over the body to absorb shock and vibration. Such armouring is of some value for astronomy, but with care any binocular can last a lifetime. While waterproof binoculars are often armoured, armoured binoculars aren't necessarily waterproof.
Many models, especially those with long eye relief, have rubber eyecups on the eyepieces. If you don't wear glasses, keep the eyecups rolled out; the cups help block stray light and keep your eyes comfortably at the right distance from the eyepieces. If you wear glasses, you roll down the eyecups to see the entire field. The rubber ring surrounding the eyepiece prevents glasses from being scratched.
The best way to improve the view through binoculars is to place them on a tripod. A steady image will reveal small details such as the moons of Jupiter and small star clusters. Many binoculars feature a 1/4-20 socket that accommodates a bracket for tripod attachment.
Even within a standard size such as 7x50, binoculars can vary considerably in weight, depending on the size of the prisms employed, body composition, and features such as armouring and waterproofing. If they are to be held, a weight under 30 ounces is best. Larger 40- to 50-ounce models are difficult to hold steady for any length of time, and for giant 60- to 100-ounce binoculars, placing them on a tripod is essential.
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