Binocular Basics
By Jon Stewart-Taylor

150mm Russian binoculars

Why Binoculars?

Binoculars are among the best instruments to start out with for many reasons. They're easy to use, give an upright image, and have a wide field of view. This makes it easy to find your way around the sky. They're easy to carry around, so you can use them often, and take them with you when you go places. Almost no setup is required, so you can start observing almost as soon as you arrive at a site.

Binoculars are inexpensive and readily available. Many people already have a pair, and even if they're not designed for astronomy, they will provide good views of the heavens. Binoculars take you about 1/2 way between unaided eyes and most amateur telescopes. Modern binoculars are better instruments than were used to discover the phases of Venus, the moons of Jupiter, and many star clusters, galaxies, and nebulae.

What Kind of Binoculars?

What kind should you use? If you already own binoculars, the answer is easy. Use what you've got. Unless they're damaged, they're sure to be good enough to start with. If you don't have any binoculars, or you want to get some specifically for astronomy, you can learn a lot from the Orion Telescope catalog. It's free, contains quite a few different sizes and brands of binoculars, and has a lot of information about the various parameter which help determine whether a pair of binoculars is right for you. Here's a quick summary of what you need to know.

The two most important aspects of binoculars are their magnifying power and their objective size. These are normally written together as power x size (e.g. 10x50 binoculars have a magnification of 10 diameters, and 50mm objective lenses). Larger objectives mean more light gathered, so larger sizes are generally better. Power is generally less important than objective size. Additional magnification can make small objects easier to see, at the cost of making it harder to hold the view steady. For beginners, binoculars in the range of 5x35 to 10x50 are the best.

Another factor influences how bright objects appear through binoculars. Every time light passes through glass, is intensity is diminished by reflections off the surfaces. Binoculars contain many lenses and prisms, and light loss can be as much as 50%. Special coatings can be put on the surfaces to reduce the reflection, and if applied to all surfaces can reduce the light lost to just a few percent. Manufactures have a jargon describing how many surfaces are coated. Coated optics means that one surface of the main objective is coated. Fully multi-coated means that all air-to-glass surfaces in the entire instrument are coated. Price is normally linked to the quality of the coatings.

Other parameters are field of view, eye relief, and exit pupil. Field of view is how much of the sky is visible through the binoculars (normally around 5-8 degrees). Wider fields of view make it easier to find your way around the sky, and extended objects like large, loose star clusters may not fit within a narrower field of view. Eye relief is how far from the eyepieces you can be and still see focused images. This is most important if you wear glasses, since you need to focus the binoculars with your glasses on. Exit pupil is computed by dividing the objective size by the power (10x50 binoculars have a 5 mm exit pupil). Exit pupil is important because peoples eyes dilate to different sizes depending on ambient light conditions, their age, and the health of their eyes. The "average" dilation for fully dark-adapted eyes is about 7 mm, although this varies substantially between individuals. If the exit pupil is larger than your dilated eyes, light will be "wasted". If the exit pupil is smaller, light will not reach as many rod cells, and you won't get as bright an image.

Choosing Your Target

You've got your binoculars, and now you're ready to observe. What should you look at? The moon is always a good choice. Observing it is good practice if you plan to take up telescopic planetary observing later. You can see more detail on the moon with binoculars than you can see on any other planet with any amateur telescope. Venus will reveal its phases, and Jupiter will present the ever-changing dance of the Galilean moons. These are easy targets, suitable for observing with any kind of binoculars under any conditions.

If you're interested in observing stars, clusters, nebulae, or galaxies, the ability to see certain objects depends on the aperture of your binoculars and your observing conditions. Most binoculars with objectives around 40 to 50 mm show stars to about 8th magnitude under suburban conditions, and to around 9th under good conditions. Since galaxies, nebula, and clusters are diffuse objects, they are harder to see, particularly in light-polluted skies. Experience will teach you the magnitude limit for your location, binoculars, and eyes. Objects at 8th magnitude or brighter should be readily visible. The Binocular Messier program of the Astronomical League classifies the Messier objects into three levels of difficulty for both average and large binoculars. These categories range from "easy" to "challenging" and will give you an idea of how difficult the different objects will be for you to observe. There are a number of books available on the subject of binocular astronomy which will aid you in choosing and observing a wide variety of deep sky objects.

Planning Your Trip

If you're looking for bright objects, you can just point your binoculars at the right area and find your target. However, good charts are indispensable to get where you're going. Which star charts are right for you depends on what you're hunting for and what your limiting magnitude is. Sixth magnitude charts like Wil Tirion's Bright Star Atlas 2000 or the Edmund Mag 6 Atlas are good for moderately light-polluted areas or fairly bright targets. The charts in Peterson's Field Guide go to about 7th magnitude. Although small, they are good for binocular use, and easy to carry. Tirion's Sky Atlas 2000 shows stars to 8th magnitude, and is good for darker sites and fainter objects.

Once you have your charts, you need to know how your binocular field of view (FOV) relates to them. A field-of-view gauge shows your instrument's FOV at the same scale as your chart. To find out what the FOV is, you can check the manufacture's specifications, or you can check it against the sky. The Big Dipper is good for this. Find a pair of stars that just fit in the FOV and your field-of-view gauge will be a circle with the same diameter as measured on the chart. You can make the gauge out of a length of wire bent into a ring of the proper diameter (don't forget to leave a handle). Some people plot field-of-view gauges onto acetate using a computer, or you can draw a circle of the right diameter on plain paper with a compass, and the use a copy machine to put it on acetate. The wire gauge has the advantage of being able to draw around it if you want to mark your chart. Once you have your gauge, tie it to your chart with a length of strong thread so it won't be lost in the field.

Ok, you've selected a target and found it on a chart. Now you need a place to starts. Pick a star bright enough to be easily recognizable in the sky, but fairly close (within 15 to 20 degrees) to your target. Put your gauge on the chart so your starting point is within the circle. Move it until you find a recognizable star or pattern of stars in the general direction of your target, with the starting point still inside the circle. If you're marking your chart, draw this circle. Center the gauge around your new landmark and continue the process until you reach your target. Now you've defined a step-by-step journey from your starting point to your target.

Steady as She Goes...

It's difficult to hold binoculars steady while standing, so you'll need a binocular mount or something on which to sit or lie. A specialized binocular chair or mount is ideal for observing, but they are expensive and difficult to move and to set up. Binoculars mounted on photo tripods work well but it is difficult to observe near the zenith. An adjustable chaise lounge is an inexpensive compromise, allowing comfortable viewing from horizon to zenith. If nothing else is available, lie on a blanket or sleeping bag to observe near the zenith.

If you don't have a binocular mount, knowing how to hold a binocular for maximum steadiness is important. Hold the binocular to your eyes. Slide your hands along the body of the instrument, toward your face, until only your pinky and ring fingers are curled around the back end of the binocular body. In this position, the binocular feels a little nose-heavy because your are supporting it behind its center of gravity. Curl each thumb up as if making a fist and flex your hands so that the second bones in your thumbs are pressed up against your cheekbones. Curl the first and middle fingers of each hand around the corresponding eyepiece. Your hands are not far from where they would be if you brought them to your face to black out stray reflections while peering through a store window at night. This make a solid connection between the body of the binocular and your face, and markedly improves how steadily you can hold the instrument.

Hop to it!

Now you're ready to go. Find the starting object in your binoculars then move around until you find the first landmark. This may take a while if you're not familiar with how the view through your binoculars compares to what you see on the charts. There's really not much anybody can do to describe it- it's just something that you learn through practice. Once you've found the landmark, center it and look for the next one following the path you've already planned out. If you get lost at any point, just go back to your last known landmark and try again.

After the last hop, when your target should be in the field, you may not see it at first. If it's bright, it will probably stand out, but if it's dim it may fade into the background, or appear to be just another faint star. Make sure you're actually looking at the right piece of sky by comparing the brighter stars to those on the chart. Look for patterns like triangles and quadrilaterals.

Once you're sure you're looking in the right place, and you know exactly where the object should be, use averted vision and hold the binoculars as steady as possible. Keep your attention on the same spot for 4 or 5 seconds to let the image "develop". You're eyes need this time to collect all the light and make the object visible. If you still don't see it, try shifting your eyes to another spot near the object. Different parts of your retina have different sensitivity to low levels of light and you'll need to learn by trial and error which parts of your eyes are best for observing faint objects. If you still can't find it, it may be that your conditions or your binoculars just don't allow observation of such faint objects.

All that seems like hard work rather than Recreational Astronomy? It's much easier to do than to describe. After you've done it for a while it becomes second nature. As your familiarity with the sky grows, you won't need to go through all of the steps. Of course nothing prevents you from just sweeping your binoculars over a likely area and seeing what you can find. Some of the most beautiful sights in astronomy are seen through binoculars while just scanning the sky.

Acknowledgements and References

The Internet Purchasing Amateur Telescopes contains lots of information about binoculars, and contains the full description of the new, improved method for holding binoculars while observing.

Binoculars and Telescopes for Sky gazers (a pamphlet available from Sky Publishing, and often included with new subscriptions to Sky and Telescope magazine) contains quite a lot about choosing and using binoculars for astronomy.

The Orion Telescope Center Catalog has lots of information describing the various parameters of binoculars and a wide range of products so you can compare prices and feature of various manufactures and models.

Backyard Astronomy (Pamphlet from Sky Publishing) has information on binocular astronomy. It also shows the Big Dipper with distances between stars marked in degrees (a useful field-of-view yardstick) and a section on deep-sky observing.

The Astronomical League Observe Guide to the Messier Objects gives locations and descriptions for all of the 100+ Messier objects and includes the Binocular Messier Club supplement.