Binocular viewers are devices that let you connect two matched eyepieces to your telescope simultaneously. This lets you observe comfortably through two eyes at once (like binoculars), instead of using just one eye to squint through a single eyepiece. Binocular viewers offer significant advantages to the visual observer, no matter what that observer is interested in observing. A binocular viewer makes for a spectacular viewing experience – of the Moon, planets, star clusters, double stars, and more.

The greatest advantage to binoviewing is simple. By using both eyes simultaneously to observe, you see up to 40 percent more detail than you do when using only one eye to squint through a single telescope eyepiece. Your brain normally combines the views seen through your two eyes into one image, instinctively using only the sharpest portions of each eye’s view to build up a single sharp image.

You can probably see this effect for yourself. Try looking at small printing on a sign or eye chart some distance away, covering one eye with your hand. Then take your hand away and look with both eyes. The chances are good that the printing seen with two eyes will be more easily readable (sharper, and with higher contrast) than the printing seen with either eye alone. This is the effect that a binocular viewer’s two-eyed viewing gives you.

Small details – lunar craterlets, festoons in the bands of Jupiter, closely spaced binaries – simply become easier to see. Your brain is able to pick the sharpest parts of two images, instead of being limited to just one. The brain corrects for small defects (aberrations) in your eyesight when given the information from both eyes to work with.

There is also more lunar and planetary contrast visible, because the bright image of a large aperture telescope is split between your two eyes, thus lowering the image brightness in each. This reduces the irradiation within your eyes that can mask subtle contrast difference.

Many observers also credit binocular viewers with creating an almost three-dimensional view of objects. The Moon seems to become a solid globe hanging in space, rather than a flat circle pasted on the sky. Jupiter also becomes a globe, with some of its moons seeming to hang in front of the planet, and others behind it. Of course, these illusions are just that . . . illusions.

Beyond a 50-foot distance, your eyes’ pupils are nearly parallel and so prevent real stereoscopic viewing. However, since our brain “knows better” (due to our experience at viewing nearby round objects, like a beach ball or an apple), we interpret the merged two images of a round object in a binocular viewer as being three-dimensional, even though the image is not true stereo. The more experienced the observer, the more this effect will be apparent.

In addition, some observers report that nebulas can appear almost solid, with brighter folds of glowing gas appearing closer to the observer than the darker areas. Again, this is a result of a lifetime of near-viewing experience . . . of the folds of window curtains, for example. The folds of curtains nearer to us are brighter than the shadowed folds between them that are further from our eye. So it may also be with nebulas. Our brain interprets the brighter folds of glowing gas as being closer to us, and the darker portions as being further away, giving the false illusion of actual depth. Whether it’s true three-dimensions or not, binoviewing is simply more detailed and aesthetically more pleasing than single-eye viewing.

Finally, it is simply physically more comfortable to use two eyes to view through a binocular viewer, rather than squinting through a single eyepiece. When both eyes are open, your eye muscles are relaxed and you do not suffer from eyestrain.