“Two eyes are better than one”, the old saying goes, but telescope aren’t exactly designed with this in mind. If you’ve endured eyestrain and pesky “floaters” in your eyes ruining your view of Jupiter or the Moon, you understand the drawbacks of “cyclops” astronomy. But there’s a solution: bino-viewers. Few other accessories can add horsepower to your telescope like a pair binoviewers, especially for observing the Moon, planets, and smaller celestial objects. Here’s what you need to know about these powerful observational tools…

Binoviewers are a packaged set of prisms that split in two the light path from your telescope and direct each beam to a separate eyepiece. There are two main types available. The less expensive is based on designs for lab microscopes. They tend to use smaller prisms which can vignette (cut off) light from some lower-power eyepieces, which are just the ones you want to use to get wide-field views of star fields. But these types are less expensive… you can get a set for $300 from Orion, Celestron, or Williams Optics. They work quite well at moderate magnification.

The more expensive binoviewers from Denkmeier use larger prisms and accommodate wide-field, low-power eyepieces, so they give crisper and more expansive views than less expensive optics. They also hold focus as you adjust the inter-pupillary distance. But they are heavier and much more expensive… at least $1000 to $1500 (not including eyepieces). Denks are a good choice if you have an excellent telescope, a solid mount, and a generous budget.

Of course, both types of binoviewers require two identical eyepieces, so this also adds to your expense.

Binoviewers attached to a small refractor telescope

What’s the drawback of binoviewers? Since they split the light into two beams, binoviewers reduce image brightness by at least a factor of two. The effect is to make an image of an 8-inch scope as bright as a single beam from a 6-inch scope, for example. But the effect is not as pronounced, because the brain more clearly perceives images from both eyes. So you hardly notice the loss in brightness unless you are observing right at the limit of your telescope.

Here’s another thing to consider. Binoviewers add another 4-5 inches of light path, so the focuser of a telescope needs enough travel to bring the image to a focus. This is usually not a problem with Schmidt Cassegrain scopes, which you focus by moving the primary mirror itself. But many refractors and nearly all Newtonians do not have enough travel in their focusers, unless you add a Barlow lens to bring the image to focus. And a Barlow lens at least doubles the magnification of each eyepiece, which means a smaller field of view.

So are binoviewers worth it?

Looking at the Moon and bright planets through a scope equipped with binoviewers is a stirring experience. The Moon takes on an almost “3D” appearance like you’re looking out of the window of a spaceship. The bands of Jupiter and rings of Saturn snap into view clearly. And looking with two eyes seems to reduce eye fatigue and the effects of distracting “floaters” that move in and out of view.

I’ve acquired a set of Williams Optics binoviewers, and I was pleasantly surprised by the views of the Moon and big planets. I can look at Jupiter for long periods of time without fatigue and see much more detail with my 4″ refractor than with a single eyepiece. And the binoviewers have renewed my interest in the Moon. I’m even following along with the notes of my own course “Lunar Observing for Beginners”, just to make sure I can find everything!

I also get good images of globular clusters and bright planetary nebulae with the binoviewers, since they are usually compact enough to fit into a single low-power field of view. With my refractor and the 2x Barlow, the restricted field of view at low magnification doesn’t work well for rich star fields and nebulae. And in my Schmidt-Cassegrain, where no Barlow is needed, the long focal ratio still meant no good wide-field views. So for wide star fields, I switch a single low-power eyepiece, or use a pair of regular binoculars.

If you have a chance to peer through binoviewers at a star party or other astronomy gathering, then take the opportunity. If you have a good quality telescope of 4″ or larger aperture, perhaps no other accessory can add as much to your enjoyment to viewing the night sky.