The Ultimate Guide to Using Binoviewers

Imagine being a passenger in a private spacecraft orbiting the Moon. As you look out the porthole, you place your hands on the sides of your temples, then gasp in awe as lunar landscapes come alive. Subtle features that were hardly noticeable before now pop out in three dimensions with detail and contrast you've ever seen before. It's as if you could reach out and touch the Moon! Does this sound hard to believe? Well, it was my impression during a public star party the first time I viewed the lunar surface through a Schmidt-Cassegrain Telescope equipped with a unique accessory called a Stereo Binocular Viewer or simply, a binoviewer.

Once the spacing for my eyes was adjusted, the views were remarkable, unlike anything I had seen before. Using both eyes just felt natural to me without any fatigue or eyestrain. The views of some of the brighter planets, nebulae, and star clusters were just as impressive.

Since then, using a binoviewer has become my preferred method for observing the Moon and other bright celestial targets. If you're looking to add something new to your observing repertoire, why not give binoviewers a try? If you are curious to learn more, read on! This guide will explain what a binoviewer is and cover its benefits and even some of its drawbacks to keep in mind. Perhaps a Celestron Stereo Binocular Viewer will be in your future.


What is a Stereo Binocular Viewer?

A binoviewer is an optical accessory that you insert into a telescope's diagonal, focus drawtube, or visual back. It uses a beam-splitter that divides the incoming light from a telescope into two fainter, separate paths. A set of prisms redirects the light towards two identical eyepieces.

Because each eyepiece receives 50% of the light compared to 100% of a single eyepiece, you might guess that the views through a binoviewer could be quite dim. But our eyes work as a team and our brains are wired to process visual information better from both eyes. So any loss of brightness is subtle—especially when viewing brighter targets such as the Moon and planets. While the image is not truly three-dimensional, your brain perceives the stereoscopic effect of the two split images. Seeing a noticeable increase in clarity and detail, you might never go back to one-eyed viewing!     

What are the Benefits?

You may have heard the old saying that "two eyes are better than one." There may be some truth to it, especially when binoviewing. Why observe with one eye when you have two? Here are just a few of the advantages of using a Celestron binoviewer:

  • Using both eyes makes the viewing experience more relaxing and enjoyable. No longer will you have to close one eye while observing with the other eye.
  • Binoviewers eliminate eye strain and fatigue. Viewing through two eyepieces is much more comfortable. You may even find yourself observing for longer periods.
  • A binoviewer is ideal to use with catadioptric telescopes such as Schmidt-Cassegrain telescopes (SCTs) and Maksutov-Cassegrain telescopes (Maks). These optical designs offer a generous amount of back focus, which can easily accommodate the extra light path from the binoviewer's prisms.
  • The Moon and planets can take on the appearance of floating orbs in space, especially at higher powers.
  • Deep-sky objects such as bright nebulae and globular clusters may appear to take on greater depth, giving these objects a virtual three-dimensional appearance.
  • Bright objects such as the Moon, Venus, or Jupiter will be easier to view because some of the light is diminished through the binoviewer.
  • The 1.25" insert barrel is threaded for filters. You’ll only need to buy one of each filter instead of one for each eyepiece.
  • Each eyepiece holder can be individually focused to match your eyes and the diopter adjustment readings are marked.

What are the Drawbacks?

Binoviewers certainly have their advantages, and many binoviewing aficionados swear by them. However, there are some drawbacks that you need to be aware of:

  • Because there is a set of prisms to split the light from the telescope, only half of the light will reach each of the observer's eyes. As a result, the overall brightness will be slightly decreased. Keep in mind, though, that our brain can compensate for some degree of light loss. We don’t recommend using a binoviewer to observe faint, deep-sky objects with low surface brightness—when gathering as much light as possible is key.
  • Depending on your budget, the cost of using a binoviewer can add up. Although the Celestron Stereo Binocular Viewer is much more affordable than some other binoviewers, you should also consider the cost of eyepieces. You will need two identical eyepieces of the same model, exit pupil, field of view, and focal length for the views to be in sync. Start with just one pair of eyepieces. As you become more experienced, you may decide to grow your eyepiece collection.
  • The binoviewer, two eyepieces, and possibly a filter can get heavy quickly, so balance or weight issues may become an issue. Extra counterweights or a counterweight system can help.
  • Because a binoviewer requires plenty of focus in-travel, it cannot function properly without a 2x Barlow or an amplifying lens when used with certain telescope types. Some refractors and most Newtonian reflector models will need a 2x Barlow or similar lens for the binoviewer to reach focus.
  • Most binoviewers only accept 1.25" eyepieces and are not compatible with 2" eyepieces.
  • Binoviewing may not be suitable for all observers, especially those who have difficulty merging the images from each eyepiece.
  • Binoviewers may get knocked out of collimation if dropped or mishandled and could need factory servicing or replacement.
  • Everyone's Interpupillary Distance (IPD) is different, and each new observer will need to adjust their binoviewer to his or her eyes. Therefore, we don’t usually recommend binoviewing with large groups of people, for example, during a star party.

About the Celestron Stereo Binocular Viewer

Celestron's Stereo Binocular Viewer is a fine accessory that will allow you to dive into the exciting world of binoviewing without breaking the bank! It provides a three-dimensional effect when observing many Solar System objects and other bright celestial objects. Its solid build with a rubberized body makes it easier to grip on cold nights. Its 22 mm BaK-4 fully multi-coated prisms allow optimum light transmission with no vignetting or light cut-off. Each eyepiece holder features metal thumbscrews to secure each eyepiece. You can focus each eyepiece individually to ensure sharp views.

At just 18 ounces, this binoviewer is relatively lightweight. It uses approximately 3.5" of back focus—sufficient for catadioptric telescopes (Schmidt-Cassegrain and Maksutov-Cassegrain), as well as a good number of refractor telescopes. (However, as mentioned previously, most Newtonian reflector telescopes will need a 2x Barlow or an amplifying lens for the binoviewer to reach focus.) The binoviewer even comes with sturdy aluminum carrying case and our two-year warranty. To learn more about the Celestron Stereo Binocular Viewer and its specifications, please click on this link

You will need a matching pair of 1.25" eyepieces to get started. Celestron offers several eyepiece lines that pair well with the Stereo Binocular Viewer and fit anyone's budget. These 1.25" eyepieces for binoviewing offer an apparent field of view of 60 degrees or more:

Some people will remember their first glimpse of the Moon or planet through a telescope. But many will remember the first time they observe the Universe from a three-dimensional perspective. Seeing objects with added colors, contrast, detail, and depth will blow you away! Is it worth the added investment of acquiring a Stereo Binocular Viewer and two or three pairs of extra eyepieces? I think so, and I have never looked back.

I have spent countless hours examining the Moon's surface, admiring all of the subtle markings that were never visible before but suddenly appeared. Seeing Jupiter as an orb "floating" in space while its Great Red Spot rotated into view and stood out like a distinct orange dot is something I'll never forget. Most importantly, I enjoyed the views without straining my eyes or having to squint. Those days are over unless, of course, I want to observe some of the faint fuzzies with low surface brightness. Then I'll have to revert to "cyclops" mode once again.

Welcome to the world of binoviewing. Enjoy the views!

Other articles you might be interested in: Ultimate Guide to Observing the Universe