Until recently, many new observers inquired about binocular viewers (binoviewers), but few actually bought them, primarily due to the expense. This is starting to change, with less-expensive binocular viewers becoming available. As with anything, you get what you pay for, so the best binoviewers will still be pricey. Also, you have to buy two of each eyepiece, plus two of any filter you might use.
How Binocular Viewers Work
Above: A typical binocular viewer, TeleVue's BinoVue
A binoviewer uses a set of prisms, similar to those in a porro prism binocular design, to split the light from the telescope. Since half the light is going to each eye, the overall brightness of the image is decreased. Binoviewers have an interocular adjustment for the spacing between the observer's eyes, just like a regular binocular. They also have a diopter adjustment on one eyepiece holder. This allows each eyepiece to be focused individually to compensate for any difference between the observer's left and right eyes. Once the diopter is set, it does not need to be changed and all focusing can be done with the telescope's focuser. However, the diopter will need to be adjusted for each new observer.
Binoviewers normally only work with 1.25" eyepieces. There are two reasons for this. For one, the prices of high-quality binoviewers is plenty without using larger prisms, etc. Also, large 2" eyepieces would be spaced so far apart they would be difficult to view through simultaneously without having your eyes surgically planted on stalks like a lobster.
Advantages of Binocular Viewers
Many people find viewing with both eyes to be more comfortable. After all, it's how you normally observe the world, so why not the night sky as well? Of course, the reason we have two eyes is for depth perception. This works because of the different angle between each eye and the object being viewed (called parallax). At astronomical distances, this angle is exactly the same for each eye, so there is no depth perception. However, the weird inner workings of the human mind to tend to produce a 3-D effect when observing through binoviewers, even if it is only an illusion.
Disadvantages of Binocular Viewers
Aside from the price issue already mentioned, there are a couple other inconveniences associated with binoviewers. One is that not everyone finds them easy to use. Some people find getting both eyes properly aligned with the two eyepieces to be difficult. Also, some long-time observers are so used to using one eye, that they actually find that to be preferable.
The main disadvantage to using a binoviewer is the reduced light. Stargazing is all about light-gathering. Observers put a lot of effort and money into big telescopes and high-transmission coatings, and high-reflectivity mirrors, trying to squeeze every possible photon out of an instrument. A binoviewer then splits the incoming light and reduces the brightness by 50%. Most observers find that the effective light loss is not so dramatic, since the brain recombines the images and seems to compensate somewhat for the reduction in brightness. Still, the image will be dimmer.
Depending on the design of the binoviewer and the type of telescope used, an amplifying lens (a Barlow or similar lens) may be required. This will increase the magnification and decrease the field of view, making wide-field viewing difficult with certain setups. Notably, refractors normally require a Barlow lens to be used with a binoviewer, unless the telescope is specially designed for binoviewers. Without the Barlow, focus cannot be reached with the binoviewer in place.
Another drawback becomes apparent when using a binoviewer for public stargazing (which we recommend only to the most masochistic astronomers). It can be a challenge enough to get a new observer comfortably observing through a single eyepiece and focusing the telescope. With two eyes, an interocular adjustment, a diopter adjustment, and telescope focus, it becomes a bit of a nightmare.
Binocular Viewer Prices
Binoviewers can cost anywhere from $200 to $2000 depending on the quality and options. The better systems have very high light-transmission to minimize the light-loss effect inherent in binoviewers. They will also have excellent quality glass to minimize aberrations. This is important because with the use of a binoviewer there is suddenly a lot more glass in the optical path. High-end models will have better mechanical quality including smoother motions of the interocular and diopter adjustments. Lastly, the better binoviewers integrate additional optical components such as Barlows or field correctors into the design, thus optimizing the entire setup.