This Celestron binocular viewer has a lot going for it, not the least of which is a very affordable price.
In many cases, the Celestron Stereo Binocular Viewer is a clear step ahead of observing through a telescope's single eyepiece. 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 an ordinary single eyepiece diagonal. Lunar and planetary details snap into focus with a clarity that makes these objects seem almost three-dimensional. Deep space objects, such as globular clusters, take on a solidity and seeming depth that renders them almost three-dimensional, as well. Of course, you're not actually seeing a true three-dimensional image (you are looking through your telescope's single objective lens, after all). However, the human brain is hardwired to assume that two eyes give you a stereo image, so binocular viewing gives many people the illusion of three dimensions. Whether it's true three-dimensions or not, binoviewing is more detailed and aesthetically more pleasing than single-eye viewing.
The Celestron Stereo Binocular Viewer is a good-looking (and good to look through) piece of precise computer-controlled machining. It is solidly built, with many nice touches you'll appreciate. Rubberized black paint coats the dimpled body to give you a sure grip on cold, dewy nights. Its fully multicoated BaK-4 prisms give you high light transmission. There is no vignetting or light cutoff at the edges of the field such as you get with lesser quality BK7 prisms. The thumbscrews holding the eyepieces are metal, not flimsy nylon or plastic that can be affected by cold weather. The individual helical eyepiece focusers have their diopter settings clearly marked, so that you can return to the proper setting for your eyesight after someone else uses the Binocular Viewer. There's no guessing about the right setting for your eyes as there is with binoviewers that have unmarked focusers. The 1.25" adapter tube is threaded for filters, which comes in handy if you decide to use a contrast-enhancing filter or neutral density (Moon) filter for long planetary or lunar viewing sessions. The Binocular Viewer comes in an aluminum carrying case with a fitted foam interior that safely cradles the 18 ounce precision optics. Dust caps are supplied for the 1.25" adapter tube and the two eyepiece focusers.
The Stereo Binocular Viewer can be used as-is with any catadioptric telescope using 1.25" accessories, as Schmidt- and Maksutov-Cassegrains typically have enough back focus to accommodate the extra light path of the prisms in the Binocular Viewer. Achieving focus with a refractor or reflector having limited available back focus might be a problem, however. Some refractors and reflectors have removable focuser drawtube sections for astrophotography. Removing the drawtube section will usually provide enough additional in-focus to use the Binocular Viewer. For those refractors and reflectors that do not have enough available back focus, a 2x Barlow placed in front of the Stereo Binocular Viewer will act as a transfer lens and usually allow the Binocular Viewer to reach focus. Any brand of physically short Barlow will usually work quite well.
An operational note: Occasionally there may be a slight misalignment in the optical axis of an individual eyepiece. This can happen with virtually any eyepiece type or brand. It is not noticeable when using a single eyepiece in a telescope. However, if you're using two eyepieces in a binocular viewer, and one is misaligned, it may be difficult to fuse the two eyepiece views into a single image. This generally happens if the offending eyepiece is misaligned vertically. The human brain can tolerate much more horizontal misalignment than it can vertical misalignment (we have a hard time looking up with one eye while the other looks down, but it's easy for our eyes to look towards each other - we do it every time we look at something close up).
If you have trouble fusing images with any binocular viewer, rotate one of the eyepieces in its holder. If it's the misaligned eyepiece, at some point the misalignment will point towards the center line of the binocular viewer, your eyes will accommodate the misalignment, and the images will fuse. If this doesn't happen, the other eyepiece is probably misaligned. Try rotating it. Make note of how the eyepieces line up in their holders. Put the eyepieces back in the same orientation the next time you use your binocular viewer and you shouldn't have any trouble getting the images to fuse.
