The Celestron Stereo Binocular Viewer gives you incredibly comfortable three-dimensional viewing, with far less eye fatigue from extended viewing.
Using two eyes to view through a telescope achieves the odd duality of being both relaxing and thrilling. It is relaxing to not have to squint or cover one of your eyes, affording longer sessions without fatigue. And the perception of depth you'll get from using both of your eyes is an awesome thrill that you'll not understand until you experience it.
Though the stereoscopic effect of using two eyes shouldn't be relevant at stellar distances, in actuality that theory is plainly dispelled. The background sky seems cast far beyond the star, an inky black canvas setting off the object of interest like never before. Subtle nuances of double stars are revealed for the first time despite dozens of previous gazes. Though less than half of the previous amount of light reaches each eye, it nonetheless seems brighter because your mind's eye "gains" much more than is lost by the mirror.
Of course, all of this comes at the expense of needing two eyepieces and this device. While not exactly inexpensive, it is a far cry less dear than similar units from other manufacturers. But if you are an enthusiastic observer after the mystical unencumbered walk through space, then this is the tool that will deliver it.
The Celestron binocular viewer is well tailored and solidly constructed, with lots of good thinking in its design. It has a rubberized and dimpled surface to both absorb knocks and provide a firm grip in damp, cold conditions. The choice for the type of prism and its coatings was not skimped a bit, for high resolution and edge brightness.
Currently the Celestron Stereo Binocular Viewer works properly with all of Celestron's Schmidt-Cassegrain design telescopes, and very likely with any Schmidt-Cassegrain. Many refractors with a removable focuser drawtube can also be used, and with those which don't have a removable tube you can generally achieve focus with a short 2x Barlow lens placed in front of the device. The binocular viewer is designed to go directly into 1.25" focusing tubes, and will not work properly while behind another prism or erecting diagonal.