A contractor once told me to always wear safety glasses when appropriate because life with only one eye is difficult. He was talking from experience; he’d lost an eye in an accident. Two eyes are good. Yet, when most people use telescopes, they sacrifice one eye and try to soak up what they can with the other.
A binoviewer can be used to offer a two-eyed view. One of the benefits of using a binoviewer is that it usually lets you see more fine detail. For example, I can see more features on the planets when using a binoviewer than a single eyepiece. A related benefit is the apparent magnification boost. When using a reflecting or refracting telescope, there’s no actual change in magnification, but the planet you are looking at still seems bigger.
Some people are bothered by floaters. These are microscopic fibers within the fluid of your eye, that tend to clump together. They make distracting stringy or lumpy, blurred shadows that drift across your view. They’re especially distracting if you’re using only one eye to look through an eyepiece. When using a binoviewer, the brain tends to ignore the floaters, and they essentially disappear.
Using both eyes is much more relaxing than squinting through a single eyepiece. Using one eye is usually okay for a quick glance, but eventually fatigue sets in, and it can be hard to keep focused.
Finally, the one hard-to-explain benefit of using a binoviewer is that objects appear more three-dimensional. We usually get a sense of the depth of field in our vision since our eyes are set apart from each other. Parallax allows the left eye to see more of the left side of an object, and the right eye, more of the right side. This is especially true for things close by, but at the distances seen in a telescope, there should be practically no parallax and 3D effect. Yet it still seems to be there!
You can see the benefits of using a binoviewer by using binoculars at night and comparing the experience with both eyes open versus a lens cap one barrel. Try it in the daytime too. With two eyes versus one, you’ll probably notice an increase in sharpness, more of a 3D feel, objects that appear slightly bigger, fewer distracting floaters, and less discomfort.
There are a couple of drawbacks with using a binoviewer. One is that you’ll need to buy two eyepieces for each needed focal length instead of one. You can avoid needing to double up on all your eyepieces by buying one good set and using Barlow lenses to change the effective power of that set. Most reflecting telescopes will require a Barlow anyway to be able to come to focus with a binoviewer. Another drawback is that the light reaching each eye is half the original intensity, since your two eyes have to share the light. Thankfully the brain seems to add the light from the two eyes together, and there’s usually not much of a perceived dimming, if any.
To get the most out of a binoviewer, it’s good to have it set up properly. Make sure the interpupillary distance is set right for your eyes by setting the width of the binoviewer to match the width between your eyes. If it looks like you’re still seeing double after adjusting the width, then you have a collimation problem. This means light is not coming straight out of the two barrels as parallel beams. One quick and easy fix is to rotate one or both eyepiece(s) in the holder until the double view merges into one. Binoviewers usually have a diopter ring for fine-tuning focus for each eye. It’s worth the time it takes to make sure the focus is perfect for both eyes by adjusting the ring.
There’s something even better than a binoviewer; it’s a binocular telescope. With that, each eye gets full intensity light, and the brain adds the two views together for double brightness above what would normally be visible.
The way to make a dim galaxy or nebula appear brighter is to use a telescope setup with a low focal ratio. There’s a problem though. The lower the focal ratio goes, the wider the beam of light entering your eye. Eventually the beam is wider than the 6 mm opening of your dilated nighttime pupil, and the light is wasted. The only way to bypass that physical constraint and increase the surface brightness is to fill both eyes’ pupils with maximum light and allow the brain to add it together. Therefore, the brightest views can come only from binocular telescopes. Unfortunately, they are the most expensive too.
