Binoviewers
Years ago when I was taught how to shoot a rifle, I was told to aim with both eyes open. Apparently that helps you relax and hold steady. When I started looking through telescopes, I tried to do the same thing. I kept both eyes open but concentrated only on what I saw through my dominant (right) eye. But wouldn't it be better if you kept both eyes open and actually usedthem both at the same time? That's the idea behind binoculars...and binoviewers.
Some thoughts about binoviewers:
1.For many, observing is more "comfortable" with binoviewers, and the view is more 3-D like. Other people, however, have difficulty merging the images entering each eye into one. Sometimes I do; sometimes I don’t.
2.Although the total light is obviously split in half, the overall brightness of the image doesn't actually decrease that much. Maybe a half magnitude is lost? I don’t notice it much.
3.Because the required light path is lengthened, some telescopes won't focus. A barlow lens must be inserted to re-lengthen the light path, thus increasing the magnification.
4.With most Cassegrain scopes, no barlow is needed, but because the distance between the primary and secondary mirror changes when you focus, the magnification is still increased.
5.Because of (3), (4), and vignetting, it’s difficult to use a binoviewer for low power, wide field views. The diameter of the cone of light entering a binoviewer is necessarily larger than the diameter of the cone of light normally entering an eyepiece. Binoviewers without a large clear aperture (the less expensive ones) reduce the amount of light reaching your eyes, especially near the edge of the field.
6.Using a binoviewer improves the quality of the image, especially for us old guys who have floaters in our eyes. What one eye misses, the other can pick up.
Conclusion: For low power views, buy binoculars! For high power views, especially of the moon and planets, consider binoviewers. Two eyes are better than one!
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