Lemaire 80mm 3x Opera Glasses circa 1890
When the moon is out take a pair of small binoculars and rest them against a wall, car top or camera tripod top to steady them. Take a careful look, noting apparent size of the moon, the extent of contrast and the amount of detail you can extract. Now, using your eye of choice, look through just one eyepiece with the rest of the binocular out past your ear. Note that the moon appears smaller, contrast and detail are reduced and the apparent size of the field is reduced. You are now using a telescope.
Observe items almost lost in shadow in a dimly lit room, first with both eyes and then with one eye covered and note that both contrast and detail are diminished with one eye covered. This can be tried with a series of grey scale images mounted on a poster. With both eyes several shades of grey can be acquired at the darker end of the scale which simply fades into the shadows using one eye.
While using a telescope, take an eyepiece with the same apparent field as the one being used to observe and as you observe, hold it up to your unoccupied eye, adjusting the eyepiece until its field coincides with the field of the observing eyepiece. If you continue to look through both eyepieces, background stars will appear in what was a blank field in the eyepiece for the unoccupied eye. The apparent size of the field will appear noticeably larger using both eyes. This is interesting, as the added eyepiece is “looking at nothing at all.”
Note, while using the binoculars first discussed and the pseudo binocular created by adding a second eyepiece, that viewing is more comfortable compared to viewing with one eye doing all the work while the other has no task.
It is generally held that a binocular telescope (whether large or small) provides about 40% improvement in contrast and detail when compared to a monocular telescope of the same aperture, magnification and f/ratio.
So it appears that the brain compares the visual data being presented by each eye and enhances data which is the same in each eye and depresses dissimilar data. This process works both ways – lots of improvement with a binocular viewer and lots of depression with a monocular. The brain seems encouraged that both eyes are at least looking onto identical stages – to the extent that, if forced, it provides a second cast of characters so both stages have the same play. Here and Here are sites discussing binocular vision.
It has been suggested that binoculars of a given aperture really have the equivalent aperture of a larger instrument due to the approximate 40% increase in detail and contrast. If so, a 6 inch binocular should rival an 8 inch monocular (6 x 140% = 8.4). In fact, my 6 inch reflective binoculars provide views as good as or better than an 8 inch monocular. But the actual resolution (Dawes limit = .57 arc sec) of the 8 inch mirror is better than a 6 inch mirror (Dawes limit = .76 arc sec) and two six inch mirrors are still limited to the actual resolution of one. I think it is more accurate to say that using a binocular allows the observer to extract a greater amount of data offered by a mirror of given aperture, as compared to a monocular of the same aperture.
How about binocular viewers? It seems reasonable to assert that a 10 inch mirror with superior resolution compared to a 6 inch mirror is a good candidate for a binocular viewer. Several manufacturers have addressed the problem of accessing the light path so far down toward the primary that the secondary mirror becomes objectionably large. Innovative optics to stretch the light path, high transmission glass and superior coatings have increased light transmission significantly. Older viewers were “light sponges” by comparison. It is my impression that views delivered either via a binocular viewer or two primary mirrors are equivalent as far as the eyes are concerned. With a binocular viewer the observer can benefit from the greater resolving power of larger aperture and the increase in contrast and fine detail from the observer’s own image processing.
I recently was given the opportunity to view through a new superior grade binocular viewer equipped with Radian eye pieces and coupled to a 20 inch Tectron scope with a well figured primary mirror. The result was outstanding and images of M51 at 250x were better than I was able to obtain with my of 17.5 inch binocular telescope. Owners of larger aperture scopes can now have “the best of both worlds.”
Whatever route is chosen, a small binocular somehow stabilized to provide undisturbed viewing, a binocular telescope, or moderate aperture coupled to a binocular viewer, the rewards of two-eyed viewing are worth the additional effort.