With a pair of binoculars, we have two objectives to collect light, and each eye will be using a separate telescope. With a binoviewer, we have only one objective to collect light, and each eye will be using one of the two splitted light cone from the same telescope.
Experiments tell us that, when observing with two eyes, the effect on light gathering will be like 1.4 times of each aperture, e.g. when you are using a 50mm binoculars, the brightness will be like using a ~59mm objective, but notice that the resolution is still in the class of 50mm. So, consider it's more like enhanced coating, i.e. it transmits more light, but it won't give more resolution.
On the other hand, with a pair of binoculars, you are using two telescopes, i.e. you will be seeing two different images with each eye. So, your brain is actually perceiving two images differ by a slightest bit of parallax (!!!), seeing, tube current. But with a binoviewer, the images are more or less identical for each eye, except otherwise at the very end of the optical chain like the floaters from your eyes. Contrast is improved as well, but the amount is again, hard to quantify.
With a 50mm telescope using binoviewer, assume 95% throughput, the brightness will be like a pair of binoculars with two ~41mm objective. Again, only brightness is concerned here, but the resolution of the system, remains like a 50mm telescope.
Actually, I am less interested in the numbers, but one thing we know is that, binoviewer does improve a lot on observational comfort and it really helps to bring a lot on contrast. We can see more on the planets and the moon with binoviewer, despite the fact that, yes, the image is dimmer.
"Cyclops" mode? People in the binoviewer community may use the word "cyclops" to refer to single eye viewing, however, I prefer to call it the mono mode instead. I found it insulting and not suitable, for example, people suffering from Amblyopia, viewing with two eyes does not make sense at all. But it's more like cultural issue than anything serious.
Images merging: It is easy to merge images with a pair of binoculars, but some people find it hard to do so with a binoviewer. This is a problem on the observer, the eyepieces used and also the binoviewer. Binoculars is an integrated system, but binoviewer is a after-work, so problem is expected.
When you find that you cannot merge the images of a pair of binoculars, you will know that they are probably out of collimation. The same logic applies to binoviewers.
A good binoviewer will hold its own collimation pretty well, but it is just part of the whole story. Binoviewer is not very much user collimateable, I think it is possible, but it's not easy either. Good vendors will check every binoviewer before shipping them.
From the user reports in the Internet, we are told that thumb screw type eyepiece holders will not work well for binoviewer, self centering holder in a compression ring style usually work the best. So, you see how critical the collimation should be, in order to have the images merged successfully.
Collimation problem can also be resulted in a pair of not exactly matched eyepieces. In that situation, rotating the eyepieces will help sometimes, after it is done, remember to mark it down their relative position, so that you don't have to re-do the process every time. Also, marking down which is left and which is right.
Lastly, it's about matching of eye balls. When individual focusing is required, and there's an accessory for this purpose. But if the difference is slight, you can loose the eyepiece holder and pull one side out a bit and then fix it at proper location again. A par-focal ring can serve the purpose to fix it in location.
One night I pointed my telescope at the moon, my father came in and he has no problem merging the image at ~50x, ~100x or even ~250x. One thing I should mention is that, he found problem observing with my Pentax 40mm SMC XL before, that eyepiece is one of the easiest eyepiece to use in the world to me. My mother also had a look that night, no problem as well. My parents rarely observe with my telescopes. They were amazed by the view like me.
Personally, I found that the key of merging is that, if you looked *for* the images in the eyepieces, you might have problem merging. However, if you looked *at* the images, you will see one image easily. I can tell my brain like this and I can see one single merged image or two separate images any time I wanted. Funny!
So, don't look into the eyepieces, but instead, look at the image. If it does not work, do a reset, i.e. close your eyes, step back, come back and try again. Pay attention to adjust the inter-pupilary distance, closer is better when you attempt to merge the images at first, once it is done, adjust it back to the most comfortable position.
The design of the eyepiece also play a part here. I found the pair of eyepieces with a nice eye cup and decent eye relief, work better than the pair without the eye cup in terms of merging.
Matching eyepieces: One of the reasons that makes binoviewing expensive is that, we need two identical eyepieces to work. If you have already gone premium, you will need to double the premium investment. Not only that, it could be hard to find exact pair, since different production runs of the same eyepiece model might result in slightly or even major difference in specifications. So, you might need to end up selling your existing ones at a lost, and buy a new matched pair in order to ensure that they are from the same production run.
Eyepieces having a longer history is going to be harder to match, for example, the Nagler 13mm. The original version and the new one differs by everything, except the focal length.
But a good news is that, there're different corrector available which can be used before the binoviewer to yield different magnification with a given eyepiece pair, just like a barlow lens. We can see them from 0.5x, 0.6x, 1.3x, 1.8x, 2x, 2.5x, 3x, 4x, 5x or even more... just to name a few of them. It helps to save some money back from having to invest in a many pairs of eyepieces.
I tend to agree with the above principle, but you also have to notice that, you won't love to use your deep sky eyepiece (lower power, wider field) to see the planets. So, you might want two pairs at least, even if they are of similar focal length.
If you are already spoiled by wide field eyepieces, give yourself a treat by staying with them. I did it, and I found the real space-walk experience which I never found so before in mono mode. The 40mm Pentax SMC XL is a bit "narrow" for my taste now.
Having said that, you might waste some field when using binoviewer. For example, when I use my 80+ degree eyepiece, I will move my eye ball or even my head a little bit to see the whole field, but with a binoviewer, you cannot easily do the same thing. People usually said around 70 degree can be seen at once, of course, YMMV.
Brightness and resolution: The beam of the telescope is splitted into two by the binoviewer, and therefore, a dimmer image is expected. Also, the more air-to-glass surface, the extra optical component, is going to reduce the overall light throughput.
So, even if the binoviewer works perfectly, you will see dimmer image. Of course, when the brain integrates back the splitted beam, the result will be brighter than either of the splitted beam, but it will still be dimmer than in mono mode.
My personal experience is that, the effect of adding two dimmer images from each eye, will not give up back even a 90% image as bright as in the mono mode. You may try it out with any binoculars, close one eye, evaluate the brightness and then open the other eye to see if the whole view would be brighten up when compared with the one eye view. If the above binoculars test won't brighten too much, it's going to be even less dramatic with a binoviewer, since we have two separate objectives in binoculars, but we got only one with a binoviewer. Personally, I found opening the other eye won't help too much.
While brightness is reduced, the contrast is improved significantly and the resolution is not affected. I don't exactly know why the contrast is improved, maybe the brain is stacking two images automagically to give more contrast. For resolution, it is a function of aperture, and less light throughput is not going to affect resolution of a telescope.
Other than contrast, the floater problem at small exit pupil (i.e. high power) is reduced by using binoviewer, and so the maximum useable power is higher than that in mono mode.
I never saw the color changing on the globe of Saturn very cleanly and easily myself in mono mode, but I found it very very easy to see the color on the individual banding on Saturn with two eyes. Magical! Dimmer but more detail. Now, it's a very clean gradual change of color across the globe instead of just some subtle changes.
Concerning the dimming effect, yes, at the lowest magnification, the image is quite a lot dimmer, but as you push the magnification, you won't see the drop very significantly. This effect is funny. I mean I don't find the view at 250x a lot dimmer than the 100x view. I found the 100x view of my 8" SCT very good with two eyes, but I begin to see it on the dim side at the same magnification when in mono mode. Another funny effect brought by binoviewer.
And one more point is that, viewing with two eyes will give an impression that the image is larger, and the field of view is wider, too. Frankly, I never got any space-walk feeling with a 80 degree eyepiece, but I do get a tunnel like feeling when I get back to a 50 degree eyepiece. However, I do really find the binoviewing view with two 80 degree eyepieces to be space-walk, out there but not from a telescope.
People reported that you won't even find the field with orthoscopics to be narrow with two eyes. I found it to be the case. Of course, do not try to compare side by side with a pair of 80 degrees here. The field really feels wider with two eyes.
Focuser travel requirements: Binoviewer is not small, it is much longer than a diagonal and it represents significantly more in-travel is required. Nearly no regular Newtonian or refractor will be able to focus, and compound telescope having a moving primary, however, will usually do without problem.
Remember that the amount of in-travel required is not just the physical length of the binoviewer, but it is the optical path length, when light travels inside a glass prism, the optical path length is actually longer than the length it travels inside a hollow tube.
Having said that, there is an accessories to fix the problem, we call it a corrector in general. Early correctors are just barlow lens, and since the length of the binoviewer is added between the "barlow" and the eyepieces, a 2x barlows when used as a corrector, will give effective magnification at more like 3x-4x. By then, the wide field usage of binoviewer is greatly reduced. That means a pair of 30mm eyepieces is acting like a pair of sub-10mm eyepiece, i.e. from low power wide field, to a medium to high power eyepiece.
Technology advancement due to the development effort from various vendors, we can now see correctors of much lower power, ranging from as low as ~1.0 something in some particular situation, to normally 1.2x, 1.3x to 1.8x, and 2x or even more. Some vendors are developing variable power corrector, multiple power corrector and even zoom corrector. Watch out for latest development.
Of course, there's no free lunch and they come with a (BIG) cost. Somehow not just an increase in price, but I have read about review such that the optical performance can get compromised, so it's another consideration than just a negotiation with your spouse, or just a bigger hole in your bank account.
For a moving primary compound telescope like an SCT, a corrector with focal reduction ability is available as well, like 0.5x to 0.6x, some particular models of SCT will not work with the 0.5x, so check with the vendor before your purchase. While those reducers are called 0.5x, when in use, they will be more like 0.6-0.7x. Due to the magnifying nature of a SCT secondary and the reduced distance between the primary and the secondary, the resultant focal length of the telescope when used with a binoviewer, is longer than the original value, so you see why a 0.5x works more like 0.6-0.7x when used in a SCT.
Notice that regular rear cell reducer/flattener for SCT will not work without special attention, because the reducer is much farther away from the eyepiece with a binoviewer in place. To make them work, the trick is to move the reducer/flattener to make it closer to the binoviewer. That is, from the normal configuration of SCT-reducer-diagonal, to SCT-diagonal-reducer-binoviewer. Binoviewer specific reducer is screwed in the nose piece of the binoviewer, so put your regular rear cell reducer/flattener more or less in the same place and you are going to make it work.
With those 0.5x corrector/reducer, it really transforms a SCT into giant medium power binocular. This is one of the most important catalysts for me to take the plunge to invest after all these years of speculating. Consider a 8" f/5-6 binocular, very attractive! isn't it!Don't expect too much here however, I found with the 0.5x, the field is still smaller than that provided by a 2" wide field eyepiece. But it's really wide enough already.
Vignetting: It is a very complex issue. To make it simple, human eyes are not so sensitive about vignetting, so unless it is very serious, you won't see the problem.
This problem originated from several sources, like the clear aperture of the prism used in the binoviewer, and also the vignetting results from an extended light path outside the focuser, and the change of focus position. It could be from under sized optical corrector. Basically, anything smaller than the light cone in that particular part of the optical path, *might* vignet the final image.
With an SCT, the primary mirror has to move much closer to the secondary in order to accommodate that extra-length posed by the binoviewer. As a result, the secondary is somehow undersized in this situation, vignetting results. Another source of vignetting is the small rear opening of an SCT, which is less than 2" for those smaller models, the baffle will also limit the maximum illuminated field size, remember the baffles are designed for normal primary-secondary spacing.
Another source of the problem is the prism size, most models have a prism size smaller than the maximum field stop of a 1.25" eyepiece. In this situation, we see vignetting again with larger field stop eyepiece, like a standard 32mm Plossl. The prism is the most expensive component of a binoviewer, so it is the most effective place to cut cost.
Like I said in the first sentence, it is a complex issue. In theory, vignetting does occur in the above cases and there are actually more different situations as well. In practice, I found it hardly noticeable. Vignetting means dimmer or more fuzzy field near the edge of the field, but such dimming effect, may or may not be detectable at different exit pupil size. People tend to find that with larger exit pupil, such effect is more noticeable, and at a small exit pupil, the effect is less. So, a 32mm 50 degree eyepiece is going to be worse than a 16mm 80+ degree eyepiece.
Sometimes people wonder whether such undersized prism will only cause vignetting or it might also cut down the effect aperture size. I am no expert, but I would guess that it won't. Will you cut down the effective aperture by using a 1.25" diagonal rather than a 2"? I never heard of that. With smaller prism, I believe only vignetting will result, that means a dimmer, less illuminated field at the edge, but not reducing effective aperture (nor resolution therefore). Note that the same argument does not apply to Newtonian secondary.
Additional Remarks: The fully illuminated field delivered by a binoviewer depends not only to the aperture of the prism, but also the focal ratio of the telescope in use. According to a discussion in astromart forum, a typical binoviewer has an optical path length of around 120mm, of course, some are longer and some are shorter. For example, with a f/10 SCT working at around f/12 with a binoviewer, the beam from the eyepiece focus point will be about 120/12 = 10mm. For every 1mm more prism aperture, we can have 1mm more fully illuminated field. Suppose you have a binoviewer of 30mm prism, you will be able to use an eyepiece with 30-10 = 20mm field stop fully illuminated. You see even if you have a 30mm prism, you will end up very limited to get wide field! But like I mentioned above, it is simply not necessary to care too much about that, human eyes are not that sensitive in vignetting unless you look for them.
Also, it depends on your target. When I looked at the moon, the gradual dimming due to vignetting is not very detectable unless you look for them, but if you look at empty dark sky, you will see a fuzzy field stop very easily.
Recently, we see some new binoviewing with larger prism than normal 1.25" binoviewer. How can it be? They do the trick by employing a 2" interface at the telescope end, and 1.25" port on the eyepiece end. Of course, there are some models having 2" in the whole design.
(to be continued)