Eyepiece selection: As usual, we need to consider the eye relief, apparent field size, eye cup design, weight, and finally "nose relief".
Eye relief is obvious, just in mono mode, you see how important eye relief is, if the eye relief is very tight, I wonder if you can use it with two eyes, and one closely related thing is the size of the eye lens, in general, bigger means better.
Except eye relief, some eyepieces are very sensitive to eye positioning, and the field will blackout easily. These are less than ideal for binoviewing. Check before you pair up.
Apparent field size seems less important in terms of feeling in binoviewing. I have said that from the above section while 80 degree eyepiece pair does give a very good feeling about space walk, people also reported that even with narrow field eyepiece like Orthoscopics, the field is far more pleasing when used in a binoviewer! So, if you find orthoscopics less than happy in mono mode, it could be acceptable in a binoviewer. Again, try it out before going ahead. Finally, I found 80+ degree for both eyes is a bit hard to ingest at the same time, something like 70 degree would be enough. And maybe that's why Panoptics have been the most popular for binoviewing.
Concerning eye cup design, we rarely take care of that unless it is really really bad, but since you are likely to get closer to the eye lens with binoviewer, one should take care if the eye cup is well designed, effective to hide you from ambient light, if there is any, and also not to help dew formation. Also, watch out for the moisture from your eyes since we tend to get closer to the eyepiece when using the binoviewer. I found eyepieces with a well designed eye cup to be better for binoviewing, especially for solar observation.
For example, my Widescan Type III pair and my 20mm Tele Vue Plossl pair are both very easy to use on any object, but the 18mm J-Ortho pair is harder to use for solar observation, despite it works very well at night, mostly because the J-Orthos have no rubber eye cup.
The concern of the weight is about balancing, and also the capacity of your mount, I hope no one would use two Nagler Type I 13mm at the same time.
Nose relief is a new term, I guess you know what it is from its name. It's less a problem for Asian people, but I keep hearing problems in international discussion groups, check the size of nose to see if you have enough clearance between the eyepieces, eyepieces like Plossl will not be problematic but I am sure Pentax XL is another story.
Observing target: Well designed binoviewer works well for planets and also for the deep sky objects, but converted binoviewer like from microscope head, will not work for deep sky objects.
For the planets and the moon, binoviewer really gives more advantages than problems. But for deep sky objects, the situation is a bit more complex. At less than 12-14" aperture, we will realize the drop of light throughput with binoviewer, it is not trivial and it is readily noticeably. It translates to a reduction in limiting magnitude.
With the observational comfort brought by binoviewer, deep sky objects look better and more contrasty, but they are also dimmer. Deep sky objects tend to be dim and so dimmer really means dimmer. Planets are bright enough, the moon is even too bright so that a dimmer image is actually a good thing!
I found myself spending more time on a particular object than when I did in mono mode. So, it helps to offset part of the loss.
On planets, the result is very obvious. Binoviewer gives much better contrast than any eyepiece used in mono mode. The color saturation is also better. I can definitely see more detail with two eyes, and I can push the magnification higher than with one eye. Similar result for the moon, and I am sure any one will be shocked. My first look using two eyes, was at the moon and by then I was attracted and I made up my mind to buy one myself.
On deep sky objects, the result is not as simple. In one night, the transparency is good (6/10), and I tried to observe M42 inside my home which is located in the city center, Hong Kong. At low power, i.e. around 50x, the two eyes view (20mm widescan + 0.5 reducer) already beated the one eye view (40mm XL). Then, I push the power further up by removing the reducer, and the improvement in contrast is very obvious. Then, I kick in the 2.5x corrector and quite a lot of nebulosity popped up, and I can actually discern a bit of the structure within the nebulosity. I am very impressed.
On the same night, I swing to M41, but in that case, single eye view was better than two eyes. I concluded that the result was due to the light loss with two eyes. Reduction in limiting magnitude actually hurt the view of star cluster in a light polluted sky. I shall check again on darker sky.
In a summer night, I observed the M6 and M7 inside light polluted home again. I found that mono view is better and also noticeably brighter. However, when I scanned around, I could detect M19 using binoviewer, which was unseen when I was scanning using mono mode. Of course, switching back from binoviewer to mono mode again reveal that fuzzy patch. It means more contrast on binoviewing.
When observing the planets, color saturation becomes much higher with two eyes, I don't find similar effect on deep sky observation. Maybe light got splitted into two eyes, so that clone cells (responsible for color) does not get over their minimum threshold. I found that seeing color on some brighter deep sky objects with one eye easier. Of course, given large enough aperture, maybe it's not a problem?
From the experience I heard, as well as my own experience, I found that binoviewing will render marginally detectable deep sky objects to be invisible. However, it does make some brighter deep sky objects more striking. The amount of detail is similar, but the two eyes mode will simply make it easier and more comfortable to see.
Personally, I found binoviewing good to brighter deep sky objects but not for those dimmer ones. Note that it's not a matter of clear aperture of the prism, which affects the largest fully illuminated field rather than the overall brightness. It is more a question on the coating, transmission figure.
Verdict? I want binoviewing and mono mode, i.e. both, all depend on the target and the situation.
Observational comfort: While it is a second nature for experienced observers, observing with one eye is not comfortable at all. An eye patch for the non-observing eye would be nice, but it cannot beat two eyes observing at all. With two eyes, we tend to spend more time on a single object, so that we can pick out more detail.
I considered myself adapted to one eye observing, but I find it hard to stare into the eyepiece for more than 2-3 minutes continuously. But with a binoviewer, 30 minutes will not be a problem.
Illusion: With a binoviewer, you will have an impression that the image is actually larger. What I mean is that, you will see thing as bigger than that in mono mode at the same magnification. Also, when light is splitted by the binoviewer and re-combined by your brain, you have a false 3D feeling, but the false feeling is really very real in some situation. ;)
Imagine the Jupiter as a sphere floating in the vast dark space with its satellites, yes, it really feels like that.
False 3D feeling varies a lot from person to person, so no guarantee here and see your luck. Such feeling maximizes for the moon.
Balancing: We know premium eyepiece tends to be heavy to cause balance problem, it is even much more serious when used in pair plus the weight of a binoviewer. To give you some idea, my telescope is mounted at 99% near the end of the dovetail when used with a binoviewer.
For a mount without friction controls or locks, when you pick out the binoviewer to insert any corrector/reducer/amplifier, the rear side is going to be lighten suddenly. Please beware of that. Even when you switch eyepieces, you may face similar problem.
Mechanical issues, mount capacity: Like what I mentioned in the previous section, the extra weight can cause problem. If your mount is marginal for your setup originally, it *will* become overloaded when used with a binoviewer.
If your counter weight is already sitting near the end of the shaft, be prepared to buy one more counter weight.
Most binoviewer comes as 1.25" format, but to tell the truth, you will find it terrible to hold a binoviewer in a 1.25" diagonal. Consider going for a 2" diagonal just to have a more rigid support to your binoviewer plus your eyepiece pair. A 2" nose piece for the binoviewer would be nice as well, since one won't want to hold the binoviewer with two eyepieces with just a single small set screw originally designed for small 1.25" eyepiece.
I have conducted an experiment, I mounted my 1.25" diagonal on the visual back of my SCT, try hard to lock it with the two set screws. And then I put my binoviewer with two eyepieces on the diagonal. I can't fix the binoviewer at all, the diagonal will rotate to point my binoviewer to the floor. The poor little diagonal is trying to save its own life by getting rid of that heavy binoviewer at its best. (grin)
Finally, the set screw on my Tele Vue Ranger failes to lock down the focusing draw tube with the binoviewer. It's so heavy.
I've conducted an experiment. Ranger can reach focus without any corrector when used without a diagonal plus the 18mm ortho eyepieces, it can also reach focus with a 2.5x corrector before the diagonal. One can also use a 0.5x SCT corrector in the nosepiece of a binoviewer to lower the 2.5x magnification factor a bit and can still reach focus.
Some accessories can be used among different binoviewers, some are specially designed, and there is no universal standard right now, so check before you buy.
Newtonian has a newtonian corrector, refractor has one for refractor, SCT has no need, but can still be used, check with the vendor.
For truss tube dobsonian users: We said that binoviewer should be used with a corrector in order to reach focus because of insufficient in-travel, so truss tube dobsonian users might want to have their truss cut shorter in order to accomodate a binoviewer without correcter?
It is a funny topic, indeed. To use a binoviewer without a corrector, cutting the truss tube is a solution, but there're some more factors to consider.
By cutting down the truss tubes, the secondary is now closer to the primary and with the original secondary mirror, it's going to be undersized, again, vignetting will result. You got wider field, but it is a vignetted field, on the other hand, like what I said, vignetting is not readily detectable. However, be aware of the problem. Also, undersized secondary in a newtonian means a reduction of effective aperture of the whole system.
Next, not only the secondary is undersized, the prism inside the binoviewer is going to be undersized, since it is now closer to the secondary and the primary. Alright, similar reasoning like the above.
So you see, a corrector does have its function to retain the maximum unvignetting field other than just correcting the focus position.
Lastly, with shorter truss, you decide yourself to stick with binoviewer. Maybe it is good to keep two sets of truss tubes.
An review in S & T mentioned that some binoviewer corrector will get into the optical path with low profile focusers which are common in many newtonians, since the whole corrector assembly is so thick, it will affect optical performance by giving a strange diffraction pattern.
Coating: Coating does not affect only light throughput or transmission, it also affects contrast. Good coating will pass the light more effectively, and at the same time, it reduces scattering.
We have metalic based coating and dielectric coating, the later is better and more durable but also more expensive. Metalic based coating on the beam splitting prism will give slightly different colors on each side of the binoviewer, but you won't notice that in actual use. Yes, you can see the difference if you do a side by side comparison.
Style and Origin: Earlier binoviewers are converted from microscopes and they are simply not suitable for astronomical usage, especially at low power. These units are usually 45 degree models, and having undersized prism. Stay away from them. There are some rare exceptions, but why take the risk?
Binoviewers designed for astronomy purpose are usually straight through, and they are to be used with a diagonal. Look no further, go ahead with this type.
Price: To me, price is a very critical factor. If price is no object, portable asides, we should really go for real binoculars, period. Consider the case if I shell out $1000 to buy a binoviewer, for example in my case, why not buy one more C8 OTA to create a pair of real binoculars?
One would say two C8 OTA will not be able to make a pair of binoculars, but to me, it can, at least on paper. Buy two diagonals plus extension tubes for each OTA, and then the eyepieces from each scope will be close enough to use for anybody. Since a moving primary will give a lot of focuser travel, by going this route, I believe the setup can still reach focus.
I have no money/time to verify this guess, but like a binoviewer, one would need to push the focus farther out like the above suggestion, adding one more diagonal will not take much more focuser travel than a binoviewer.
Support and Services: Binoviewers have been made very generic these days, but we must still pay attention to make sure it matches your existing equipments.
For examples, Celestron 9.25" and Meade 14" are reported to be some special cases where you will need to use 0.6x reducer instead of 0.5x. Some exceptionally lower power corrector may not work with all telescopes. Also, collimation of binoviewer is not quite user do-able currently.
All these mean good support and services are very essential.
Return Policy: Having said all these, maybe you take every precaution to avoid lemon, to avoid poor devices, there could be exceptions. So, a "no question asked" return policy would be the most important thing you should look for. Binoviewers are not cheap, if you got stuck, no one can help. Quality products should be guaranteed with a return policy, if one vendor cannot provide, better ignore it altogether.
Which Brand: People said, the refractor and newtonian debate will never end, the Celestron and Meade "discussion" also. But I tell you, these are not the worst topics in the field of amateur astronomy.
When I started to find a good binoviewer for me, I go ahead to find in the Internet like usual. To my surprise, you see only one brand, yes, one only. No, to be exact, there are more than one, but in most places, you see only one single brand.
To me, Internet means freedom. Free(dom), open source software originated from the Internet, free discussions among so many people in the world are on the Internet, you see this article in the Internet. So, how come we can see only one single brand?
I don't exactly know, until one day, some good guys privately told me, one brand is prohibited, and there's a very long story behind, and they told me it's wise to stick with the other brand which is widely discussed in the Internet. Oh...
Yes, sometimes, we see the prohibited brand somewhere, but there will also be flame associated with it. The signal to noise ratio, is simply making no useful information available easily without going through a huge pile of trash.
So, I give no preference on which brand, I love good performance and good price. I keep one eye closed here.(Remarks on 2008: there are far more players in the market now, and the amount of user reviews which you can find on the Internet is good enough, just search!)
Some of the information above were collected into my mind throughout all the days when I shop for a binoviewer. I tried to verify them myself before writing this article and it took me over one year to finalize everything. However, I know I will make mistake and some information here will soon before obsolete as technology moves on.
I've no special interest with any manufacturer or company. I am only interested in good products, something that can enhance my experience in this wonderful hobby.
No matter how, binoviewing is a wonderful experience, especially on brighter targets. Comfort, the enhancement of color saturation (give the light level is above the threshold of your cone cell), the improvement on contrast, are all hard to match with only one eye.
I tried to list every single consideration from my mind when I made my own decision here, just wish to save some effort for others to go through the huge pile of information over magazines, internet, your friends, etc. However, nothing replace your own experience. By all means find your club members, ask your local folks, try everything you can if possible.
Good luck and enjoy!