Binoviewers - Enjoying the sky with TWO eyes!
an equipment review, by Todd Gross
8/15/96, updated 1/22/97, and 9/16/98, and 1/26/99, 8/24/99, 4/26/00, 12/23/02, 3/26/03,5/13/03, 7/9/03


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It sounds so simple, split the view into two beams, and turn your telescope into virtual binoculars! Well, in practice, it's not quite so simple. Binocular viewers (binoviewers) for telescopes come in several different styles, can throw your focus into a tizzy, and work well with certain eyepieces and not others. Throw into the mix different people's ability to "merge" two images, increased magnification due to changes in focal length on a Schmidt-Cassegrain scope, changes in focus when different people view through some units, limited clear aperture on most, barlow options, and various misconceptions, and you have one big mess to sort out!

Meanwhile, as many folks are getting into Astrophotography, and CCD Astronomy, I have decided to, at this point, focus my attention on the cutting edge of "visual" astronomy (gee, does anybody still look through their scopes with their EYES?) and this of course includes great eyepieces (such as the Naglers), great filters (such as the Lumicon UHC/OIII) and of course, binoviewers..... sooooo here we go:

Let me start by telling you that looking through a really good binoviewer (like the Televue unit) is absolutely wonderful. Star clusters which you are accustomed to seeing with 10x70 binoculars as fuzzballs, are now "alive" and resolved, similar to the way you would view the Pleiades in a regular pair of binoculars. Planets seem to show more features, and nebula are easier to dwell on. In fact, the impression you get is exactly the same as looking through binoculars. You can't see 3-D in binoculars when looking at objects at infinity, and you can't see 3-D in binoviewers either, although it does have a false "3-D" effect, just like binoculars pointed to the sky, which is very impressive.

EYESTRAIN RELIEF/FLOATER REDUCTION:
I will get to the controversial "light sharing" issue in just a moment, but first, let me pose this question to you: If you were told for now on, to only look through ONE of your eyes when using your pair of regular binoculars, how would you feel? Certainly you would make out almost as much with one eye, but how do you feeeeeel? The answer is: just plain uncomfortable. The main advantage of a binoviewer, has nothing to do with the obvious ability for two eyes to be able to perceive more than one, it is more the lack of eyestrain than anything else that impresses me! This is the key reason why I am no longer able to use my telescopes without the binoviewer comfortably.

Additionally, at high power, eye "floaters" often distract observers. There is apparently about a 75% reduction in the noise level from eye floaters when using two eyes, another bonus when it comes to binoviewing.

THE LIGHT SHARING ISSUE:
I want to dispel the myth about not being able to use binoviewers for deep sky objects right off the top here. As you will read below, certain units, such as the Televue binoviewer, are designed to be able to be used at low power, with a wide clear aperture and large prisms that enable you to use 1.25" eyepieces down to 35, 40mm without any vignetting. The most commonly quoted disadvantage of binoviewers (other than price!) is that they split the same beam of light into two beams, thus decreasing the amount of light you are seeing by at least 50%. While this is true on paper..... Just as you have an additive advantage when using binoculars over a monocular, you take away very little light gathering with a binoviewer, especially in larger aperture, as your brain seems to somehow account for this split. In fact, since you are perceiving the image better with two eyes anyway, you end up almost where you started from, after splitting the beam, and then viewing with two eyes. True, I have measured between a 1/4 and 1/2 magnitude starlight loss in an 8" scope, and at first glance globulars are significantly less resolved, indicating less light gathering, but the amount of the loss seems far less than 50% in practice. Perhaps closer to 25% effectively, or thereabouts. I have written about this in detail before on the newsgroup sci.astro.amateur on the Internet. Try this experiment:

In a somewhat darkened room, look at a piece of white paper while covering one eye with a piece of cardboard. Take away the cardboard and now view it with two eyes. Do this several times. Did you notice the shade of white seems to "brighten" a bit as you use both eyes? Do the same now with a regular pair of binoculars (this is additive unlike binoviewers, which subtract light, but it will show you the point) View a starfield with one eye, then two. You can make out MORE stars with two eyes! A good pair of 70mm binoculars will give you a better view overall, light gathering wise, than a low power 70mm telescope of the same star field.

And then of course, two eyes can perceive more detail, which also helps to off-set the light loss.

Anyway, the bottom line is that you will have a more enjoyable view, but indeed slightly dimmer using a binoviewer. Thus on my 10" scope for instance, I noticed that deep sky objects are not quite as bright as without the binoviewer, but certainly brighter than my old 8" scope without the binoviewer. I have effectively lost about an inch or so of aperture light wise, although this is purely subjective. I have noticed the same thing with a 4" refractor, I am seeing more than a 3" scope, but not as much deep sky brightness as with the 4" scope without the viewer attached. The greatest hindrance to deep sky views using the binoviewer, by the way, appears to be on extended galaxies, which are quite dim to begin with. However, there is a magic number around 15" aperture. Once you get beyond that, views are equal to, if not better than single eyepiece views in just about every way, on every object. On my 18" reflector, I am now finding I purposely switch to a binoviewer to bring out detail in galaxies now, rather than vice-versa! One has to question why you would buy/build a huge binocular telescope of two 13" scopes, when you can get similar results using a binoviewer in an 18".  (although one sometimes has to employ a barlow to reach focus in the binoviewer as discussed below)

KINDS OF BINOVIEWERS:
There are three basic kinds of binoviewers all with different "quirks":

1. Microscope-head style binoviewers (Celestron-Orion -University Optics): Adapted from Microscope heads, small prisms with limited clear aperture, 45 degree angle rather than straight through or 90 degree angle. Most will plug right into a 1.25" star diagonal, some like Orion's screw right onto the back of a Schmidt - Cassegrain scope. You can adapt Celestron's binoviewer to fit onto a schmidt-cassegrain like that using their standard visual back.

The disadvantage of this binoviewer is that you cannot easily use eyepieces much over 18 or 20mm before you will see significant vignetting. The general light throughput is also not quite as good, and the 45 degree angle can be somewhat distracting. You cannot easily use these at all on reflectors.

The other major disadvantage is the problem with focus shift as you adjust the two eyepieces (push-pull) for the different distances required for different sets of eyes. In fact, even with just ONE user, you sometimes have to change this interpupilary distance to accommodate different sets of eyepieces, and here again you will need to refocus every step of the way as you adjust the width between the eyepieces.

The advantage of this style is the price, they tend to run around $500-$600 or so new, and a bit less second hand. (Orion's list is $649) They come in and out of availability and when I checked several months ago, the Celestron unit was not available, the Orion unit was.

The other advantage is that on the Moon and planets at high power, you seem to be able to get as good performance, or just about anyway, as the more expensive units. Like all binoviewers, it takes a lot of focus travel to be able to accommodate them, see further down in this article for details.

One should note that the Takahashi TWIN VIEW unit, although it fits in this category, is in a league of it's own for 45 degree angle binoviewers:  It has high light throughput, and larger clear aperture. PLUS, it includes a built in barlow, which boosts magnification to exactly 2.1X which is nice and low (most barlows are 4x when mated to binoviewers). It completely makes up for the additional light path, when mated directly to the scope without a diagonal, so it will work in any refractor, no problems with focus travel at all. In fact, Takahashi did the job too well, as you may need an extention tube to reach focus in a few scopes, since the focus position is pushed back an additional 2.5" from the normal focus point. (In other words.. if you mate the unit to the scope directly, you have an extra 2.5" of focus travel relative to where the scope would normally reach focus, so you may need a 2" wide ext. tube that is 1-2" long. Alternately, if you use the unit in a 2" star diagonal, you have to pull IN an extra 2" which will work on some but not all scopes) If I haven't confused you enough already, note that this plentiful amout of focus travel does not apply to Newtonians. Refractors take into account that you are going to use a star diagonal, Newtonians don't. Thus, the TWIN VIEW doesn't reach focus in Newts without an additional barlow of some kind employed! Like any of these other 45 degree angle units, it is a bit uncomfortable to use in a Newtonian anyway, although not impossible. In March, 1999, I received this additional note from Steve Foltz:

>Todd, I was just reading your review of Binoviewers and noticed that one important aspect of the Tak model
>was left off that you might want to insert.The Tak bino can be used wilthout its built in barlow (it can be
>unscrewed). I use it like this for even wider field of view observing. I don't know if you've tried it like this or
>not, but it' s really cool!

There is also very little if any vignetting, in the Takahashi, unlike the "cheaper" units above. Resolution is superb as well. The Takahashi, while not as versatile as the next group of binoviewers to be listed, is a big step up, if you don't mind being limited to somewhat high power. (but at the convenience of a versatile focus position in refractors). It also has the best barlow solution, as the other units, listed below in the next group, either have built-in barlows which do NOT make up for the additional in-travel, or the barlows are add-on and less convenient. So at least in that sense, the Takahashi has a "one up" on all the other binoviewers in this entire article.

2. True binocular style binoviewers: NOTE: THIS TOPIC IS BEING REEVALUATED DUE TO THE RECENT ADDITION OF GREAT OFFERINGS (2003) The Astrophysics/Baader/Zeiss binoviewer, the Televue Bino Vue and the BW Optik/Lumicon are the only ones that I know of that fit this style, currently in production.  4/03 Update: Harry Siebert's custom binoviewer also fits in, and has now been improved to compete with the Televue.  I have devoted a separate page to it. (CLICK)
5/13/03 Update: Denkmeier is currently offering straight-through units as well, and a variety of optical correction systems for them, which I am currently evaluating. I am finding the Denkmeiers to be on the cutting edge of the binoviewer offerings. Click on this special write up on Denkmeier.

This style of binoviewer is very expensive.. running almost $650.00 - 1100.00 (includes a 3.8x barlow lens with the Televue unit) I am not sure what goes into the production of these binoviewers, but it does seem extraordinarily expensive. However, the performance improvement is quite substantial...and worth the price.

The advantages of this style of binoviewer is firstly, that they are built just like binoculars. That is, you can adjust for the distance between your eyes in the same exact fashion, with no change in focus, as you adjust. The less expensive pairs are more of a push-in/pull-out proposition, which throw off your focus as you adjust for eye distance.

The second BIG advantage is that this style of binoviewer has full size prisms which fully illuminate 1.25" eyepieces. No vignetting is noticeable even at low power, or with 20-40mm focal length eyepieces. The light throughput also seems to be higher on these units, and makes them totally suitable for deep sky observing. The Lumicon / BW Optik binoviewer has 22mm clear aperture, so there is  vignetting on lowest power eps only. I comparison, the clear aperture on the Televue unit for instance, is 26mm. The Lumicon/BW Optik however, performs just as well, if not better at high power than the Televue and A/P Baader/Zeiss units. (did a side by side with the televue) but does not have readily available barlow options to bring the scope to focus in the necessary spot on Newtonians, refractors, etc. (although they can be found from Markus Ludes I am told) THIS LINK TAKES YOU TO MY BW OPTIK/TELEVUE COMPARISON.

The next advantage is that these units are NOT built in a 45 degree style, you can use these binoviewers either straight through, or utilizing a star diagonal. (The A/P unit has a built in diagonal, the Televue unit requires a 1.25" (or 2") star diagonal). This allows you to use this type of binoviewer in a Newtonian reflector, which is difficult with a unit built at a 45 deg. angle. However, coming to focus on a Newtonian will require barlowing (see below, there is an exception)

Also.. viewing straight-through on the Televue unit, and even in it's "diagonal" position on the Astrophysics unit, it is possible on many refractors to actually reach focus without introducing a barlow lens before this type of binoviewer. This makes it possible to enjoy truly low power viewing. (more on this later, reaching focus on some scopes is a snap, others quite difficult)

3. The alternative style... right "in between": As I mentioned, BW Optik, has less clear aperture than the Zeiss or Televue Units, but belongs more or less in the same category above, and is a superb unit.  However, another  from Russia, LOMO, falls right in between the first two alternatives. Featuring a binocular style that will not lose focus as you adjust for the width between your eyes, they appear to have somewhat more clear aperture than the cheaper units, but less than the more expensive ones. Apparently they are around 20mm clear aperture. The LOMO unit was a 45-degree style binoviewer. It does lack inter-ocular focus problems, but is still more similar to the Celestron type binoviewers than the Televue type when it comes to performance. Planetary and lunar images were as sharp (but not as bright) when tested head to head with Televue's binoviewer. NOTE: The LOMO unit, in it's most usual position, appears "upside down", which did not affect the view, but just took a bit of getting used to.Kind of like using binoculars upside down.

"MERGING" THE IMAGES:
The way binoculars, and binoviewers work, is to collimate each side perfectly with each other, so that your eyes can "merge" the two images into one. Some people, it turns out, do this more readily than others. High power eyepieces make it more difficult to merge images, than low power ones, as explained later.

COMING TO FOCUS WITH YOUR BINOVIEWER, and BARLOWING:
Perhaps the greatest inconvenience in using a binoviewer, is that it requires that you rack your focus way in to account for the extra travel path of the light to reach your eyepieces. This travel length averages 5.5 inches. In fact, on most scopes, you simply CANNOT reach focus without using a barlow ahead of the binoviewer, to counteract this. That doesn't sound like a big problem at first, but because the distance is also greater between that barlow, and the eyepieces after travelling through the binoviewer itself, the barlowing is "magnified" to somewhere between 3 and 4 times, on a regular 2X barlow! For some scopes, this will strap you to such high power, that wide field and low-medium power viewing is impossible. There are several exceptions, and tricks to help dodge this issue: (I really got good at dodging this!)

1. On many refractors, the (no-longer-available) Astrophysics/Zeiss binoviewer will come to focus w/o a barlow. Without modification however, it will not come to focus w/o a barlow in most reflectors. There seems to be less focus travel required for this unit, perhaps because of the special color-correcting lens that comes with it.

2. On many refractors the Televue binoviewer will come to focus w/o a barlow if used "straight through", and not in a star diagonal. Inconvenient, but it works.

3. Reflectors have, and can be custom made or re-configured to come to focus with a binoviewer. In fact, on truss Dobs, you can get an extra set of truss poles made some 5.5" shorter, for instance, to accommodate binoviewer use. Of course, this can be inconvenient switching back and forth, and requires some design consideration as well, involving the light path. (ie.. making sure you diagonal is large enough) I didn't particularly like this solution.

4. Cadiatropic scopes such as Schmidt Cassegrain that come to focus by moving the primary mirror will likely have no problem coming to focus with a binoviewer... in most configurations. (provided you don't try to use it with a reducer-corrector which already throws off your focus position substantially)

5. You can use a lower power barlow to try to minimize the magnification. A 1.8X Televue or a 1.5X 2" Vernonscope Barlow (called 2X barlow, but that is when it is used ahead of a star diagonal, in reality it is 1.5X) will help. I have done some crafty "mixing" of barlows to reach 2.6x. Using a regular 2x barlow often results in magnifications close to 4x when employed with the binoviewers.

6. Televue has come out (9/98) with a 2X magnifier that is parfocal! (No in-travel) It only magnifies the image exactly 2x and allows the Binovue to be used in almost all scopes since any extra in-travel is minimal. Televue has now (1/99) made this special barlow "standard" with every binoviewer they sell. This means that you no longer are strapped to the 3.5X - 3.8X that came from using their regular 2x barlow with the unit, as it was previously supplied. The only disadvantage is that their new barlow (or "magnifier", whatever) creates some vignetting which is noticeable on lowest power eyepieces only, mostly when a fast scope is stopped down off-axis.

7. Zeiss/Baader  has a 1.7X magnifier that reduces focus travel significantly.  It does not completely solve the in-travel problem however... leaving nearly 3" more in-travel needed. If you can figure out how to mate it to your particular binoviewer, (it's internal) and work with other considerations as outlined above - you may be in business. Markus Ludes is currently selling a 2.6x that he apparently claims works to make many binoviewers parfocal. I have not tried that option.

8. As mentioned above, the Takahashi Twin-View has a built-in 2.1X barlow that reduces focus travel and
allows it to reach focus in many (but not all) scopes.

9. Harry Siebert has put a lot of effort in getting his binoviewer AND the Televue binoviewer to work with reduced magnifications. For instance, he sells a corrector/barlow that only has need for 1/4" of in-travel, which means it can be used with almost all telescopes. It is long at 3 1/2" so it is not great in a diagonal, but works wonders in a fast dob. In fact, are you sitting down................... The magnification from the unit is only around 1.35X !!!!!!!!!!!!! It works exceptionally well, and I am now using it instead of my 2x televue magnifier/barlow/corrector with the Televue binoviewer that I own. I frankly couldn't believe my eyes when I used it. It is that good.

Even with all of the above, finding the exact focus position that works with your scope mated to your binoviewer with, or without a barlow can be a bit tricky. I personally find the latest Televue solution the most sure-fire.
 

The SCHMIDT-CASSEGRAIN ENIGMA:
The most versatile scopes to use a binoviewer with are the standard Schmidt-Cassegrain scopes such as those widely available from Celestron or Meade. The reason is that the primary mirror is shifted forward and back to reach focus, allowing a very wide latitude of focus adjustment, which is exactly what these lengthy units require.

However, whenever I looked through my LX200, S-C scope with a binoviewer, I could swear that I was much more magnified than when viewing with a regular star diagonal and the same eyepiece. I couldn't, at first, figure out why. It turned out that indeed the Astrophysics unit I was using included a special correcting lens which boosted magnification by 20%. However, I still noticed a 15-20% increase in magnification on my Televue binoviewer when using the LX200, but NOT while using a refractor. The reason, of course, turned out to be the change in focus position. It turns out that whenever you re-focus most schmidt - cassegrain scopes, you are moving the main mirror, and because of the shape of the secondary mirror, you are changing your focal length a little bit. However, the primary has to be moved in so much to compensate for the length of the binoviewer, that on a 10" f/6.3 scope, I was seeing 15-20% growth in apparent focal length. (This supposedly does throw a bit of spherical aberration into the mix as well, as S-C scopes are designed so that only ONE focus position holds the minimum spherical aberration for the scope, but this is unraveling a whole different topic, and I could not detect a difference in image quality)

The bottom line, is that with a C8 as an example, running at f/10, you may very well find it harder to reach low enough power for many objects even without barlowing. Why? Well first off, you are limited to 1.25" eyepieces, so a 32mm 50 degree field eyepiece is about as wide as you can get. With your now increased focal length, the eyepiece is going to act more like 27mm or so. (comparing it to without the binoviewer.) The problem gets even worse at 10" f/10, and 12" f/10, etc, which already are limited in their wide fields without 2" eyepieces. Making the problem even worse, is that you cannot compensate this by throwing on a f/6.3 reducer-corrector, as that unit requires you to bring in your mirror substantially to reach focus, and therefore you cannot reach focus using a binoviewer PLUS a reducer corrector.

I guess, what I am leading up to, is that it is best to go with an f/6.3 scope if possible if you are planning to use the binoviewer for medium-low power, and wide field viewing. You may find yourself kind of boxed in at high power with an f/10 scope.

CHOOSING EYEPIECES FOR YOUR BINOVIEWER:
Selecting eyepieces for your binoviewer is a tricky proposition. The first problem you will immediately stumble into is the price! You have to match eyepieces EXACTLY, and double up on your supply. You can use your barlow to reduce the number of eyepieces, but the doubles can run into some serious money.

The second problem, I have run into several times, and you will not realize it until it is too late... eyepiece designs change with time, and you have to match the exact STYLE of that eyepiece! For instance... both the Meade Superwides and the Televue Naglers have undergone design changes in the last few years..in fact, the Naglers have undergone 3 or 4 design changes, the latest of which brings the top lens flush with the top of the eyepiece. You MUST match exact designs, so buying eyepieces second hand becomes very risky when putting pairs together.

The third problem is also obvious.. size and weight. If the eyepiece is over 50mm wide, you will start running into problems with them being too large to use, and if they are too heavy, they weigh your scope down, throwing off balance. The 22mm Panoptics are the largest possible eyepieces to use in practice in most cases with these binoviewers (but the weight may still deter you.) The 22mm Panoptics, by the way, offer an unbelievable wide field view, with good eye relief, akin to the most expensive binoculars on the market.

The fourth problem is selecting eyepieces carefully with consideration to eye relief. If the eye relief is too long, then holding the view becomes even harder when you attempt this with two eyes. 35mm-40mm eyepieces typically exhibit this problem. Eye relief that is too short, also becomes just a bit more bothersome than normal with a binoviewer. Mid-range eyepieces in the 10-25mm category usually work out just fine in terms of eye relief.

The fifth problem is the field size. Unfortunately, you can comfortably only take in about 60-65 degrees of apparent field before having to seriously shift your head around. This means that on the 7 and 9mm Naglers, for instance, you will sacrifice some field when using the binoviewer.

The sixth problem has to do with high power eyepieces. Turns out that the lower the focal length of the eyepiece, the harder it is for the brain to "merge" images. The critical alignment of each side of the binoviewer can be best maintained with eyepieces of 10mm focal length or higher. I was NOT able to merge images on a pair of 4mm Vixen Lanthanums as an example! Some people merge images more easily than others.

As an aid, in your selection, here is a sample of some of the eyepiece pairs that I have tried:

42mm Celestron Ultimas: Widest possible true field for 1.25" eyepiece, but tunnel-like-view due to apparent field of around 38 degrees. Somewhat hard to hold the view as you shift your head around due to the jumbo eye relief. Great performance, including planetary when barlowed.

35mm Celestron ultimas: Widest possible field, however, hard to hold the view. Great performance, including planetary when barlowed.

32mm Televue Plossl: Excellent, bit hard to hold view. Preferred low power eyepiece by many users. Great deep sky performance. Didn't try on planets.

30mm Celestron Ultimas: Perfect, even a tad better than the 32mm TV plossl in a side by side, same true field size too. Great performance on both deep sky and planetary. (some users prefer the 32mm televue plossl to this)

24.5mm Meade Superwides: Almost the widest possible field, very comfortable view. Soft towards the edges. Nice performance all around.

22mm Panoptics: Incredible wide field views. Very comfortable eye relief. The problem with this eyepiece is that for children it may be too large, (their eyes are closer) and the weight is a real problem for many scopes.

19mm Panoptics: All the advantages of the 22mm panoptic, with none of the disadvantages.. lightweight, 14mm eye relief, stunning perfect field of view. The absolute BEST eyepiece for the binoviewer, hands down. Planetary performance is good, but you can do even better with radians, or orthos (see below)

18mm Meade Superwides: Wonderful in all ways, bit soft towards the edge compared to the 19mm panoptics.
Great alternative to the 19 pans. Performance excellent on deep sky and even planetary.

17mm Orion Sirius Plossls: Excellent viewing, lightweight, quite soft towards the edges

16.5mm Docter widefields: One of the absolute best sets of binoviewer eyepieces. Small and lightweight, similar to the 19mm Panoptics. Stiff fold-down eyecup, allows you to get "lost in the view". Decent eye relief, about the same as the 19 Pans. (just barely good enough with thin glasses) Very high contrast, just a tad soft near the edge of field (barely) and seems to be a great planetary eyepiece too performing similarly to the Radian line. A bit lighter than the 14mm Radians.

16mm, and 12mm Naglers: Quite large and heavy, can't take in the whole field. Not that good, especially for folks with eyes that are closer.

15mm Panoptics: Excellent, small and light, just a bit shy on eye relief. Nice performance, sharp at the edge.

14mm Radians: Excellent, although a tad heavy, and a tad of "blackout" as you move your head around. This can be totally worked out if you get the sliding eyecup set perfectly.  Ultimate performance on all objects, and sharp at the edge.

13.8 Meade Superwides: Very good, just a bit tight on eye relief. Fogged easily. Superb performance on deep sky and planetary , although soft towards the edge.

12.5mm UO Optics Orthos: Very nice, small, light,excellent performance, enough eye relief w/o glasses, slight strain to merge the view for some reason in the pair I tried. (not perfectly matched?)

12.5 Celestron Ultimas: Nearly perfect, plus the "horn" style eyecups work well with binoviewing . Tight on eye relief. Not for use with glasses. Great deep sky and planetary. Small and light.

12mm Pentax Orthos (in 1.25" adapters): Great stuff, enough eye relief without glasses, and no strain. Only 45 degree afov.Needs a good deal of in-travel. Best planetary and deep sky performance.

10mm Zeiss Orthos, .965, 1.25" adapter: Good view, but too short on eye relief . Small , light, in-travel required

10mm Radians: Excellent, a bit heavy, and a bit of "blackout" areas if you don't position your head perfectly. This can be reduced or eliminated by carefully placing the eyecup in the right spot. Superb/ultimate eyepiece even for planetary.

9mm Vixen Lanthanums: Very good, although the eye relief is actually a bit "long" for binoviewing, and the view is just a tad hard to hold..but this is minimal. The non-widefield version that I tried had a fairly narrow afov.

9mm Naglers: On the heavy side, and it is very hard to take in the whole view, about 70 apparent degrees realistically. The eye relief is a bit short too. Still a good view.

8mm Televue Plossl: Nice, small, light, but short on eye relief

7mm Naglers: More comfortable than the 9mm Naglers by far, but still just a bit short on eye relief, making it even harder to take in the whole view. Nice wide field deep sky views. Planetary performance quite good, although there are better.

5mm Takahashi LE ED: Best for short focal length eyepieces, good eye relief for a 5, 52 deg. fov. Slight strain merging the view. Superb planetary performance, along with deep sky. Very small and light. Nice short eyecup.

NOTE: The 12, and 16 Naglers work, as do the Pentax XLs, but are awfully heavy and inconvenient.

As with single eyepieces, eye relief of over 15mm or so is necessary when using glasses.

BINOVIEWERS ON PLANETS & THE MOON
As already hinted, the traditional use of binoviewers has been on the Moon and planets The less expensive binoviewer adapted from microscope heads, works just fine on them. There is a two, or even threefold benefit from using a binoviewer on these brighter objects.

a. Even though the amount of light that you "perceive" is not reduced greatly, the splitting of the beam into TWO eyes results in a less glaring object, especially on the Moon.

b. Two eyes can perceive more detail than one. If you ever took an eye exam, you know this is a fact, you can read the fine print easier with two eyes.

c. The most important difference is the comfort factor. Eyestrain becomes a non-issue, and it allows you the time, and relaxation if you are seated comfortably (and no mosquitoes are biting you) to really pick up on detail that you would have otherwise missed. This is the definitely the most important difference to me.

BINOVIEWERS ON DEEP SKY:
Deep sky viewing can be achieved on all binoviewers, but best results, especially at lower power is with the more expensive units. Here are some of the wonders I have observed using both eyes:

a. Firstly, like planetary and lunar viewing, objects take on a false, but very truly awesome three dimensional look. Although this effect struck me strongest when first using a binoviewer, as the moons of Jupiter appeared to pop out at me, I also have found that this takes place while viewing nebula. Star clusters seem to have some dimension, and are much, much more interesting.

b. Galaxies, and nebula become something that you WANT to dwell on. Passing over them for another object becomes less appealing. Brighter nebula are particularly fascinating. I picked out intricacies on the Orion nebula that I would never had normally looked for...and really studied the subtle blue-green color differences within the nebula. The lack of eyestrain makes this possible. Best views though of galaxies are with larger aperture, as the split of  the light beam does take away a bit of the brightness in small and even mid-aperture. From 16" on up, the view is better with two eyes in every way than with just one, using the better binoviewers.

c. Globular clusters at first glance are a bit less resolved than using a single eyepiece.... however, they seem to throw a punch right out at you like you have never seen before when you do the right things.

1. Get the magnification up, for better resolution
2. Use a scope with more aperture (10" or larger)
3. Darkest skies, and higher dark adaptation help a lot.

Unfortunately, the view is so great breaking up globulars with two eye vision, that it leads to the worst possible case of aperture fever, as greater aperture allows you, in general to resolve globulars much more than smaller aperture.

OTHER CONSIDERATIONS:
Optically, in terms of quality, I found that on all units that I tried (the Celestron Binoviewer, the Televue Binoviewer, the Astrophysics Binoviewer and The Russian Binoviewer, views were equal to or better than single eyepieces on the Moon and planets. The more expensive units such as Televue's also showed no aberrations on stars and deep sky. Apparently the special correcting lens in the Astrophysics unit also was responsible for color correcting, and holding aberrations down. (Televue indicated that this was not necessary on theirs). I did notice however, that with certain eyepiece/barlow combinations, that annoying reflections, almost like ghost images, showed up nearby bright objects such as Jupiter. I noticed this on the Televue and Astrophysics binoviewers most, especially the Astrophysics.

At highest magnification, on a large scope, in a side to side test, one user did report to me that the Astrophysics unit held up the best on planets (we are talking about 600 to 800x here) However, in all fairness, this could have been a fluke, as I could not verify this.

Finally, the weight of the binoviewers, even with small eyepieces, will require that you think about appropriate ways to counterbalance you scope.

SUMMARY:
As the word about dual eye viewing spreads, more amateur observers are getting in the act. With the growing number of large binocular telescopes being built by amateurs each year, it is only a matter of time before they become more commercially available. Yet, one has to wonder if there is an easier way to view the sky with two eyes, than to use and/or pay for a dual-telescope system. For instance, a binoviewer utilized on a 12" scope will likely yield views equal to,or perhaps even better than two 8" scopes combined as a binocular. Put in that perspective, the price of a really good binoviewer may be well worth it.

If you want to feel like you are flying over the moon, more easily observe the festoons of Jupiter, or to be "wowed" by a seemingly 3-dimensional Wild Duck Cluster (M11), with some careful consideration and planning, you too can turn both eyes to the sky through your scope, instead of just a lonely one.

PARTIAL/LIMITED LIST OF CURRENTLY AVAILABLE UNITS:

Preferred:
Televue binoviewer
Baader/Astrophysics Binoviewer
Siebert Binoviewer
Denkmeier

Very Good:
BW Optik/Lumicon
Takahashi Binoviewer
LOMO

Others:
Orion (telescope and binocular center) binoviewer
University Optics

5/13/03 NOTE : OTHERS HAVE COME OUT - I AM LOOKING INTO INCLUDING THOSE AS WELL

Thanks for reading my explanation about binocular viewers. Please note however, that while I believe all the information included to be accurate, it is likely that I have unintentionally omitted, or mis-stated some point of fact. If so, I welcome feedback....

This article, and others can be found on my home page on the Internet:
http://www.weatherman.com

1998/1999 Addendum: I have recently tried the Zeiss/Astrophysics unit on a 16" dob, and was not able to reach focus in any circumstance with, or without a barlow. However, other users have reported having success.. It is a great unit for refractors, reaching focus on a 5" Takahashi (and A/P scopes) even in the diagonal position without a barlow. Be sure you have a "return" option on these units just in case.

Clear Skies,

TODD