Review of Orion BinoViewer (with Comments Applicable to Most Binoviewers)
by Tom Fowler
The Orion BinoViewer (item 52071) is a very nice, well-made binoviewer with extremely good optics. Optical images are bright and clear, with no vignetting. The angle between the eyepiece tubes can be adjusted smoothly, and each eyepiece holder has a damped adjustment for individual focusing, to accommodate any differences in the user’s eyes. The binoviewer comes in a metal case, and includes a 2x Barlow which is exceedingly useful if not essential for reasons to be discussed below. It takes standard 1¼” eyepieces, and will fit in any 1¼” focuser. Figures 1 and 2 show the binoviewer with and without the Barlow, and Figure 3 shows the binoviewer in its case, which I have slightly modified to hold a pair of eyepieces. (As of the time of this writing, Orion has discontinued this binoviewer, but most of my remarks will apply to any binoviewer since they all use the same basic design. A similar model is available from Celestron, and Orion binoviewers are available periodically on the secondary market). I observe from an urban location, and have about 50 years of experience in amateur astronomy. I have two scopes, an Explore Scientific 127mm APO refractor and an 11” Celestron SCT.
Figure 1. Binoviewer on top of case.
Figure 2. Binoviewer with 2x barlow lens attached
Figure 3. Binoviewer in case, with eyecups and two eyepieces stowed
Binoviewers use a prism to split the incoming light into two paths, each of which feeds into an eyepiece. Obviously two identical eyepieces are required to use the binoviewer. The purpose of any binoviewer is to allow the observer to employ both eyes. Celestial objects are too far away for a true 3-d view, but the 3-d like views in a binoviewer are wonderful. More importantly, I have found that using both eyes rather than just one has significant advantages, especially for resolving detail. Not only is it much easier to look at an object--say Jupiter or a crater on the moon--for a long period with both eyes in use, I can see more detail this way than with a single eyepiece. So for solar system objects, I preferentially use a binoviewer and find that it makes my observing much more pleasant and productive. The views of objects such as the moon though the binoviewer are really breathtaking—you will not be disappointed!
For deep sky objects, the fact that the light from the telescope objective is split means that each eye gets only half the light it would when using a single eyepiece and no binoviewer. This would seem to reduce the faintest objects seen by about ¾ of a magnitude. In practice, due to the way that the brain processes visual information, I have found that the reduction is much less. Though difficult to gauge exactly, I would put the effective magnitude reduction at about 1/3 magnitude. For some objects this would be unacceptable, but for others it is perhaps outweighed by the incredible, 3-d like view of the object through two eyes. As an observer, you will have to decide in each case.
This brings up the question of eyepieces. Obviously duplicating one’s entire eyepiece collection for binoviewer use would be cost prohibitive for most observers. However, I found that the advantages of using two eyes go a long way to ameliorating the need for super-high quality eyepieces. I purchased a pair of Orion Highlight Plossl eyepieces (in 40mm and 17mm sizes), which work extremely well, and are very reasonably priced. I already had a pair of 25mm TAL Plossl eyepieces. I did duplicate my Explore Scientific 11mm and 6.7mm eyepieces, which have wider fields of view and better eye relief than short focal length Plossls. These eyepieces are also reasonably priced so I did not have to spend a fortune to get a good range. One issue: the Explore Scientific eyepieces have long shafts, and the insertion depth of the binoviewer is limited, so for them I found it necessary to purchase parfocalizing rings from ScopeStuff (item #PFR1) to ensure a good fit. The other eyepieces work with no problem. I would not recommend zoom eyepieces unless they have carefully calibrated click stops. The binoviewer requires the eyes to work together, and they cannot do this well if the images are of different size.
One other practical matter. In-focus distance can be a problem with binoviewers. For this reason Orion (and most other binoviewer makers) include the 2x Barlow, because without it many refractors cannot bring the binoviewer to focus (this is true of nearly all binoviewers). I have an Explore Scientific ED127, which has 10cm of focus travel, but even with the Barlow, I am down to about 2mm or so. Without the Barlow I cannot bring it to focus in that scope. There is a useful accessory that can help with this problem, and I highly recommend it if you have a 2” focuser. It is the Negative Profile Eyepiece Adapter, 1.25" to 2", item #NPEA, from ScopeStuff. This is designed to give more in-focus when using 1.25” accessories in a 2” focuser. It is much handier and gives a couple more millimeters of focus travel, which can be important in case your eyepieces need a little more in-focusing. Note that this accessory requires you to use the Barlow, as it will not attach to the binoviewer without it. See Figure 4 for a photo of the binoviewer with this accessory attached.
Figure 4. Binoviewer with 2” Adapter Attached
On my Celestron Edge HD 1100, the binoviewer will come to focus without the Barlow, but it does require significant inward travel, near the in-focus limit of the scope. This is a disadvantage because the SCT design is optimized only for one position, about the middle of the focus range. So there is some sacrifice of visual sharpness when the binoviewer is used this way, at least in my experience, though not a great sacrifice. However, using the Barlow with the binoviewer allows me to set the focus very nearly at the center of the focuser travel. The result is that images are extremely sharp. In fact, I have found that my sharpest views of solar system objects come with the binoviewer and the barlow. Of course, using a 2x Barlow on a scope with 2800mm focal length means fairly high magnification, which is the reason for having 40mm eyepieces! They yield 140x on my scope, but I can assure you that the moon at 140x through a binoviewer really has to be seen to be appreciated. The 25mm eyepieces give 250x, which is quite useable on many nights and is great for detailed study of lunar features. Occasionally the air is steady enough to use 17mm or even 11mm eyepieces in the binoviewer with the Barlow, though at least in my area those nights are not common. For cases where the absolute best sharpness is not required, e.g., most deep sky objects, use of the binoviewer without the barlow may be desirable in order to reduce the power of the optical system and increase the field of view.
Shorter focal length eyepieces are more useful with my refractor (focal length 952 mm) and would be so for all shorter focal length scopes. On the Explore refractor, 40mm eyepieces in the binoviewer give a modest 48x with about 0.9 degree FOV, good for wide-field viewing. The 11mm Explore Scientific eyepieces give 173x with nearly 30’ FOV—quite spectacular on the moon and the sun, and very good for planets. Generally focusing of eyepieces individually is more critical with shorter focal length eyepieces. My procedure is to focus using just one eye, and when a sharp focus is achieved for that eyepiece, I use the individual focus adjustment on the other eyepiece to bring it to sharp focus.
There are some caveats with binoviewers. First, it takes some getting used to positioning your head and eyes properly. Unlike a single eyepiece, where you just have to center your eye over the eyepiece, with a binoviewer you have to center two eyes, which means that your head has to be oriented properly in order for both eyes to be centered over the eyepieces. After some practice this comes fairly naturally. At first it might be useful to purchase eyecups. Orion used to sell these, but they are probably available from other suppliers. The eyecups assist in positioning of the eyes (see Figure 5). Many eyepieces sold today include eyecups. Second, like binoculars, the binoviewer has to have the angle between the eyepiece tubes adjusted for each user. The action on this binoviewer is very smooth, but correct adjustment does take a bit of patience, at least at first. If multiple persons are viewing, this can be a bit of a hassle, and it makes the binoviewer less suitable for public star parties and similar events. The angle between the eyepiece tubes is not shown on a scale, as it is with many binoculars. This makes setting the binoviewer to a specific angle more difficult, and slows the process of switching between observers. Third, the binoviewer is fairly heavy (18 ounces, about the same weight as a large 2” eyepiece). This may be an issue for some telescopes, though I have never had any problems on account of it.
Figure 5. Binoviewer with 25mm Plossl Eyepieces and Eyepcups
In summary, I highly recommend this (or any other high-quality) binoviewer, especially for solar system observing and brighter deep-sky objects. It is well worth the cost and the expense of purchasing some additional eyepieces. The views are spectacular, and in my experience allow you to see more detail on many types of objects. Using two eyes is also much more pleasant than squinting to use one.