For those new to this term, a binoviewer essentially allows you to use both eyes to view an object, a notable departure from the traditional monocular view provided by most telescopes.
This guide aims to delve deep into the world of telescope binoviewers, shedding light on their features, benefits, and how they compare in today’s market. Whether you’re considering an upgrade or just curious about this intriguing addition to the astronomical toolkit, we’ve got you covered on everything you need to know about the best telescope binoviewers.
The Basic: How Do Binoviewers Work?
Like any standard telescope setup, everything begins with the telescope’s primary mirror or lens collecting light from a distant object. This light is then directed towards the eyepiece. Instead of channeling this light into a single eyepiece, a binoviewer contains a beam splitter, or prism system. This system essentially divides the incoming light into two separate, equal paths. These two separate beams of light are then channeled through the binoviewer’s optics, ensuring that each beam maintains a parallel path and is properly aligned. This alignment is crucial to producing a coherent, merged image when observed.
The parallel beams of light are directed towards two eyepieces. This means you’ll need two identical eyepieces for your binoviewer. These eyepieces allow each eye to observe the image, offering a binocular viewing experience. By allowing both eyes to participate in the viewing process, the brain merges the two slightly different perspectives into one. This results in an image that many users find to have improved contrast, depth, and a more ‘three-dimensional’ feel compared to the flat, monocular view of traditional eyepieces.
While binoviewers introduce a unique and often more comfortable viewing experience (reducing eye strain for many), they do come with considerations. For instance, by splitting the light into two paths, there’s an inherent reduction in the light reaching each eyepiece. This might make faint objects more challenging to observe. Additionally, the added weight and balance considerations of a binoviewer setup should be factored in, as well as the additional required focus travel of most binoviewers, which requires one to often use a Barlow or relay lens of some sort to reach focus when used with many telescopes, particularly Newtonian reflectors.
Advantages and Disadvantages of Binoviewers
One of the primary benefits that attracts many to binoviewers is the enhanced contrast, especially when observing the Moon and planets. Using both eyes allows for better differentiation of low-contrast features and allows your eyes/brain to process out floaters, glare, and even some of the effects of bad atmospheric conditions. The result is a richer, more detailed view that many astronomers find unparalleled compared to traditional single-eyepiece viewing.
Furthermore, observing with both eyes can be more comfortable and natural, significantly reducing eye strain during prolonged viewing sessions. The ability to keep both eyes open can make the experience more immersive and less tiring.
However, binoviewers come with quite a few caveats. For starters, not all eyepiece models are compatible with binoviewers. Your inter-pupillary distance (IPD)—the distance between the centers of the pupils of the two eyes—plays a role in determining which eyepieces can be used. Eyepieces’ barrels have to clear both each other and your nose. This rules out the majority of 2” oculars and some wide-bodied 1.25” eyepieces. The inadequate size of the prisms in cheaper binoviewers will also lead to vignetting with some low-power eyepieces.
Many binoviewers’ physical length also means that you may not have enough in-focus travel with your telescope, which would lead to binoviewer not reaching focus. Catadioptrics with moving-mirror focus systems can usually accommodate a binoviewer, but many refractor and reflector telescope users will need to either modify the telescope to get greater back-focus (unless you can simply remove an extension tube, etc.) or use a Barlow lens, which limits the maximum field of view achievable with a binoviewer down to an even smaller part of the sky.
As mentioned earlier, by splitting the light into two separate pathways, there’s an inherent reduction in the amount of light reaching each eyepiece. This reduction can make it challenging to observe fainter deep-sky objects, as the image might appear dimmer compared to single-eyepiece viewing.
The construction and internal makeup of a binoviewer play a crucial role in determining its performance, compatibility with eyepieces, and overall viewing experience. In the competitive market of binoviewers, there’s a spectrum ranging from top-tier models with expansive prisms, like the premium Denkmeier and Tele-Vue units, to more economically priced options such as the William Optics and Celestron binoviewers.
Important Terms to Know
The field stop is the physical diameter of the field of view your eyepiece takes in, usually limited by a metal ring that holds in one of the bottom lenses. You can usually just find the field stop diameter from the manufacturer of your eyepiece, though whatever the stated field stop is may be inaccurate. With simpler eyepieces like a Plossl, you can just take a caliper if you have one handy and measure the diameter of the bottom lens. The trouble is that many eyepieces use compound designs where the field stop is not necessarily right at the bottom of the eyepiece, where you can easily measure it.
Thankfully, the field stop can also be calculated from your eyepiece’s focal length and apparent field of view, provided the manufacturer’s proclaimed specs are completely accurate, which they rarely are to a precision beyond +/- within a few degrees or tenths of a mm. The formula is simple:
AFOV = 57.3 x (FS / FL)
Where AFOV is the apparent field of view in degrees, FS is the field stop diameter in mm, and FL is the focal length of the eyepiece in mm. This can be worked out like any algebraic equation. If you have the physical measurement of the field stop, you can figure out the actual apparent field of view of your eyepiece, which is how we’ve verified that the redline and goldline eyepieces, for instance, differ widely from their claimed 66-degree apparent field of view.
The clear aperture of a binoviewer refers to the size of the prisms. The size of the clear aperture corresponds to the largest field stop of an eyepiece you can use without vignetting. With an undersized prism in a binoviewer, you will notice that the apparent field of view of your widest-angle oculars appears to be cut off around the edges, and it is due to the inadequate physical size of the prisms. A binoviewer with only 21mm of clear aperture, like the cheaper units from Celestron and William Optics, will vignette slightly with eyepieces that don’t even maximize the field of view of a 1.25” barrel, such as a 25mm Plossl eyepiece with a roughly 55-degree apparent field of view and a 24mm field stop.
A 27mm clear aperture prism, however, will fully illuminate any 1.25” eyepiece, even the widest-angle ones such as a 32mm Plossl or 24mm Panoptic. This principle applies to seldom-seen 2” binoviewers as well.
As mentioned, many telescopes can’t physically rack their focusers in far enough to reach focus with a binoviewer (mainly Newtonians). A Barlow lens, either one you already have or one supplied with the binoviewer, brings the focal plane of the telescope out enough to reach focus with a binoviewer—at the cost of forcing you to use higher magnifications. Screwing the Barlow lens cell directly into the nose of the binoviewer generally produces a milder effect. However, the best option is to either physically shorten your telescope, its focuser, or get a binoviewer that comes with a relay lens that will allow you to reach focus without physically modifying the telescope or dealing with the consequences of a Barlow.
Our Top Binoviewer Picks
Best Binoviewer Under $300 Budget: Celestron Stereo Binocular Viewer
Celestron’s binoviewer won’t win any awards, but it features the same multi-coated prisms and diopter adjustments as any good binoviewer and sharp images at the eyepiece. The 20.2 mm clear aperture of the Celestron binoviewer undoubtedly presents certain considerations regarding eyepiece compatibility. Oculars up to a 25mm or so field stop will work okay, but there is definitely some vignetting at field stops significantly wider than 21mm. Celestron doesn’t provide a Barlow or relay lens, but for non-catadioptric telescopes, you will almost certainly need one to reach focus.
Choice for $300-$400 Budget: William Optics 1.25″ Binoviewer Package
William Optics’ binoviewer is the same basic product as the Celestron unit but includes a 1.6x Barlow to allow one to reach focus with almost any telescope and a pair of rebadged 20mm redline/goldline eyepieces. The 23mm field stop of these eyepieces aligns well with the 20.2mm clear aperture of the prisms, showing essentially no vignetting. You would end up paying essentially the same price as this William Optics kit if you were to attempt to equip a Celestron binoviewer yourself, so we would highly recommend you just get this one unless you already have an ample eyepiece collection and a suitable Barlow lens to use.
Choice for $400-$600 Budget: Long Perng 1.25″ Linear Binoviewer
Long Perng is a popular OEM for many brands, and as such, you might see this binoviewer offered under other names, including Orion. Nonetheless, Long Perng has created a very unique product with this binoviewer, which utilizes an unusual design for its beam splitter, based around a mirror rather than prisms.
The Long Perng Linear Binoviewer introduces no additional optical path length, eliminating the necessity for major focus adjustments or the use of a relay or Barlow lens to focus with your eyepieces. This is accomplished with a set of seven relay lenses on each side of the binoviewer.
While this binoviewer is unparalleled in its convenience and offers sharp images, one should note that the internal clear aperture is around 17mm. As a result, using wide-field eyepieces with a field stop exceeding around 19mm will lead to obvious vignetting. This means that you can’t get the widest possible field of view with a 1.25” eyepiece. A 15mm redline/goldline or a 20mm Plossl is about the widest you can go before running into significant vignetting issues. As such, the lack of a need for a Barlow lens is somewhat obviated by the field of view loss.
First Choice for $600+ Budget: Baader MaxBright II Binoviewer Set
One of the standout features of the Baader MaxBright II is its capacity to utilize the maximum field stop of 1.25″ eyepieces thanks to its full-sized 27mm clear aperture prisms. Examples of such eyepieces are the 24mm with a 68° wide field or the common 32mm Plossl. With this capability in hand, you’re primed to savor the most expansive true field of view a 1.25″ eyepiece can present with a given telescope without any visible vignetting.
Depending on your specific setup, the MaxBright II will probably require a Baader Glasspath corrector (essentially a relay lens) or a Barlow to reach focus. The appropriate Glasspath corrector is determined by the backfocus of your telescope. Since Schmidt-Cassegrain telescopes and most Maksutovs usually offer more than enough back focus to accommodate the MaxBright II binoviewer, they won’t need a Glasspath.
Second Choice for $600+ Budget: Explore Scientific BinoViewer for Telescopes
Explore Scientific’s linear binoviewer is merely a Long Perng linear binoviewer unit with a new paint job and two 25mm 52-degree eyepieces included. These oculars’ 23mm field stop means that they do vignette slightly around the edges of the field of view, but they are a good match for most telescopes and would cost a bit extra if purchased individually with the Long Perng-branded unit.
Best Binoviewing Eyepieces: Our Recommendations
Obviously, there are a lot of eyepieces that can be used with binoviewers, but some are better than others in terms of comfort, clearance, and suitability for different prism sizes. Here are a few of the eyepiece lines that are among our favorites.
The Nagler Type 6 eyepieces are nearly unrivaled in their sharpness, apparent field of view, and compactness for 1.25” oculars; the 16mm are ideal for a fairly wide field at medium power with almost any binoviewer.
The Plossl-like design and long eye relief of the ES 52-degrees, combined with their long eye relief, make them fairly comfortable to use with a binoviewer, while they’re also fairly affordable and offer sharp images. The 20mm is ideal for low-power binoviewers with smaller field stops. Those with wider field stops can try the 25mm or 30mm units.
The entire series of redline/goldline oculars is affordable and great for binoviewing. The 20mm and 15mm max out the field of view with many cheaper binoviewers and perform fairly well in slower telescopes or with the use of a Barlow, while the 9mm is of course a fantastic high-power ocular.
While normally not a product we strongly recommend, mainly on account of its narrowing field of view at the low-power end, the Celestron 8-24mm Zoom is extremely convenient in a binoviewer, while the 17mm field stop at the low-power end ensures no vignetting even with the narrower field stop binoviewer options.
