Binocular Viewers

Jack Kramer

For many years, binocular viewers have been expensive and slightly exotic telescope accessories, labeled as a "gimmick" by some. But consider that the binocular microscope is now a standard piece of equipment in most laboratories. Recently, more telescope "binoviewers" have been appearing in equipment ads. They're no longer exotic, though they still aren't cheap. Are they worth the cost?

LOMO mirror-type binocular viewer on 4" refractor

Most people initially get a binoviewer for the "three-dimensional" view that it presents. This is not a true three-dimensional image - using both eyes just tricks your mind into a certain amount of depth perception. It does give the appearance of greater depth to whatever you're observing, and planets in particular seem to be suspended out in space. The 3-D impression is especially pronounced on the moon. How it works is that as the cone of light comes out of your focuser, it is split into two images, each one going to a different eye. This is accomplished with either prisms or mirrors. The splitting of light makes each image somewhat fainter than it would be if you were using a single eyepiece. But your brain enhances the image by having two channels to work with, so the result is often better than a single eyepiece at the same magnification. It's been suggested that for telescopes of 10-inch or less aperture, a mirror binoviewer is preferable because the light throughput is greater. There is some total light loss, so it works best on bright objects. Typically faint deep sky objects need every photon your scope can grab.

One practical benefit is that by using both eyes, your visual sensitivity is increased and you are able to detect fainter details. According to Geoff Gaherty, an amateur astronomer in Toronto, "What finally convinced me to try a binoviewer was trying to observe a naked-eye sunspot. I noticed that I could not see it with either eye alone, but when I used both eyes it popped into view. Once I'd borrowed a binoviewer and tried it, I found it just about impossible to go back to one-eyed viewing at high magnifications." My own favorite target for the binoviewer is the moon; it definitely gives you a feeling of flying over it in a space capsule. In the 4-inch refractor, the best view I ever had of the craterlets on the floor of Plato was with a binocular viewer.

Another advantage is the highly recommended practice of observing with both eyes open, which relaxes your facial muscles and helps visual acuity. The use of an eye patch over the unused eye is a common practice, but with a binoviewer, you're keeping both eyes open naturally and comfortably.

Some observers also report that they have a problem with floaters in their eye getting in the way when using high magnification. But a binocular viewer minimizes this. Paul Gustafson said, "I have horrible floaters, large enough that I often have to flick my eyes to move them so I can read the newspaper, and they nearly disappear with the binoviewer. Long viewing sessions are much more comfortable as well. For me, it is an indispensable part of my observing equipment."

As with almost any telescope accessory, there are some drawbacks. One is cost. Most binoviewers list for $700 to over $1000. I did not even consider a binoviewer until I saw the Russian-made LOMO viewer from Russell Optics at a (relatively) bargain basement price of $325 for the mirror-based model. That's only a little more than you would pay for a good super wide-field eyepiece. Of course, you also need two identical eyepieces to use with it. (Another cost!) The thing that finally encouraged me to buy one last year was that Russell Optics was having a limited time special promotion - buy a binoviewer and get two free eyepieces. These are Koenig eyepieces that Gary Russell makes himself, and they turned out to be pretty decent, even when used individually for one-eyed viewing.

Another consideration is that because the light path is extended so much, in order to attain focus with the binocular viewer, a great deal of in-focus travel is required. In fact, most telescopes don't provide nearly enough in-focus travel, so a Barlow lens is required. When used with a binoviewer, it's referred to as a "transfer lens". (So here's another cost if you don't already have a suitable Barlow.) I purchased the standard 2.2x transfer Barlow with the binoviewer, but even that proved insufficient to attain focus on any of my scopes. So Gary Russell swapped it for a custom 2.7x Barlow, at no additional cost. How's that for customer support!

The transfer Barlow increases the magnification of your system. With my 4-inch refractor and the Russell 25mm oculars, I estimate the power to be about 200x. The Barlow is used to relay the focal plane far enough out to compensate for the long optical path of the binoviewer (5" or so). This means that the distance between the Barlow and eyepiece is much greater than it would be without a binoviewer. As a result, the true magnification factor is actually greater than the Barlow's stated rating. While some people use binoviewers on deep sky objects, most find that they perform better on the moon and planets, where higher magnifications are the rule.

There is also a learning period in order to get the hang of using this thing. A few people are never able to use them successfully because of an inability to "merge" the two images. The binoviewer has a small exit pupil, so the pupil of each of your eyes must be lined up exactly on the axis of each eyepiece. If not, your brain is befuddled by two separate images that don't quite overlap. The answer is to:

  • find the exact distance from the eyepieces to place your eyes when viewing,
  • focus each eyepiece properly,
  • make sure the inter-pupilary distance is correct for you.

After using it a few times, I found it increasingly easy to merge the two images. Apparently, these are situations common to all binocular viewers.

Before buying a binocular viewer, I had read glowing reports from various users. The comments were similar regardless of the make of the binoviewer. The only negative comment was from a person who was never able to merge the images and ended up selling his binoviewer. But he seems to be the exception rather than the rule.

The LOMO is built like a Russian tank; it's very sturdy and also very heavy. This means that your scope has to be rebalanced when using this viewer. I found I had to use it in the straight up-and-down position in my Celestron refractor, rather than being able to point it off to one side, since the weight caused the star diagonal to turn around, even with the set screw firmly tightened. On two occasions, the star diagonal turned around and the binoviewer fell out and onto the ground. (Luckily there's that Russian tank durability!) Now I often lower the tripod legs on my refractor so it's easier to peer straight down into the binoviewer. I've also made it a practice to use it with my Vixen prism-type star diagonal, which has a groove milled around the circumference of the drawtube. This ensures that the diagonal won't slip out of the focusing tube and provides a more secure fit. A 2-inch star diagonal holds it much more solidly.

One design variation relates to the effects of adjusting the inter-pupilary distance between the eyepieces. On some viewers, the eyepieces slide back and forth, which increases the light travel and necessitates refocusing. Others, including the LOMO, vary the inter-pupilary distance by rotating the eyepiece barrels in an arc around the central tube that goes into the focuser. This method doesn't affect the light path, so refocusing is not required when it's necessary to adjust the spacing between eyepieces.

This review has been based on the experiences of others and my own use of the LOMO binoviewer. Is the performance comparable to the far more expensive units such as those from Tele Vue and Lumicon? Judging by comments from others, the more expensive binoviewers are slightly lighter in weight and some claim that the images appear to be a bit brighter with less light scatter. But from a price/performance standpoint, the LOMO is a clear winner.


To my knowledge, Gary Russell is the only one who imports LOMO binoviewers. This is not intended as an advertisement, but since Russell Optics is not widely known, here is the contact info: P.O. Box 263, Meadview, AZ 86444-0263; (520)564-2886; web site:

Recently, Gary Russell has stopped carrying the mirror-type binoviewers largely due to increased costs from his supplier. Although many people really liked the mirror type, The LOMO prism type has received better reviews and they now have blackened edges and a built-in baffle that basically eliminates internal reflections completely.

Published in the June 2001 issue of the NightTimes