Courtesy of
Sky & Telescope
September 2005

S&T test report

So You Want Giant Binoculars...
Two eyes are better than one.  The RB-66 from JMI offers a two-telescope approach to two-eyed observing.

By Alan M. MacRobert

     WE VERTEBRATES EVOLVED with two eyes for several good reasons, as astronomers using one-eyed telescopes may not always appreciate.  True, our two-eye depth perception is useless at astronomical distances.  And unlike our animal ancestors, most skywatchers these days run little risk of losing an eye in a fight or injury.  But two eyes working together have real benefits for seeing dim, doubtful objects in the dark and for noticing subtle, slight contrasts.
     What's happening here is a signal-to-noise effect.  A dim galaxy in the dark, or a borderline low-contrast feature on a blurry planet, tends to disappear into your visual system's "noise": the faint, random patterns you see when you close your eyes in pitch darkness, or the slight irregularities you suspect in a blank surface.  According to the laws of statistics, when you combine two similar images you improve the signal-to-noise ratio by the square root of 2, or 1.41 times: 41 percent.  And indeed, experiments show that with two eyes people can detect stars nearly 40 percent (nearly 0.37 magnitude) fainter and can see contrasts about 40 percent weaker.
     That's why binocular views feel a little surer and more realistic.  You're perceiving more reality and less noise.
     Astronomers can get binocular views two ways.  You can view through two telescopes, one for each eye.  Or you can use a beam-splitting binocular eyepiece viewer on a single telescope.  In the latter case, to get a view matching the brightness of the two-telescope view, you'll need a telescope with the same total light-collecting area — that is, with 1.4 times the aperture.  I'll have more to say on this later.
     Amateurs for more than a half century have occasionally built big binocular telescopes, but the challenge is formidable.  You not only need two whole telescopes; they also need to be optically identical, they have to direct their light in parallel into eyes only about 6 or 7 centimeters apart (and this spacing has to be adjustable), and they have to stay aligned with each other extremely well.
     For the last three years JMI Telescoes (a.k.a. Jim's Mobile, Inc.) has ventured where no commercial vendor previously dared to tread, by offering 6-inch "Reverse Binoculars" consisting of two 6-inch f/5 Newtonian reflectors side by side.  The light beams are sent, by way of an extra reflection, to a pair of eyepieces as shown here.  You look comfortably down at the view while the telescopes point up past your ears.  Recently JMI has announced 10- and 16- inch Reverse Binoculars as well.
     How well does this design work?  We obtained a 6-inch Reverse Binocular on loan from JMI to see what we could see.

Mechanical Assembly
Our RB-66 came in the optional molded-plastic carrying case ($199.95).  The case is big, 36 by 28˝ by 13 inches (91 by 73 by 33 centimeters) including external hardware, and it weighs 72 pounds with the RB-66 in it, more than some people can carry.  It does have small wheels, but it was too wide to wheel through some of my doorways.  The wheels don't roll well on rough ground.
     Most of the RB-66 comes pre-assembled, but instructions for the remaining setup were confusing and ambiguous.  To sum up, the unit has the design feel of something from a small workshop; don't expect the engineering elegance you're use to from mass-market consumer products.  Using common sense, though, I eventually got everything together right . . . I think.  But after a tough evening I was ready to dismiss the RB-66 as an unwieldy white elephant.
     Then I took it outside.

Under the Stars
Omigod, what a view.  Lyra was overhead my first night, and using the supplied pair of 20-mm Plössl eyepieces for 38×, I landed on Vega and its surroundings.  I've surveyed this double-star-rich area with binoculars countless times, but not like this.  The little Ring Nebula, the globular cluster M53 — it was as if I were up there looking out a window cruising by.
     Sort of.  The Reverse Binoculars need a lot of adjusting, and all of it is done by motors with, inevitably, a little time delay and overshoot.  A toggle switch runs the spacing of the two eyepieces together and apart.  But changing the spacing also changes the focus.  The focus is controlled by four pushbuttons.  But tweaking them much is liable to put the eyepieces at different heights, which requires (as the instructions explain) that you run the eyepiece spacing all the way out to its far end to clear the discrepancy, and then start over.
     Then there's the alignment.  Getting the optical tubes precisely aligned so two high-power images fuse naturally seems to be the biggest problem with binocular telescopes.  The alignment of the RB-66 is done by working two rocker switches; the rear end of the left telescope tilts slightly up and down, and the rear of the right scope moves side to side.  But your eyes automatically try to correct misalignment by themselves, so the process is uncertain.  I found that it helped to use an old trick for checking ordinary binoculars' alignment.  Center on a bright star, them move your head away from the eyepieces until you're looking at the star in just a small field of view.  Relax your eyes, and your two images of the star will probably drift apart.  This makes it easier to use the controls to bring them properly together.  It also helps to do the vertical alignment before the horizontal, as the instructions say.
     But then you may find that the interocular spacing isn't quite right.  Change this, and you have to refocus.
     During many nights out with the RB-66, I found myself spending a lot of time tweaking the seven controls, trying to touch up a view that often seemed just a bit out of focus somehow or otherwise slightly off.
     But the views drew me back, especially when everything clicked.  The flyby of the near-Earth asteroid Toutatis gave a lesson in the benefit of using two eyes.  I had just one hazy night's shot at it, when it was 10th magnitude near my south horizon.  Bright moonlight lit the haze, casting a gray pall over the field of view.  With one eye closed I could glimpse Toutatis only occasionally and briefly.  But with two eyes, I often held it steadily for several seconds at a stretch.
     The altazimuth mount for the RB-66 is smooth and fairly steady.  The downward viewing arrangement is as comfortable as observing can be.  You can sit and watch the sky as easily as reading at a desk, especially if you have a variable-height observing chair, while moving the scope by its big handlebars.  To use the supplied 1× red-dot finder sight you do have to get up and walk around, but JMI offers its MAX digital setting circles to help you aim at objects in a database from a readout as you sit.
     This is not a planetary telescope.  All the problems of a binoscope are magnified at high power, and JMI recommends no more than 150×.  Even that might require a matched pair of 5-mm eyepieces; the star diagonals cannot take standard long barlow lenses.
     Star testing at 125× showed the mirrors to be very good.  They also arrived in good collimation.  I appreciated this, because collimating either telescope (as opposed to aligning the two with each other) requires removing the white outer cover and using a hex wrench.

Considerations to Weight
For whom would the Reverse Binocular be a reasonable buy?  In my opinion, you should be a true binocular nut, someone whose heart pounds on reading "38 × 150".  You should be mechanically inclined and ready to tinker.  You should plan to specialize in low-power, rich-field viewing; the RB-66 would be ideal for comet sweeping.  And you should have muscles and like to use them, especially if there are stairs between where you'll store the RB-66 and where you'll use it.
     And what about the alternative?
     An eyepiece binocular viewer on a normal 8- or 9-inch telescope will give the same bright, two-eyed images as the RB-66, though probably not allowing such low magnifications and wide true fields.  (See page 98 of last March's issue for a list of astronomical binoviewers.)  Even if you purchase an 8-inch Schmidt-Cassegrain telescope and binoviewer for this one purpose alone, you may still come out ahead price-wise — and you'll also have a general-purpose telescope with other capabilities, such as high-power observing, tracking, and potential for imaging.  You won't have to fiddle with the optics, since binocular viewers are aligned at the factory, and your focusing and eye-spacing adjustments will be faster, smoother, and surer, as in normal binoculars.
     But if you're a true binocular nut, the lure of a 6-inch telescope for each eye may be irresistible.

ALAN MACROBERT, author of Star-Hopping for Backyard Astronomers, uses his trusty 10 × 50 binoculars all the time.



Scenic low-power viewing
Ideal observing comfort


Weight and Bulk
Frequent adjustments with finicky controls


JMI's RB-66
Reverse Binoculars

With altazimuth tripod mount and two-20mm Plössl eye-
pieces.  Carrying case and digital setting circles extra.

US price: $2,795

JMI Telescopes
Jim's Mobile, Inc.
8550 W 14th Ave, Ste 200
Lakewood, CO  80215


JMI RB-66 Reverse Binoculars
Optics  ««««
Mechanics  ««
Overall  ««˝
Sensibly perfect.  No meaningful improvements possible.
Any shortcomings will go unnoticed in normal use.
Problems noticeable but do not seriously affect performance
Problems noticeable during normal use – performance compromised.
Problems so severe that the equipment is virtually unusable.
Ratings are intended to convey performance compared with equivalent equipment and should not be used to predict the relative performance of instruments having different apertures or optical designs.
Bottom-line summary:
A cleverly designed "binoscope" offering breathtaking wide-field, low-power views while the observer is seated comfortably.  It will appeal to avid binocular enthusiasts willing to put up with its mechanical quirks.