Sky & Telescope
S&T test report
So You Want Giant
Two eyes are better than
one. The RB-66 from JMI offers a two-telescope approach to two-eyed
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
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.
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.
author of Star-Hopping for Backyard Astronomers, uses his trusty 10 × 50
binoculars all the time.
WHAT WE LIKE:
|Scenic low-power viewing
|Ideal observing comfort
WHAT WE DON'T
|Weight and Bulk
|Frequent adjustments with
With altazimuth tripod mount and two-20mm
pieces. Carrying case and digital setting circles extra.
US price: $2,795
Jim's Mobile, Inc.
8550 West 14th Avenue
Lakewood, CO 80215
RB-66 Reverse Binoculars
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.
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.
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.