Astronomy Technology Today
JMI 14.5-inch f/4 Reverse
What happens when you view
through twin telescopes - one for each eye!
by Tony Hallas
years ago I gave up doing deep sky visual observing – the 25-inch and
17.5-inch Dobsonians proved to be too difficult to easily transport and I was
tired of looking through eyepieces with only one eye. Although on small
objects and the planets a large Dobsonian with a binocular ocular is impressive,
the field of view was too narrow to affect a true "Binocular"
Then, two year ago, Jim Burr brought his binocular
telescopes to RTMC - specifically his 16-inch and 10-inch versions. I was
walking by at night, convinced that nothing would ever entice me to become more
than a casual observer again, when I took a look at M13 in the 16-inch
binoculars. I was stunned! M13 looked like a 3D ball of stars, not
something flat ... and the view was "alive" ... there is no other way
to describe this. It's what happens when you view with both eyes through
dedicated telescopes – one for each eye.
My first JMI Reverse Binocular was a 10inch. I
modified it by adding larger secondary mirrors to prevent vignetting of the
primaries and enhanced coatings everywhere. This was a good
"starter" binocular – it provided some amazing views and I learned
how to quickly merge the images and collimate the optics. It was so light
that Daphne and I could lift it into the back of the truck. Our new
14.5-inch binoculars roll up a ramp, but, like the much smaller 10-inch
binoculars, these benefit from no setup or tear-down ... you arrive, they roll
out, you're ready!
All JMI Reverse Binocular telescopes have one thing in
common: everything is accomplished at the push of a button. The individual
telescope tubes are mounted via a bearing that allows each to move either
"up and down" or "left to right." Unlike some
binocular telescope where merging is accomplished by tweaking the collimation,
each set of telescope optics of the JMI Reverse Binoculars is first perfectly
collimated, then each tube is moved via electric motors to merge the
images. With this technique, the collimation of each tube always remains
The ability to merge the images via electric motors
cannot be overstated. With any telescope system, major swings in scope
orientation can produce minor flexures that have the potential of splitting the
images apart. With the JMI binocular telescopes, a few seconds of tweaking
alignment using the push button controls is all that's necessary to remerge
them. This becomes especially important when viewing at very high
magnifications. Focus for each eye is also electronically controlled, as
is interocular distance adjustment. System electronics also feature
optional digital encoders that were installed at our request together with a
computer for quick location of objects in the night sky.
Once we had the 10-inch binoculars, it wasn't long
before old habits resurfaced and aperture fever set in. So, I got in touch
with Jim and asked whether it would be possible to build a pair of 14.5-inch f/4
binoculars. These would have exactly twice the light gathering power of
the 10-inch binos, but would still be remarkably compact and portable. Jim
decided that the 14.5-inch size would make a nice alternative to the larger
16-inch binoculars that JMI already offered and, about a year later, they were
finished. We returned the 10-inch binoculars and took delivery of the
These new 14.5-inch binoculars are prototypes, although
much was borrowed from the 16-inch design. I ended up making a few minor
adjustments to balance them more perfectly and to tighten up the reduction gears
in the drive motors a bit. Once these adjustments were made, the new
binoculars were everything that I had hoped for.
The collimation is adjusted via a "push-pull"
set of bolts. Once it is perfect and everything tightened down, it does
not change, because the mirrors are glued to the supports in the mirror
cell. The mirror cells, and a small portion of the mirrors themselves,
hang out of the back of the telescope tubes – a great feature, because this
allows the optics to equilibrate to ambient temperature very quickly. Each
telescope tube of the binoculars is equipped with a back cover that is, in turn,
equipped with fans to force cooling if there is no breeze, but I prefer to leave
the mirror backs exposed to the night air whenever possible.
A word about eyepieces: when I first got the 10-inch
binoculars, the included "standard" eyepieces were designed to provide
a 50 degree apparent field of view. While adequate, these did not provide
the "space walk" feeling that you want to experience with
binoculars. So, I bought a set of Televue Panoptics and the improvement
was immediate and significant. Edge correction improved and the 68-degree
apparent field of view was terrific! In all, I equipped the 10-inch
binoculars with set of 24-mm, 19-mm, and the now discontinued 15-mm Panopics.
These served us well with that binocular.
The f/4 optics of the 14.5-inch binocular proved to be
a little fast for the Panopics so I contacted David Nagler and asked if we could
borrow a couple of sets of 1.25-inch Naglers to try out – specifically the
16-mm Type 5 and the 11-mm Type 6. When they arrived it was cloudy – I
had to wait a few days until we had a clear night – but, when we finally got
to enjoy the view through the Naglers, it was as if someone had turned on the
lights! The edge correction was almost perfect, despite the fast optics,
and, with the 82-degree apparent field of view, it was truly like floating in
space! More important, the 16-mm Type 5 Naglers provided the same actual
field of view as the 19-mm Panoptics – over 1/2 degree! Satisfied that
the 1.25-inch Naglers are ideal for the 14.5-inch, f/4 optics, we acquired a set
that is now dedicated to that scope.
All the optics were made and coated by Discovery
Telescopes. The matching mirrors are superb and the enhanced coatings
brilliant. Once again, I have chosen oversized secondaries to make sure
that 100 percent of the light provided by the primaries gets to the
eyepieces. In this case, the secondaries measure 4 inches in minor axis to
better accommodate the steep, f/4 light cone. The custom optics and
coatings cost a little bit more, but are worth every penney!
The ease of transport, the beautiful and inspiring
views and the compact size make these binoculars special. They are not for
someone who wants a turnkey, no-brainer telescope – these have to be studied
and understood. But, it only requires a few nights to master what does
what – after that, it's almost reflex.
I anticipate some lines at stare parties – you
simply cannot look at these and not want to look through them.
- OTA Type: Dual Newtonian reflectors.
- Mount: Alt-azimuth mount attached to a
- Mirrors: 14.5-inch f/4.0 primary
mirrors, 3.5-inch diagonal secondary mirrors.
- Diagonals: Erect image or standard at
- Clearance Between Light Paths: 8
- Eyepiece Spacing: Variable from 2
inches to 3.25 inches.
- Alignment: Motorized "x" and
"y" axis for optical tube alignment.
- Power: 6vDC operation with 4.5
amp-hour rechargeable battery with 110vAC/60Hz or 220vAC/50Hz charger.
- Binocular Weight: Approximately 155
- Pier Weight: Approximately 27 pounds.
- Binocular Height: 61 inches at Zenith
(measured from top of binocular to bottom of tripod with binocular in
- Binocular Length: 52 inches.
- Binocular Width: 45 inches.
- Binocular Depth: 22 inches.
- Two 2-inch RCF-1 Focusers with motors
for motorized focusing.
- Two 30-mm Wide-Angle Eyepieces.
- Star Pointer Finder Scope.
- 6vDC battery (includes AC charger).
- Built-In Handle Bars permit easy
movement of the scope.
- Ease of use - no bulky, complicated or
expensive mount. Just look down into the binocular to see the
sky behind you.
- Six motors for adjusting inter-ocular
(eye) spacing, focusing and optical tube alignment with the touch of a
- Modified Reverse Crayford focusers.
- Fold-away motorcycle-type handle bars
for pointing the instrument.
- Town bar and wheels for easy movement
of the instrument.
- Compatible with MAX computers.
- Battery operation.
- JMI's celebrated quality construction.