JMI 14.5-inch f/4 Reverse Binoculars

What happens when you view through twin telescopes - one for each eye!

by Tony Hallas

     Some 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" experience.
     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 perfect.
     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 14.5-inch.
     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.