BIG BINOCULARS

A few years ago a friend bought some 15x80 binoculars. Wow, were they beautiful! The views of the Milky Way in Sagittarius were breathtaking! But these binoculars were expensive. Since I could not afford a pair of nice giant binoculars like his I scrounged up some parts and began to work. My first experiment with large binoculars involved a pair of identical 60mm Pentax refractor tube assemblies attached to a 10x50 binocular assembly with the original objectives removed. I simply attached the refractor assemblies to the binocular body as you can see in the picture. The result is a 30x60 giant binocular.

This is the perfect instrument for viewing the moon at low power. Notice the metal coupling hardware near the end of the tubes. This is the means for adjusting the co-colimation of the instrument. It was a lot of work to get these binoculars to maintain co-colimation when they were adjusted for the interpupillary separation. This is, I think, the most significant difficulty with all large binoculars. This instrument requires that the user look straight through the instrument, which can make astronomy difficult  because the targets sometimes ride very high in the sky.

Here is one solution to that problem. Attach the binoculars to a stand which has a reasonably good first surface mirror placed at a 45 degree angle in front of the binoculars. The resulting image is mirror reversed, of course. But that makes little difference for most astronomical observing. And the increased comfort is well worth the modification. In many cases these stands have been set flat on a table. I designed mine to tilt and swivel to reach any target. I can even adjust the angle to be more or less than 45 degrees. I use this with ordinary 10x50 binoculars.

SIX INCH BINOCULAR TELESCOPE

JANUARY 2003

Though I have built several large binoculars and am very happy with many of them, like the 18 x 100's described below, I thought that these kinds of instruments could offer wonderful views in  6" or maybe even 8" apertures. My experiments in the late 90's with a 13 inch binocular telescope led me to conclude that the bigger the aperture the more difficult these things were to build and use. See my page on GIANT BINOCULARS for further discussion of this topic.

I am still obsessed with the question of the optimum size for home-built binocular telescopes. So my experiments continue. I have been looking for a binocular telescope that does not cost a fortune, holds alignment under most all circumstances and delivers delicious low-power views. Hopefully it should be easy enough to build so that many other ATM's can make one for themselves!

In 2002, JMI introduced a six-inch binocular telescope. I had been saving a pair of 6-inch f/6 closely matched mirrors for several years and the JMI product lit my fire! I decided to use my experience with the 13-inch binos and put all the painful lessons learned in those experiments to work.  Everyone knows the well-deserved reputation of JMI for impeccable quality in all of it's products. I wanted to see if I could "nearly" duplicate the undoubtedly superior performance of the JMI binocular in my garage. This is the result:

These beauties deliver a wonderful view of the heavens, especially the Moon. They are a pleasure to use. The set-up time is minimal because they are very robust in construction and hold their alignment nearly perfectly. The interpupillary adjustment is simple, intuitive and does not affect the alignment!

I made nearly everything you see except for the mirrors, secondaries, original mirror mounts, secondary holders, ball bearings and the focusers. So, if you have some ATM experience, you may wish to consider building something like this. It's a doable project if you have built a few scopes and you have modest metal-working skills.

Learn all the secrets of building this instrument by visiting Six Inch Binos - Construction .

I hope to try some of the JMI binoculars some time, though I cannot afford to buy a pair. It will be interesting to see if mine compare favorably with those beauties. My six-inch binocular telescope is a lot of fun. I operate it at about 44X magnification and the Moon takes your breath away. You seem to see much more with both eyes. It's not so much that you see more as that you RECOGNIZE more! Your brain realizes that the phenomenon is actually real. Much more validity is automatically bestowed on an image when it is seen with both eyes. When you finally get the adjustments perfect the image "sings sweetly". You wonder how you could have ever thought you enjoyed monocular telescopes!

LAZY DIAGONALS 

Many years ago I read an article in Sky and Telescope written by Tatsuro Matsumoto describing a nifty trick he devised which used two mirrors to provide an upright and left-to-right correct image using standard 1.25" eyepieces. This system has only two reflections per eye and a "correct" image (as with an Amici prism) in binoculars. This article appeared in "Sky and Telescope" magazine in November,1982. Working from his idea I devised and constructed a set of  four "lazy diagonals" which, when coupled together two at a time, deliver a 90-degree viewing angle and involve only two reflections, much like an Amici diagonal. I experimented with this system and suffered many crossed eyes before I abandoned the concept for  home-made large binocular telescopes. My construction techniques were just not up to the exacting optical requirements of long focal length instruments. However tatsuro Matsumoto is now producing a commercial version of these. You can learn more at Tatsuro Matsumoto's Page. A skilled amateur telescope maker has used the product in a couple of scopes. See Joe Castoro's page.  Visit  Giant Binos as well.

This is my attempt to integrate the system with a pair of 4-inch f/10 refractors:

The derivation of the proper angles for the "lazy diagonals" required a bit of trigonometry. Colimation was a nightmare. And the interpupillary distance is not easily adjustable, though I discovered that I could rotate the lazy diagonals a bit (introducing a minuscule amount of field rotation ) to tweak the interpupillary distance a mite.  This also turns the eyepieces a bit, so that their axial alignments are not perfect. The longer-than-usual focal ratio (f/10) may make the system a bit more forgiving of such abuse of the optical train. It may also be that the Plossl design is more lenient.  

The views through this binocular were breathtaking, when it worked. The image was correct (not a mirror image), right-side-up and quite bright. The idea was to make something portable with a middle to higher range of magnifications and aperture for binoculars that would be very robust and hold it's colimation well. The last point was the downfall of these binoculars. It simply required a major readjustment every time I touched the instrument. It was an agony of twiddling set screws and getting crossed eyes every time! It seems to be IMPOSSIBLE to overbuild such instruments!

The weird "lazy diagonals" gave the instrument an interesting appearance:

LAZY DIAGONAL CONSTRUCTION DETAILS

The diagonals are called "lazy" because they don't make it all the way to 90 degrees and they seem to be just a bit "lazy".

You can get a good idea about how they work by study of an "amici" diagonal which involves a roof prism. In essence, this is two reflecting surfaces at a perfect 90 degree angle with respect to each other. If you look into this assembly from an angle such that you are seeing an image reflected by both surfaces, the image will be a correct (non-mirror) image that is not inverted. (With prisms, you are limited in these angles by certain characteristics of the glass.).

I convinced myself of these facts and experimented with angles by building a small cardboard 90-degree corner (see photos below), placing mirrors on the opposite surfaces and looking "into" it from many angles. I was able to look through any angle from much less than 90 degrees to nearly 180 degrees. The fact that my assembly was made of mirrors rather than solid glass, as in a prism, meant that I could really look at about any angle; I was not limited to 90-degrees. A study of the geometry of the situation and the construction of a working model will convince you that the only important thing about these "lazy diagonals" is that they all be the same in your system. You can select any (reasonable) angle for any desired resultant viewing angle.

I wanted a  nearly 90-degree diagonal  but  since I was not limited in my choices I finally settled on an angle of about 80 degrees as an optimum viewing angle. The geometry is outlined in the diagram. The required reflection angle for each individual mirror is about 125 degrees and the "chiral" (rotational) angle is about 77 degrees. I built this as two adjustable "lazy" diagonals, each one at an angle of about 125 degrees with an adjustable "twist" between them. I got the approximately 80 degree bend with only two reflections. The image was correct and right-side-up. Each mirror was mounted on small adjustable mirror mounts to permit fine adjustment.

The process of making the adjustments to each small mirror was time consuming. I assembled all the components, attached them to the tube assemblies without the objectives in place and tweaked each one iteratively until I had everything collimated (I was looking straight down the CENTER of each tube) and each view was a perfectly orthogonal. I used a window screen to make sure all the horizontal and vertical lines were the same in each eyepiece. In operation, I do not make any regular adjustments to these small mirrors. This was a one-time process.

The entire optical assembly MUST be as robust  as possible to keep the optical components perfectly rigid. When each telescope is adjusted for parallism using the small set screws in the mounting rings, they must maintain that perfectly. This was the downfall of the instrument. Remember this point if you build any large binoculars: the longer the focal length the more sensitive the instrument will be. Very slight adjustments can be made by fiddling with the eyepieces, but this as at the cost of introducing possible optical defects to the image.

Here's an image of my jig for setting parallelism and centering with the lazy diagonals. This is a matter of an hour or two and much trial and error.

If you decide to build such a project, please let me know how it works out. Best of luck!!

COMPARISON TEST

On several occasions I did careful comparison testing of this instrument. I compared it with a 6-inch f/10 refractor using a binoviewer. The effective light gathering area of the 4-inch  was about 25 square inches and that of the 6-inch was about 28 square inches. The 4-inch had 10mm Televue plossl eyepieces yielding 100X and the 6-inch had 17mm Televue plossl eyepieces for 88X. I made the comparisons in my backyard under urban skies and viewed the Moon, Jupiter, Saturn and M42 (the Trapezium stars and modest amounts of nebulosity were visible).  By the way, this testing complements my research with 13-inch binoculars under a dark sky. See my Giant Binocular page for details.

The results were consistent with my experiments using larger binocular telescopes. I found that the views through the binoculars were superior, but not not by very much. The brightness, at least for these experiments, was about the same. The detail was also similar. And the esthetic experience was delicious in both cases  as long as everything was working properly.

As for the binoviewer option; my home-made giant binoculars were always in need of tweaking while binoviewers were very simple, effective and relatively hassle-free. One of the distinct advantages of a binocular telescope is that it can effectively operate at lower magnifications for the same aperture because you can get more light into your eyes without increasing the exit pupil. But, if a binocular telescope carries a hefty price tag then that slight advantage quickly evaporates. Rather than buying or building a giant binocular telescope, it might be smarter to invest in a binoviewer and a larger aperture instrument to go with it. The most substantial payback of binocular viewing, seeing things with both eyes, is available to you with much less hassle and  probably at a lower cost if you buy a good binoviewer.

Assuming you have decided that you MUST have a giant binocular telescope: the key point to be made for anyone considering whether to build or buy giant binoculars is the substantial difficulty of setup and constant need for adjustment of amateur made instruments (at least for amateurs with my level of technique). I have had the opportunity to check out  the 5" binoculars from Orion. I find that they meet the exacting requirements for mechanical and optical precision in giant binocular telescopes beautifully! They were a pleasure to use and had wonderful views. In my opinion they are well worth the price. I am sure that many other makes are good too. I'll bet the Fujinons are breathtaking!

Binoculars have a valuable role in astronomy...the traditional role. The lower power of binoculars is a distinct advantage in any effective aperture. A pair of fine quality giant binoculars is an exquisite luxury. They excel at quick, fun trips through the sky. If you buy Phil Harrington's book Touring The Universe Through Binoculars you will have a great time with a pair of binoculars whether they are 50 mm or six inches. Anyone considering the binocular telescope option must realize that you should stick with large aperture monoscopes to really haul in the photons and deliver the resolution. You can  taste the best of both worlds with a good binoviewer, if you can afford one. And if you simply must have the biggest binoculars possible, be prepared to pay dearly for them.

FOUR-INCH BINOCULARS

Another effective solution is to use existing binoculars designed with a 45 degree tilt and increase their aperture. I acquired a classic pair of  Japanese 7.5 x 60 WW2 binoculars. The exit pupil on these was designed for young eyes and is inappropriate for me. (As you get older your dark adapted eyes will only accommodate an exit pupil of about 4 or 5mm.) Also I wanted greater light gathering power for astronomy. So I replaced the original objectives with new 4-inch f/6 .3 objectives. These triplet objectives were selected because of their low chromatic aberration and short focal length. One of the important lessons I have learned in my experimentation is that the longer the focal length the more rigorous the requirements for mechanical robustness. (See my discussion about LAZY DIAGONALS  above for more details on this topic.)  In fact the requirement for mechanical stability and strength increases exponentially with focal length. I wanted this instrument to be reasonably portable with good images so it had to be short. I was able to obtain the brand new f/6.3 triplet objectives for a reasonable cost as components in imported telescopes, probably made in China.

Construction of this18 x 100 binocular telescope required extended tubes, as you can see. Out of respect for the historical value of the original instrument, I made no substantial change to  it. It can be restored to it's original condition with virtually no visible signs that it was ever modified. I spent some time matching the paint and patina of the old section on the new tube assemblies. I suspect even somebody who knows binoculars would not be aware of the modification at first glance.

These binoculars are a pleasure to use. They maintain their colimation well and deliver very pleasing wide field views. The mount is simple and effective.  At 18X the instrument offers a nice  magnification and aperture for low power astronomy. At last I have something better than my friend's 15x80's. Unfortunately my friend has now obtained 5-inch binoculars from Orion!

These are the originals and a photo of some large WWII originals for comparison.