How I built a 20-inch binocular

How I built a 20-inch binocular

Since we were intended to use both eyes instead of just one, the idea of making a large-aperture binocular telescope always appealed to me. After successfully completing a 13.1-inch pair, I found the views impressive and began to wonder what even greater light-grasp would offer. Struck by the common malady of "aperture fever," I starting planning a binocular that would be as portable as possible while having substantially larger primary mirrors.

Because I was going to construct most of this scope out of wood, I signed up for a woodworking class at a local junior college. Meanwhile, I enlisted Pegasus Optics of 2301 W. Corrine Dr., Phoenix, AZ 85029, to make my matching 20-inch f/4.5 primaries.

In any binocular, proper eyepiece separation is important so that all the incoming starlight will enter the eyes. Different people's eyes range from about 56 to 72 millimeters apart, so a way to adjust the eyepiece spacing is required in order to share views with friends. While some telescope makers have designed binoculars for their own eyes alone, an adjustment is still advisable to allow for construction tolerances. Improper spacing can be very tiring and lead to eyestrain and headaches. (Like binocular, rangefinder can help you see everything which is far from you. Click here: for the best rangefinder reviews.)

With these thoughts in mind, I designed the upper-end tube assemblies to rotate individually. They are skeletal frameworks of plywood with sheets of 1/8-inch-thick bendable poplar glued and stapled around the outside, giving them excellent rigidity. The flange on which they rotate consists of two sheets of 3/4-inch plywood, glued together, in which I carefully routed two large circular openings. The upper-end tube assemblies are readily detached from the flange for ease in transport. The whole upper unit, once unbolted from the truss poles, fits down within the lower box.

I knew the lower box would be very difficult to lift into the back of my pick-up truck without taking out the 50-pound primaries. So I went the modular route, designing each mirror cell so it could easily detach from the scope. This way the main optics can ride in the cab with me, where they are protected from the dust of dirt roads and can easily be locked up for security purposes. Masonite slats protect the front and back of each primary from accidental damage.

Since the optical axes of the two 20-inch reflectors need to be perfectly parallel in order for star images to mesh, I included many ways to fine-tune the collimation. For example, under each tertiary mirror are three screws that I use for push-aligning it with the corresponding secondary. The primary and secondary mirrors have center dots for checking their alignment, and the primary mirror supports have the usual three adjustment screws for controlling tilt.

In addition, I devised a turn-screw system on the primary-mirror slings so that either mirror could be raised or lowered with respect to the other. When initially installed, the mirrors could also be pushed a fraction of an inch left or right. Once I had positioned the primaries properly, I put small drops of silicone sealant on the 18-point mirror-support pads to prevent further movement.

Using a Lazer-Mate collimator purchased from Astronomical Innovations (P.O. Box 14853, Lenexa, KS 66285), I am able to align all mirrors precisely. Now that the bulk of these adjustments have been made, only minor tweakings are needed each time the scope is set up to give perfectly meshed and beautifully crisp star images across the entire field.

If a binocular scope is not rigidly constructed, images can "unmerge" during the simple act of turning from one star to the next. For this reason, when constructing the bottom box I used double thicknesses of 3/4-inch plywood on areas I thought crucial. I also opted for a lazy-Susan azimuth bearing instead of the usual Teflon pads. Such a bearing lets the instrument swing around easily, so it is less likely to flex from the force applied.

I feel these precautions really helped, for I can observe at more than 300 power without the collimation being affected when I slew from one deep-sky object to another. The bottom box and the rest of the scope are covered with Formica, which has a much nicer surface finish and greater durability than bare wood.

For anyone who is considering a binocular scope like mine, here are some important points to remember:

1. Be certain the focal lengths of your primary mirrors match as perfectly as possible. This helps ensure identical magnifications and, even more importantly, keeps the eyepiece heights the same. A 1-inch difference in focal length will mean a 1-inch difference in eyepiece height after you have focused.

2. Get secondary and tertiary mirrors with enhanced-reflectivity coatings. Too much light -- more then 20 percent, in fact -- will be lost if you use standard aluminum coatings with 88 percent reflectivity on all mirrors.

3. Make the scope as rigid as possible at all critical joints.

4. Design your scope so all mirrors may be adjusted independently. This makes collimation easier and allows some leeway for minor construction errors.

5. Shop around for small parts like eyepieces and focusers. You'll be surprised at how much prices vary, and in a binocular all component costs are doubled! I bought much of what I needed from out-of-state vendors, thus saving hundreds of dollars on sales tax alone.

Although I don't recommend a project of this type or size to fainthearted ATMs, those who do undertake a large reflective binocular will be more than rewarded for their efforts. The views, to say the least, are awesome. The Moon actually appears three-dimensional, as do other objects because of the powerful psychological influence of two-eyed vision. Faint details and extensions in objects become much more obvious, for there is a significant gain in contrast using two eyes instead of only one.

Sweeping the summer Milky Way with two 16-mm Nagler eyepieces and my 20-inch binocular is an experience I won't soon forget.


A 20-inch portable binocular was designed and manufactured using plywood as frameworks and flexible poplar sheets as casings. The large primary mirrors were able to collect 5,000 times more light than the human eye. Moreover, the parts, which were readily aligned, could be easily disassembled and transported to different places.