A 12.5" Binocular Telescope

Designed and built by Jerry Oltion

Featured in the January, 2015 issue of Sky & Telescope Magazine


Jerry with binocular scopeWhen you start grinding mirrors, you wind up with more mirrors than you know what to do with. After I made the mirror for my 12.5" trackball, I picked up a second mirror of nearly the same focal length at a swap meet. I joked about making a big binocular scope, then put away the mirror and worked on other projects for most of a year. From what I'd heard, a binocular scope was way too difficult for someone of my skill level. Conventional wisdom held that they were fussy, temperamental things that would make your hair fall out and increase the national debt.

But Frank Szczepanski, a telescope maker in my home town of Eugene, Oregon, had built an 8-inch binocular that knocked my socks off every time I looked through it, and he said it was relatively easy to build and to use. Just for grins, he made another one out of a pair of 4" mirrors he got from Surplus Shed. That one went together in about a week, and it worked beautifully, too.

Admittedly Frank is a guy who can put together a scope in an afternoon (I am not exaggerating), but his success encouraged me to give it a try. My circle of telescope-making friends, the Scopewerks, egged me on, thinking, I expect, that I would either fail in a spectacular but educational way or that I would succeed in an unconventional but mundane, seat-of-the-pants sort of way. That does seem to be my trademark, so I went into this project with that in mind.

First off, I needed to make both mirrors the same focal length, so I shortened the second one's focal length until it was within 3/4" of the first. (I could have gotten it closer, but I naively thought it would shorten by that much in polishing and parabolizing. Not so! As any mirror maker more experienced than me will tell you.) They wound up 57.9" for the first one (f/4.63) and 58.6" for the second one (f/4.69). That's a 1.2% difference, well within the 3% difference that common wisdom says your eyes can compensate for, so I was okay.

That done, I started thinking about the design of the scope. All the horror stories I'd heard about binocular scopes centered around keeping the images merged. It was apparently really tough to keep the scope rigid enough that both optical trains would point at the same spot in the sky from zenith to horizon. The designs I'd seen tended to be of two types: two telescopes strapped together side-by-side or two optical trains in a framework that looked a little like a truncated Eiffel Tower. I thought about that, and about my friends' (and my own) expectation that I would come up with something completely new, and decided to do something completely conventional. I decided to think inside the box for a change. A big, honkin' box. Two of them, in fact, one on top and one on the bottom. I'd connect them with eight trusses, two to a side, making in effect eight triangles to provide stiffness between the two boxes. If that bugger flexed more than a few angstroms, I would eat it.

Binocular mirror adjustmentsThat still left me with a lot of decisions to make. In order to align both optical paths, I would need to be able to adjust the aim of at least one, if not both optical trains. Again, the 'net was full of cautionary tales and people proclaiming that you must do thus and so or you will be doomed, doomed to fail. The most reasonable advice that I could find seemed to be that I should make the primary mirror mounts adjustable horizontally as well as tiltable. A horizontal motion combined with a little tilt to both the primary and the secondary would move the optical train's aim while keeping the mirrors collimated.

In order to do that I had to make floating mirror cells. I came up with all sorts of Rube-Goldberg ideas, but finally settled on simplicity itself: make the collimation screws run all the way through the mirror platform to rest on the bottom of the mirror box, and use springs to pull the mirror platforms down so they wouldn't fall out when the scope was tilted toward the horizon. The lateral motion then became as easy as putting in four screws, two for each mirror, that could push the mirrors sideways. They double as the mirrors' side supports, which should be 90 degrees apart from one another anyway. I can move each mirror an inch or so in any direction; way more than necessary.

The mirrors are full-thickness pyrex, so I only need three points of support. That means the "cells" (if you want to be generous and call them that) are just plywood triangles with cork pads for the mirrors to rest on. I've got clamps that reach up over the top to keep them from falling out.

I put metal plates underneath the collimation screws so they wouldn't dig into the bottom of the box. The metal was shiny, so I covered it with construction paper disks. They don't look all that black in the photo, but they're plenty dark at night.

The collimation adjustment rods are just all-thread. They stick up a ways so I can reach them easily when I'm merging the images. (More on that below.) I made the right ones taller than the left because I only use one mirror to merge the images. I leave the left one alone so that optical train stays collimated and becomes the reference train for the other one. (Otherwise I could wind up moving both optical paths way out of collimation if I wound up chasing a phantom sweet spot on a bad merging day.)

That left me with one more major decision: How was I going to change the interpupillary distance? People's eyes aren't all the same distance apart, so you need to be able to move the eyepieces closer together or farther apart to account for that. With hand-held binoculars, you just twist them together or apart, but I figured that might be a little difficult to do with two 12.5" scopes. That left me with two choices: move the eyepieces sideways with focusers or rotate the secondary cages.

Using focusers had its appeal. For one, I could bolt down the secondary cages and keep my rigid framework all the way to the top. But the more I played with it, the more complicated and fussy it seemed. It would require four focusers, two to move the eyepieces sideways and two more aimed vertically to actually focus the images. Worse; every time you changed the interpupillary distance, you would have to refocus. And you would have to refocus a long ways. Plus all four focusers would have to be rigid as rocks. Any flex there would be just as bad as flex in a moveable secondary cage.

Rotating the secondary cages began to look like the better option. I had already come up with a design I liked for my 12.5" trackball, and one of its features was a nice round bottom ring. I had made that one out of lightweight foam to save weight, but I wasn't worried about weight for this scope so I decided to make its cages out of plywood. I used 1/2" Baltic Birch for the rings and 1/8" doorskin for the panels. I cut the rings with a router on an arm pinned to the center, which made them nearly perfect circles.

The secondary cages are held onto the top platform with only four bolts each. The bolts have nylon spacers between the wide washers on top and the platform, so the bolts don't pinch the bottom rings of the cages. The cages rest on Delrin pads so they'll slide smoothly. There's a thousandth of an inch or so of play to the cages, but that barely affects collimation or image registration. Your eye compensates way more than they move.

The wire spiders give me another neat advantage: I can move the secondary mirrors around in the cages just like I can move the primaries around in their box. I can move them up and down, sideways, and change their tilt just by tightening and loosening various wires. I also have tilt adjustments at the center hub so I don't have to mess up the other adjustments for simple collimation.

Notice the flattened faces on the top rings just above the focusers. That's forehead space. I wanted to keep the light path between secondary mirrors and eyepieces as short as possible (to keep the secondaries a reasonable size), so that meant getting the observer's head in as close as possible.

Focusers and diagonalsI ran into one snag early on: the tertiary mirrors — actually just diagonals like you'd use in a refractor — wouldn't come close enough together for most people's eyes. The corners banged into one another!  So I did the only logical thing: I ground off the corners. Now they come together just fine.

I made the tertiary mirrors adjustable, too, by putting foam tape between the mirror plate and the diagonal housing. I can tighten or loosen the mounting screws and get a little tilt that way; enough to help collimate the entire system. (I collimate secondaries and primaries first, then adjust the tertiaries to match.)


Interpupillary adjustorAt first I tried just rotating the secondary cages by hand, but it was way too easy to over- or undershoot, so I started thinking about how I might be able to make a screw drive for it. I mentioned my problem to the Scopewerks guys, and a couple of days later Frank Szczepanski said, "I've found your adjustor." It's a cargo strap tightener. The far end of it used to have a big coil of cargo strap on it, and you would tighten the strap by turning the handle and pulling on the strap anchors. I cut the strap holder off and left the tightening arms, and connected the arms to the secondary cages with a couple of aluminum bars and some vertical rods made out of a knitting needle. The linkages allow the upright rods to get closer together or farther apart as the cages rotate. It works beautifully! It's intuitive enough that I can tell people at star parties "This is where you adjust it for the width of your eyes," and that's all the instruction they need.

The focusers are odd-looking little things. I didn't really want to buy two commercial Crayfords. Besides the expense, they're heavy. I wasn't worried about weight at first, but as the design progressed I began to realize that I was creating a monster, so I decided to see if I could keep the focusers relatively light.

I had made a really light one for my 12.5" trackball, but that design was a little too flexible to hold up under the weight of a diagonal and an eyepiece. So I decided to think inside the box again. How about a square set on its side? The bearings could go in two of the faces, and I could use a stiff wire for an axle. Turns out that worked pretty well. So well, in fact, that Sky & Telescope magazine ran a separate article about the design, and I've got a separate web page for that here.

 Flex Rocker and ground ring
The flex rocker is pretty conventional. I used three roller bearings and one teflon pad on the bottom, and four teflon pads on the top. The latter turned out to be a little too stiff, so I replaced the front two pads with roller bearings. (This photo still shows the teflon pads.)

The altitude bearings are half-inch Baltic birch with aluminum strips for runners. They're removable so the mirror box can sit in the back seat of a car. I made them a little smaller in diameter than they should have been for perfect balance, figuring that would reduce the amount of "fin" sticking out in front. The operator has to stand inbetween them to view but go around to the back of the scope to use the finder, and I didn't want to be tripping over the altitude bearings all the time in the dark. I compensate for the lack of balance with springs on either side, and that works really well.

The ground ring is from the 20" scope that Kathy and I bought from Mel Bartels. I was glad to see that we could re-use that, saving me the trouble of building (and storing!) a second one.

I stained the wood "Early American" and varnished it with Varathane. I think the finish is one of the things that surprised me most about this scope. It turned out pretty! Who knew? I sure didn't expect that.

In a fit of energy not long after, I built an observing chair and stool to match. They turned out pretty, too. (See top photo. Yeah, that's glow-in-the-dark tape around the edge of the stool.)

How it works

That's pretty much the design. So how does it all work?

In a word: "WOW!"

It takes me about 15 - 20 minutes to set up. The collimation stays pretty tight even after tear-down and re-assembly, so I normally just have to tweak that a little at the beginning of the night, stick in a couple of eyepieces, and merge the images by tilting the right-hand primary mirror. That throws off the collimation of that side a bit, but hardly enough to matter. If it ever gets too bad, I shift the primary sideways a smidgen and recollimate. It took me a while to figure out which direction to move things in order to improve things rather than make them worse, but once I did I realized it's surprisingly intuitive. The big bugaboo about binocular scopes — merging the images while keeping the scopes collimated — turns out to be no big deal.

I don't even have the adjustment rods extended up to where I can reach them while looking in the eyepiece. I know from experience how much motion and in which direction a turn of each screw will give me, so I look in the eyepiece, figure out where the right-hand image needs to go, and reach down and move it that much. I usually have to fine-tune it with another bend-down. The horror! It takes at least five seconds' work! Sometimes ten.  (I will undoubtedly raise the rods someday just to eliminate the bending down, but it's really not a big deal.)

Once the two images are merged, the sky takes on a depth and richness like nothing you've seen before in even the largest single-mirror scope. It's simply stunning. Star clusters look three-dimensional, rather than like salt spilled on velvet. Nebulae look like they're floating right out there in front of you. And the Moon! OMG, the Moon! If that was the only thing I ever looked at with this scope, it would be worth it. It looks like a big bumpy rock floating about five feet away. The horizon curves away from you, and the shadows along the terminator have depth. I used to feel let down when the Moon would get into first quarter, but now I'm eager to get out and observe on moonlit nights just so I can look at it in stereo.

The distance even to the Moon is way too great for real parallax, of course, but the image processing parts of your brain don't know that. You've got two separate images going into two separate eyes, so the wetware does its magic and you see everything in stereo.

The rigid design does its job: the images stay merged from zenith to horizon. They stay merged when I swap out eyepieces, too, except for my highest-power ones (4.7mm Explore Scientifics, which give me about 315x). For some reason, those eyepieces don't like the same settings as all my other ones, so I have to tweak the merging when I use them. Like I said above, it's the work of a few seconds to do that, so it's no big deal, but I keep trying to figure out why they're different. I think it may be their tapered barrels interacting with the set screws differently than the straight barrels of the other eyepieces.

I occasionally have to tweak the merging even when I don't use the high-power eyepieces. Stuff gets bumped, or temperature changes shift the tolerances, or cosmic rays addle my visual cortex. I've learned that different people's eyes have different abilities to merge. Some people are slightly cross-eyed, while others are slightly wall-eyed. Most of us can meet somewhere in the middle, but it doesn't take much deviation before you start dropping the outliers off the bell curve. I've learned to ask them "Which way does the right-eye image need to go?" and I run the merging knobs while they call out "left, right, up, down" and we quickly dial it in for them. (After which it's usually wrong for everyone else.) Fortunately this only happens for maybe 1 out of 20 people.

The light-gathering ability seems to be about the equivalent of an 16 or 18-inch mono scope. I've been able to see 16th-magnitude galaxies through it, which I can't do with just a single 12-inch mirror. Planetary nebulae like the Ring and the Dumbbell are amazingly rich in detail through this scope.

All in all, it's an easy scope to use and the view through it is way, way more exciting than through a mono scope. It's rapidly becoming my favorite.
Mirror covers
People  kept telling me I should name the scope "Gemini" and call one mirror "Castor" and the other one "Pollux." I liked the idea, but my favorite star names are over in Libra instead. I put the names on the mirror covers.

How to contact me

email graphicI'd love to hear from people who are interested in this scope design. Please feel free to email me at the address on the right. (Sorry you can't click on it or copy and paste it; it's a graphic file to thwart spambots that search the internet for addresses to send junk mail to.) I have no idea how much mail this idea will generate, so I can't guarantee a response, but I'll do my best to answer everyone who writes with a genuine question or comment about the design.