Restoring and using German 10x80 Flakfernrohr binoculars

Last year I came across some German second world war 10x80 binoculars in a used camera store. The war and a half century of civilian use had left them in poor shape, so the asking price was reasonable and I bought them. Initially, I was inclined to leave them as they were out of respect for their historical value and my lack of repair skills. More and more, however, I noticed the paint flakes inside the binoculars and the stains on the prisms that looked like dried drops and streaks of rusty water. Leaving them alone meant allowing them to deteriorate further, so I decided that they should be disassembled and thoroughly cleaned. This could be done professionally (at great cost) but the construction of the binoculars is so sturdy and straightforward that it can be done by anyone with the time and care to devote to the job.

Since these are now excellent binoculars for astronomy (see Markus Ludes' article on this website for a more experienced astronomer's assessment of them), I decided to write about my experience to help anyone else interested in such a project. It should be emphasized, however, that not all military binoculars are candidates for restoration by inexperienced owners like me. Many are much more complicated (waterproof construction, different collimation procedure), and some are rare with great historical and monetary value.

Collecting Nazi memorabilia is a shameful hobby, but second world war German optics will always be of interest to many people because of the exceptional quality of their design and construction. Well preserved examples should probably be left as they are. If you are lucky enough to have a pair of 8x60 U-boat commander glasses in your closet, please don't take them apart and respray them after reading this article. The 10x80s, however, were deployed by the thousand in the military and in anti-aircraft defense all over Europe. Many are still in perfect shape and there is no shortage of collectable examples. I believe astronomers should be encouraged to take one of the many that are in poor, repainted, or incomplete condition and turn them into a truly great binocular for looking at the night sky. So much talent in German industrial design and manfacturing was misused during the 1930s and 1940s; if some of its products can have a more peaceful second life now, all the better.

After a background discussion of the 10x80s, most of this article will concern disassembly / cleaning / reassembly. Readers interested only in impressions of how the binoculars work may wish to skip to the final section.

History of the Busch 10x80 binocular

I knew next to nothing about these binoculars when I bought them, and I am greatly indebted for the information in the following section to Dr. Hans Seeger, whose book Militärische Ferngläser und Fernrohre provides a detailed description of the development and use of the 10x80 as well as dozens of other binoculars. Dr. Seeger's book will be of use and interest to anyone interested in military optics (not only binoculars, not only German, and not only second world war). It is in German, but there are over 900 photographs among the 537 pages, and the detailed captions are all translated into English. A couple of the chapters are in English, including one by Kevin Kuhne on German and Japanese large aperture binoculars. Mr. Kuhne restored one of the 3 existing examples of the 20/40 x 200 Zeiss binoculars built near the end of the war and the pictures of this behemoth are spectacular. Don't take one of these apart, either, if you find one.

In 1936, Emil Busch AG of Rathenow won a contract to produce a 10x binocular for air observation for the German military. Leitz and Möller each produced competing prototypes, but the Busch model was chosen for its lighter weight (6.5 kg vs. 9.25kg for the Möller and 8.5kg for the Leitz) and greater field of view (131m at 1000m vs. 105m/113m for the Möller/Leitz models). Production began soon afterwards and continued through the war, when the Busch design was also produced by other firms in Germany and occupied Poland. The markings of the different factories are listed in the following section. All models have identical optical systems: 80mm cemented achromatic objectives with 280mm focal length, 70-deg eyepieces, 45-deg Schmidt roof prisms. Due to allied bombing, the 10x80s were primarily used for identifying attacking aircraft and directing the large 4m rangefinders, searchlights and cannons of the anti-aircraft batteries around German cities. They also proved useful on the battlefield, and a version with 20-deg inclined eyepieces was produced for use at sea. A single half of the 10x80 binocular served as Z.F. (Zielfernrohr) 10x80, a sight for the 8.8cm artillery.

Markings (found on right prism housing)

The D.F. engraved before 10x80 stands for Doppelfernrohr (double telescope). Only the early Busch models will have the manufacturer's name, others will have the three letter code beh (E. Leitz, Wetzlar), dkl (Schneider, Bad Kreuznach), cro (R. Fuess, Berlin Steglitz), bpd (C.P. Goerz, Vienna), eug (Optische Präzisions-Werke, Warsaw) or cxn (E. Busch, Rathenow). According to Seeger, people with knowledge of the various models find them to be of identical optical quality, although the Schneider models appear to have been especially robustly constructed, while the Leitz 10x80s were not as well built as the others. In addition to the serial number, you may encounter some other symbols. KF is short for Kältefest, (cold weatherproof) meaning that the cold weather grease Invarol has been used and the device should function to -20-deg C. My 10x80 also has a dark F and a red X, about whose meaning I can only speculate. A light blue circle indicates grease no. 1416 or similar, functional to -40-deg C. Both cold weather greases apparently proved inadequate in Russia, and from winter 1942 all optical instruments of the German military were lubricated with grease no. 1442, again supposedly functional to -40-deg C, and indicated with a light blue cross. A triangle engraving later in the war indicated a further improved lubrication. These markings could of course be added during repair or regreasing, so they cannot be used to date the binocular.

How to choose the right pair

At any given time there are usually several 10x80s up for auction on Ebay. It makes sense to wait for the right ones to come along. The condition of the optics is most important. While a few of these were coated, the vast majority were not. Given the number of optical surfaces (doublet objectives, 45-deg Schmidt prisms, 5 element Erfle eyepieces) and the fact that uncoated surfaces can pass at best only 90% of the light reaching them, coating would seem to be highly desirable. The coatings used were poor by today's standards, however, and are likely to be in deteriorated condition. Since the prisms are likely to need heavy cleaning/polishing, coated optics are of questionable worth even if you do find them. Make sure the prisms are not badly cracked - such damage is serious and non-repairable. When I bought my pair, both prisms were stained, one so heavily that looking through it was like looking through a glass of muddy water. They were intact, though, and after cleaning (described below) they work fine. Avoid binoculars with separation in the cemented objectives or eyepieces. This will look like a wavy or rainbowy patch, often near the edge but sometimes more extensive.

Eyepieces or (especially) objectives with only small areas of separation are useable, but this condition will likely worsen with time. It is fixable, but this is a job for professionals with specialized equipment. Large chips in eyepieces or lenses should also be avoided, but scratching (even quite heavy) is less of a problem than you might think. Most 10x80s will have at least some cleaning scratches; dewy or dusty lenses were more likely to be wiped clean with a dirty shirtsleeve during the war than with a tissue paper and cleaning solution. A few scratches, even deep ones, won't make a visible difference. If you don't believe this, take a camera with a telephoto lens and move a pencil in front of it. The obstruction needs to be surprisingly large before it has an effect on resolution and the same is true for a objective lens flaw in binoculars. A large amount of small scratches is a problem, since this will lower resolution by scattering light. One of my objective lenses (the one on the right below) has lots of heavy scratches which does make the view a little less sharp through one side, but not enough to be objectionable for me.

front view, notice the multiple strong reflections from the uncoated glass surfaces

There are also a few other things to consider in choosing your pair. The removable and often missing dew/glare shields are important - with uncoated glass you need all the help you can get. The rubber headrest (which needs a holder to attach to the dovetail on top of the binocular) looks nice, but I don't use mine. Although it is in very nice condition, with age the ends have bent in and they now press uncomfortably against the user's temples. Since the exit pupil is so large and the eyepieces are not prone to black out, I don't miss the headrest at night. It would probably be more necessary if a few anti-aircraft searchlights were panning around next to me, but thankfully the neighbors lights are quite that bad, yet. Another consideration is the robust but intricate and sometimes broken mechanism that controls the alternating orange/green/neutral density/clear filters. If this is broken, it is no tragedy for astronomers, since the filters are for cutting down glare and improving contrast in daylight.

If broken they can be simply removed (as described below). The right eyepiece had a removable light to illuminate the reticle, but it is almost always missing. Finally, it is rare to find a pair complete with its double yoke mount and tripod. Through a great stroke of luck, this is not a problem. Removing the dovetail mounting plate on the base of the binocular will reveal two holes which are metric drilled but by chance extremely close to _ inch / 20 standard tripod threads. This allows the 10x80s to be mounted directly on a fluid video head.

Step-by-step disassembly and cleaning

I took all of the pictures for this section during reassembly after stripping and refinishing the parts, so they don't show the full messiness of the project, but at least the process should be clear. First, remove the dewshields. Mine were well stuck on and needed to be pried off with a screwdriver. Next, remove the lenses. Because the interocular distance is adjusted by moving one side of the binocular (with the knob on the top of the housing) while the other remains fixed, the removal of the two lenses is different.

The lens cell holder at the bottom of the picture above (right-eye objective) simply comes off once the four screws attaching it to the main casting are removed. The other, conical shaped, one is more difficult. Unlike its sibling, it needs to have the actual lens cell taken out. Remove the outer retaining ring (but not the inner one) and pull out the lens cell. Before doing this, it is a good idea to mark the glass with a dot in the 12 o'clock position so that you can put it back later without disturbing collimation. The collimation of my pair is not at all sensitive to rotating the lens cells, but it never hurts to be safe and keep everything in its original position. The cell is held in place with a large amount of grease as well as the retaining ring. This may have frozen up, making it hard to pull out the lens cell. Warming the cell may help, but be careful - oven heat might cause the doublet objective to separate. Once you do have it out, wipe off the old grease so that you can replace it later with new (the thicker the better). Looking into the conical tube will reveal six or eight screws that need to be removed with a long handled screw driver. Be careful here not to slip with the screwdriver or a greasy screw and damage the exposed prism face. The tube should now pull out, to be set aside with its sibling. A look into the other side of the binocular will show four screws to be carefully removed. These attach the body to the right prism housing, which may now simply fall off - catch it if it does.

To remove the prism housings from the body, take off the brass end plate shown above. Next take off the L-shaped plate, held by three screws, pointed at by the screwdriver below.

Both prism housings should now slide freely out in the direction of the brass plate. If you have trouble sliding the second one, help it along by turning the interpupilary distance knob on top of the binocular. Again, remove the grease on all faces so that you can put new grease on before reassembly. Notice how incredibly light the empty body of the binocular now is. Although it is probably covered up with ugly paint, it is a very elegant and complicated one-piece aluminum casting. No binocular maker would go to the expense of making something like this today.

Now that the prism housings are off, it's a good time to remove the eyepieces. These should simply unscrew from the housings, but they may well be stuck. The best tool to use is one of those jar openers that consists of a simple tightenable rubber loop attached to a plastic handle. Set the eyepieces aside with the lens cells. Next, the prisms need to be removed from the prism housings so that they and the filters within can be cleaned and the housings refinished, if desired. The prisms are actually attached to the thin base plates on the housings, which can be taken off simply by removing a few screws on the thin base plate (facing the objective). Do not remove the three screws shown still in place in the picture below - these attach the prisms to the base plate and removing them now could cause the prisms to simply fall into the housing, potentially a very bad thing.

Once you have separated the two parts of the housing, it should be clear how everything is put together and removing the prisms themselves is easy. Be careful, though. I don't think there is any difference between the two prisms, but it is best not to switch them around. Sixty years of sitting against the metal housing will have left a faint circle where the prisms open into the body of the binocular, and this hole will not be in the same place if the prisms are switched or turned upside down. The picture below shows the two parts that need to be removed first: the top plate I am holding and the bottom plate pointed at with the screwdriver. Once these are removed the screws pressing into the sides of the prisms need to be loosened or removed before the prisms can be lifted out.

Below are the screws holding the prisms from the sides - during reassembly be careful not to tighten these any more than necessary, as you could shatter the prisms.

Cleaning the prisms and lenses

This will be necessary in every pair of 10x80s, unless a recent owner has done it for you. The problem in my pair, and I suspect many others, is the steel top plate. Although of very high quality (notice how little rust is apparent after 60 years), it and other fittings inside the housing are not made of aluminum like the main body of the binocular. Because the designers attached more importance to synchronization of the filter mechanisms than to watertightness, moisture inevitably will have entered the prism housings where the mechanism connects. Over time, oxidation from the fittings and dirt will have seeped down and stained the prism faces. Mine were particularly bad - no witch's brew of acetone, windex, ammonia, alcohol or mineral spirits had any effect whatsoever. Toothpaste did remove some of the bigger flake-like deposits, but both prisms were still more rusty-brown than clear.

The solution turned out to be a cerium oxide glass polishing compound used by car window repair shops. A jar containing more than enough powder for a dozen binoculars cost about $20 from Delta Kits, who have a website. The orange-brown powder mixes with water to form a non-toxic paste which Delta Kits recommended applying with a circular cloth drill attachment. I decided to see how rubbing the prisms with a rag soaked in the paste and wrapped around a finger would work before I resorted to power tools on the prisms. After about 5 minutes of hard rubbing, I saw a noticeable difference. After 10 minutes the first prism looked almost as good as new. After working on both prisms for awhile longer, there was still some faint staining that looked like tarnishing, but the difference was like night and day. The faces of the prisms are a bit wavy looking in the places where serious rust deposits etched them, but they both now perform much better than I had ever hoped they would. I sure the binoculars viewed better 60 years ago, but not a lot better.

If you have serious problems with your objective lenses, the same treatment might help. I decided to leave mine alone, because the views are great now and there is more to lose than gain by polishing the lenses. Polish too hard and you could change the figure on the lens. There is also the potential for the cerium oxide paste to leak down into the lens cell and damage the bond between the two glass elements. On the other hand, polishing could really help some badly scratched lenses.

Eyepieces and filters

Assuming you don't have any separation of elements in the eyepieces, there is not a lot to do here. I choose to remove the optical flat containing the reticle, reasoning that I didn't need to aim at anything and that it would be better to be rid of two uncoated glass surfaces. Removal is simple and makes the view through both eyepieces identical (although it will mean that one eyepiece will probably reach focus further in than the other). Just unscrew the retaining ring and the reticle flat will drop out.

removing the reticle. the slot visible on the housing is for light from the illuminator

The filters are a more complicated job, so complicated that I advise not messing with the mechanism if it isn't broken. My filters were stained, but since I don't use them, this was not so important. Still, you want the clear (knar) filters to be as clear as possible. One of mine was beyond hope, so I crossed my fingers and simply knocked it out of its brass holding ring by rapping on it with a screwdriver handle wrapped in cloth. It popped out cleanly, and not having it does not seem to affect the views in the slightest. I was puzzled as to why it was there in the first place, considering that the two extra uncoated glass surfaces can only make the view dimmer. Seeger, however, informs us that without a filter, the focus position is slightly changed. During a battle or while scanning the skies for aircraft it would be undesirable to have to take the time to refocus each eyepiece when changing filters, so the clear filter was added. For astronomy they could safely be removed, as long as your eyes will still reach focus on infinity without them. This can be verified before removing the filters by turning the selector dial just past one of the filters, then tying back the mechanism within the housing with dental floss or string. After reassembling the binoculars you will be able to make sure that you can still focus a star to a fine point without the filters in the light path.

Restoration/alteration

Now that disassembly and cleaning is complete, there are a few things you might want to consider doing before putting everything back together. First, flocking: the optical tubes and prism housings were originally painted a not-very-flat black, which with time has probably flaked off to a greater or lesser extent and will continue to do so. I s****ed off the loose bits, but paint continued to fall off and inevitably came to rest on the inside of a lens or on a prism face. After taking them apart and recleaning a couple of times, I decided to flock all the inner surfaces with black velvet self adhesive paper that can be seen in the third picture. This velvet paper is inexpensive and can be bought in large rolls from the supply company McMaster-Carr online. Cutting the right shapes is a fidgety job but worth the time. Some places inside the prism housing cannot be flocked because the paper will not leave enough room to reattach the two parts of the assembly. If you end up with extra paper, you could flock the insides of the dew shields as well. I also put a little on the underside of the plate holding the prisms in order to delay future staining (see seventh picture). Flocking was an afterthought for me but it turned out to be the most effective thing I did with the binoculars other than cleaning the prisms. Painting flat black would also be effective, but more time consuming.

Purists may want to simply reassemble the binocular at this point, and if yours are in nice shape this is probably the best option. If they have already been repainted (most likely poorly, without disassembly) you may decide to strip them and repaint. This is time consuming, as there are few flat surfaces. I found that 60 grit sandpaper was the best for removing paint, but as you start to work on the nooks and crannies you may prefer one (or ten) of those flexible sanding pads. As more and more bare metal is removed, go lightly and switch to 120, 150, 220, and finally 320 grit so that you don't leave big scratches in the metal. Be prepared for this part of the job to take a long time - it took me the entire baseball playoffs and world series, listening to almost every game while sanding. When done, I was surprised by how attractive the steel, aluminum and brass construction was, and decided not to repaint them but simply to clear coat the parts for protection against oxidation. To my eye, this highlights the beauty of the binocular, but you may feel differently and decide to return to the original desert tan or dark grey/green over red. If you have a headrest, the rubber can be protected and moisturized by rubbing in a little silicon based plumber's grease. Avoid petroleum based grease as it will harm the rubber. Since the dew shields are steel rather than aluminum, I decided to repaint these in satin black, although the original black was glossier.

Reassembly

Presuming you haven't lost any screws, the only difficult part of reassembly will be synchronizing the filter mechanism. In the fourth picture, you can see a articulating ball joint extending from the first prism housing. It fits into a socket in the second prism housing so that one set of prisms turns in coordination with the other. Before reattaching the prism housings, make sure they are both have the same color filter fully and exactly engaged. Then slip the ball and socket joint together, trying to move everything as little as possible. It make take a few tries, but eventually you'll get it. At the very least don't do as I did and reassemble the entire binocular before finding out that one eyepiece has the green filter engaged and the other has orange. This made for some creative cursing. Otherwise reassembly should be easy. Now you get to use them!

What are they like to use?

Markus has a lot more experience with binoculars than I do, so read his review on this point. For both daytime and nighttime use the 10x80s are impressive. Sure the Erfle eyepieces have some distortion at the edges, but then all binoculars have some edge distortion. These are well-corrected, and the 70-deg apparent field of view means that the field is so wide that the pincushion distortion is not distracting - you have to look for it. The exit pupils are 8mm, larger than any human being's pupils could possibly dilate even after a week in a cave. This means some light is wasted, but the advantage is that the view through the binoculars easy for your eyes to grasp and hold. This would have been helpful if you wanted to use the binoculars while your tank was bouncing across a field, but it also means that the 10x80s are exceptionally comfortable. There is no trick to viewing and even inexperienced users will find them easy and pleasurable to look through for a long time.

Daytime views are fun, but I rebuilt these binoculars to use for astronomy. I primarily observe with a telescope, but on many nights it is more enjoyable to lie back in the grass and look up with a pair of 7x50 or 10x50 binoculars. It is surprising how many deep space objects you can see this way, and of course Milky Way star fields are fantastic. For me, this is the real appeal of binoculars for astronomy, and I wanted to try it with a larger pair. This meant mounted binoculars, since I find it hard to hold even 10x50s steady enough to enjoy for long periods. Despite the attractiveness of the Fujinon 70mm binoculars and the many new inexpensive 70-100mm Chinese models, in order to view at or near zenith, they need an expensive parallelogram mount that will always be slightly awkward and bulky. There are a few 45 degree models that solve this problem, notably from Miyauchi and Vixen, but these are expensive and have a relatively small field of view. The only binoculars that fit my requirements of extremely wide field of view and 45-deg viewing for comfort were these 60-year old flak binoculars. There simply is no modern equivalent.

I am satisfied with how they work under the stars. Despite the enormous light loss through all of the uncoated optical surfaces, there are a lot of stars to be seen with these 10x80s. The field is fantastically wide - looking at Orion last night, for example, not only did all three belt stars fit in the field of view, they would have fit twice over. The belt stars and M42/M43 are framed nicely with plenty of room in the same field of view. The 10x80s are also the only binocular I've used other than the Fujinon 7x50 that will fit Alpha, Beta and Gamma Cassiopeia in the same field. These are the kinds of views you just can't get with a telescope. On the down side, although stars do focus to points, there is a kind of distortion that I am not knowledgeable enough to label. Unless you hold your head very still, stars seem to shift around a bit and move in and out of focus. With your head held still and looking directly at a star it is in focus, but when you look to another star on the other side of the field of view the first star seems to defocus. In any event, they are still a pleasure to use.

I hope this article will be of use to others. I enjoyed my 10x80 project, and would like to thank several people who helped me. When I was at a loss as to what to do with the prisms, Jaroslav Zaharchuk suggested the cerium oxide which worked wonders. Tom Mengel was also kind enough to offer his advice on cleaning even after I ignored his first suggestion to leave the binoculars alone. John Van Stelten of Focalpointlens.com, who has a great reputation for recoating and repairing old camera lenses, made the valuable toothpaste suggestion, which I would not have trusted from a non-professional. Finally, I am indebted to Hans Seeger for the wonderful book referred to throughout this article and to the Allister St. Claire of Cloudy Nights for his time and effort in creating this great site for amatuer astronomers.