An essay, published in Amateur Telescope Making Journal. Number 11, 1998, pp. 26- 27; followed by posts from on-line on the subject. RANGEFINDERS AND STEREOSCOPIC TELESCOPES In 1893, Ernst Abbe, working for Carl Zeiss, applied for a patent on their new prism binocular, but it was denied because of the earlier Porro prism glasses from several European makers. A revised patent was submitted for a prism binocular with enlarged objective distance, with the increased separation between the objectives being the protected feature. This was approved, and for 15 years no other optician could make a Porro prism binocular with objectives more widely spaced than the oculars. The rapid development of prism glasses by other quality makers caused the energetic Zeiss publicity works to seize their unique characteristic and proclaim its advantages in advertising. There is a real, if minor, increase in sense of depth that follows this increase in inter-objective distance, which is probably perceptible at close focus with standard, hand held binoculars, although there is wide variation in individual ability in stereopsis. Zeiss used the term 'plasticity' to describe the enhanced sense of depth, and it is a very apt term, since nearby objects appear modeled or sculpted. This characteristic was quantified, with 'specific plasticity' being defined as objective distance divided by ocular distance, and 'total plasticity' as magnification times specific plasticity (higher magnification adds to the effect.) Increased perception of depth does allow the observer to distinguish between objects that might otherwise be of very low contrast, and this advantage was the subject of many studies, papers, advertisements, and brochures around the turn of the century. Zeiss also made theater glasses with closely spaced objectives for portability, and they were not shy about publicizing the advantages of this configuration. They claimed that in the theater, diminished depth perception is useful because the spectator will see the live actor as part of the painted backdrop. While these concerns are of minimal import today, the effects are real, and were a very important part of the introduction of binoculars to the public. The Zeiss prism binoculars of 1894 were the first commercially successful, the first mass produced, and the first high quality binoculars. At the same time, Zeiss offered 2 prism binoculars with objectives 12 inches apart (8 power,) and 16 inches apart (10 power.) A hinge between the oculars allows them to fold in half, leading to the generic term 'Scherenfernrohr' or scissors telescope. These were called by Zeiss, "Relieffernrohre," and were not successful. The 8 x 20 model was offered from 1894 to 1906, and the 10 x 25 from 1895 to 1908 and through 1918 for military use. They give spectacular views of terrestrial objects, greatly magnifying the perception of depth in a scene and the appearance of modeled relief in an object. Here there is no exaggerating the effect. They were used as rangefinders in both World Wars, by several service branches of most of the participants in the conflict. Hand held instruments were about 6 x 30, with objectives 18 inches apart, and a folding hinge to reduce the length for transport. Tripod mounted instruments could have 50mm objectives, for use at dawn and dusk. These were used by artillery forces to approximately judge distances. The smaller sizes were needed for quick judgments on shell bursts, when a large instrument or more complicated rangefinder could not work quickly enough. These 'battery commander's rangefinders' can occasionally be found at gun shows or military collectors' meetings, and there are a few optical repair shops remaining that can correct their typical out of collimation condition. Truly remarkable instruments were used by the U.S. Navy (among others,) from prior to WWI through the 1980s, for controlling the large guns of their ships. Some of these rangefinders used coincidence sighting, where two images were brought together in the viewfinder and the distance read off a scale. Others were stereoscopic rangefinders that gave a true stereo image of the target. A reticle for each eye was fixed in the tube, and formed a stereo image that appeared to move towards & away from the observer when optical wedges were rotated. When the image of the reticles (an arrangement of diamond shapes,) seemed to be at the distance of the target, the actual distance to the target could be estimated. There was extensive research and development on these fire control instruments during the 1920s, and they were the primary tool used to aim naval guns through most of this century. The longest recorded distance for optical rangefinder controlled gunfire, successfully firing on a moving target from a moving battleship, is 26,400 yards, achieved in 1940 by the British. These rangefinders were designed around a particular gun, and the distances at which they were accurate were determined by the range of the gun. In the U.S. Navy, the Mark 41 (1930s) and Mark 75 (1950s) had objectives eleven feet apart, a near focus of 1200 yards, and maximum useful range of 20,000 yards. These were made by Keuffel & Esser, weighed about 1200 pounds, and had 147 glass elements, including lenses, prisms, wedges, reticles, mirrors, and frosted elements. There were 15 foot models, weighing about 1500 pounds, in a motorized mount that was connected with servos to a gyroscope, to maintain the horizon at a level. The 11 and 15 foot models could be targeted on aircraft, and longer instruments were used to range ships and targets on shore. Larger models were made by Bausch and Lomb, including the 26.5 foot used with the common 16 inch guns. The Mark 52 consisted of a 25 power system with objectives 46 feet apart, weighing 10,500 pounds and costing about $100,000 during World War II. Near focus was 5,000 yards, maximum use at 45,000 yards. One interesting aspect of later rangefinders is that they were gas charged with helium, since it is the only gas with an index of refraction that does not change in the temperature range encountered by these instruments, and the extreme length of the rangefinders mandated this stability. The use of helium necessitates yet another level of maintenance for personnel; one source notes that it can leak through steel, and no doubt all seals & joints are somewhat porous to helium. These instruments were closely held secrets during their era (still used in foreign fleets,) and their size and weight ensured their dismantling on retirement. Very few persons have had the privilege of viewing through one, and the effect can only be imagined. Bernard Merems of Patagonia, Arizona, is an ambitious ATM who is constructing a binocular refractor with a widened base between the objectives. At Riverside '96, Bernie described his half-finished project. Two B & L telephoto lenses, 5 inches in diameter, and 40 inches in focal length, are mounted onto a prism housing so that the objectives are 16 inches apart. Light from the objectives enters the housing and strikes first surface mirrors, mounted at 45 degrees from the lens' optical axis, to converge the light into prisms at the center. The first prisms are standard 90 degree reflecting prisms, to direct the light back towards the oculars. These prisms will serve as collimators, by rotating around a vertical axis, and are an unfinished aspect of the instrument design at this point. Collimation of twin telescopes is quite difficult, and it is likely that adjustment about a single axis will not suffice to correct all collimation errors. The light exits these prisms in the same horizontal plane that it entered the instrument, having been reflected twice, and giving a correctly oriented image. However, it was desired to direct the oculars downwards, at 60 degrees to the horizontal, to allow comfortable viewing of the sky. This created many complications in anticipating final image orientation. Bernie finally consulted R. Buchroeder of Tucson on the subject, and was advised to purchase two toy periscopes, that allow rotation between the two mirrors. When held vertically, in using position, and the upper half rotated 360 degrees to scan the entire horizon, the image rotates to upside down when pointing backwards and back to right side up, when pointing forwards again. Continued perusal of this phenomenon is thought to give an intuitive grasp of the complexities of designing image erecting systems. The final image erecting design, if first light does not force any revisions, adds a deflecting prism with two silvered surfaces, with the light exiting at 60 degrees to the horizontal. A total of four reflecting surfaces in this orientation should give an inverted and reversed image. For terrestrial use, part of the assembly will be replaced with a single prism or flat. Inter-ocular distance will be adjusted by mounting all four prisms and the oculars, as a pair of assemblies onto a sliding track, with right and left handed threaded rods to change separation. Two inch oculars will be used. The 16 inch objective separation places a distant limit on the close focus of the instrument, for viewing nearby objects would require the telescopes to swivel inwards towards each other. This reduces the required eyepiece travel, for they will not have to rack out for focusing on nearby objects. Many such details must await final assembly of the instrument. Telescopes are typically used to increase resolution, contrast, and light gathering ability. Their potential for enhancing stereoscopic perception of depth is a fascinating and overlooked subject. Any input on the topic is welcome. Peter Abrahams, e-mail: telscope@europa.com ============================================== 27 Apr 1999 From: Robert Wohleb Subject: Re: ATM Home brew rangefinder? If the land is reasonably flat you can always find the distance using the height of the scope, angle from level ground, and simple mathematics. Distance = Height / (TAN Angle) even if the land is on a slope, the slope is rather constant, and you know the slope, you can claculate the distance using a variation of the above equation. ------------------------------ 27 Apr 1999 From: "Michael P. Lindner" Subject: Re: ATM Home brew rangefinder? beamsplitter---->/ /<---mirror o=======================o<---fulcrum (hinge or dowel) eye---->v lever--->============/ Take a long sitck. Mount a beamsplitter prism on one end so you can look straight ahead and also see an image straight down the stick. take a second stick, almost as long, and attach it to the first with a hing or dowel through both sticks. Mount a mirror on that (any mirror, doesn't have to be special) oriented so that the view through the beam splitter down the stick is reflected forward again.You may want to put a third stick from the beamsplitter to the ground, to steady the device (I've seen people make a little yoke to rest the thing on their shoulders instead). To calibrate. Take a third stick longer than the first one, and make 2 marks on it, the exact distance from the center of the beamsplitter to the center of the mirror. place the stick some distance away, and lok throught the range finder. adjust the mirror's angle by moving the long lever until both marks are superimposed. That is infinity. Note the separation of the stick and lever. If you want, you can attach a short piece of wood perpendicular to the stick, and a pointer to the lever, and mark off the distance scale on the wood. To calibrate the rest of the distances, you will use a single mark, at a known distance, and set the lever so the two images of the same mark are superimposed. The triangle made by the two ends of the lever and the stick is similar to the triangle formed by the two ends of the stick and the object being measured, so we have 50m/length of stick = length of lever/distance moved for the total travel. If both the stick and lever are 2m long, then 50/2 = 2/D or the total distance from the infinity position to the 50m position is 4/50 or 0.08m or 8cm, and the total distance from the infinity position to the 500m position is 8mm. Accuracy? A +/-1mm error in the measurement of the distance between the stick and lever results in a -56/+71m error at 500m, and a -5.6/+7.1m error at 50m. You can make it more accurate by more accurate measurement of the lever distance, or by making the stick and lever longer. You can pick up both the beamsplitter and mirror for under $10 from some place like Surplus Shack (http://www.surplusshack.com). Mike Lindner ==================================== rangefinder: if a simple telescope with very precisely calibrated focusing mechanism may not be good enough in this case. Maybe a 2 inch diameter, F:8 objective working at about 20 to 30 magnification. Preferably with a cross hair and two focusing mechanisms. One between the eyepiece and Xhair and the other (the calibrated one) moving both Xhair and eyepiece. The calibrated distance ring must be moveable so it can be set to infinity before use on some convenient distant object. In fact, the spotting scope may possibly be turned into the rangefinder. Peter Smith. ============================================ ---Guido Thürnagel recently I submitted to this ng because I am a fan of stereoscopical rangefinders. During the blow out sale of the NVA equipment, I got an OEM-2. It is made by Carl Zeiss Jena and the best I have ever seen in daylight-optics. In addition to the oem-2 I have got the pre oem-2 EM-61 with an optical base of 0,9 m. - just another green box... But if you take seat at the rim of the harbour of hamburg for example you know why doing this! ---DGoncz Yes, I am interested in stereo rangefinders, and in enhanced or simulated depth perception apparatus in general. Doug Goncz Replikon Research (PO Box 4094, Seven Corners, VA 22044-0094) ----Dr. Mark W. Lund I also have an OEM-2. I got it for Christmas from my brother. I have always been interested in rangefinders. I haven't had much time to play with it, and the manual is in German, even my German mother-in-law can't make heads or tails of it :) best regards mark ----- Peter W It is just that I calculated the accuracy on rangefinders some time ago and I assumed that their accuracy depended on the acuracy of angle measurment, and I had no good data on that, so, to get the data for real ones (optical base and the estimated range error at some distance) should help. I looked in edmund catalog they are selling small ones but I have a hunch that one can do better than the edmunds range finders. some meters in 15 km shows that I was right. I just made a quick estimate: ~10 m uncertainty in 15000 m and 4 m base requires a very accurate angle measurement ~0.04 arcseconds! is that really possible do you think? Peter Weijnitz ----Guido Thürnagel here are some data from the "Mindestfehler (MiF) ((minimum average mistake??))"oem-2.(optical base is 0,52m, magnification is 14x) (sorry, I don´t know the technical term) distance in m MiF in m +/- 400 1,1 600 2,5 800 4,4 1000 6,8 1500 15,3 2000 27,4 2500 43,0 3000 61,5 3500 84,0 4000 110,0 5000 171,0 6000 248,0 7000 336,0 10000 685,0 The mistake is reduced by 1,7 if you use the electrical pendula. - A "special" of the oem-2. ----Peter W Peter Weijnitz I get a angle measurment acccuracy of about 1.3 arcsecond from your data, it seems reaonable?? ----Guido Thürnagel "What's this electrical pendula?" there is an electrical mechanism integrated that makes the sight move for- and backwards - like a pendula. And it is officially called so. The effect is that the user gets a better impression of the locality of the sight. 8