Binocular telescope FAQsWhat is a binoscope? What is a binoscope?The word binoscope is a colloquialism used by amateur astronomers for binocular telescope, with either refracting or reflecting objectives (although giant binoculars seems to be more popular when describing refracting binoculars). However, the word does have another meaning. Some in the medical community use binoscope to refer to a binocular microscope, or a microscope with a single objective and two eyepieces (i.e., similar to the optical arrangement of a Dobsonian telescope equipped with a binoviewer). Stereo microscopes are microscopes with two objectives and two (or four) eyepieces. They provide true 3D images. Unfortunately, there is no such thing as a stereo telescope (except perhaps for NASA's STEREO spacecraft launched to observe the sun4). Astronomical objects are too far away to yield 3D views, no matter how big the objectives are. What is a Newtonian binocular telescope?It's a reflecting telescope with two Newtonian optical trains, arranged so the two eyepieces are next to each other. Two telescope configurations are typically used: side-by-side or stacked vertically. Side-by-side requires extra diagonals to make their focal planes available. Stacked vertically, the bottom telescope's secondary must be positioned closer to the primary for the same reason. In side-by-side configurations, you look towards the primaries with your back to the object. In the stacked approach, the eyepieces are located in the typical Newtonian fashion — along the side of the tube perpendicular to the primaries' optical axes. What do you mean by "convergence"?Before the binocular works, you have to be able to adjust the telescopes so they are parallel optically. Since the word "collimation"1 has been used historically to describe the process of aligning monocular Newtonians, we need another word to describe the unique process of achieving optical binocular parallelism. Some ATMs use the word "co-collimation"; I use the word convergence. A properly converged binocular lets the user's brain fuse the two images into one. Individuals vary in their ability to fuse images. Some people have considerable tolerance for optical misalignment, while others are equally intolerant. Higher powers obviously magnify binocular misalignment. How do you converge a binocular telescope?After you align the finder with one side, then look through both eyepieces. Probably you'll see only one image; if you're lucky, you'll see two images that too far apart to be fused. You must determine how the two telescopes are misaligned: are they cross-eyed, splayed outward, offset vertically, or some combination of these errors? Determining the which type of misalignment is not difficult; thinking about the geometry of the error helps. For example, if the telescope has to be moved to the left to find the image in the left side, then the two telescopes are cross-eyed. Successive corrections usually solve the problem; however, the adjustments for final alignment are very subtle. Often, a quarter turn of a thumbscrew will suffice. The adjustment you use for convergence should not change either telescope's collimation. You should design a way to make your two telescopes parallel that doesn't involve the same adjustments used for collimation. Finally, if you design your convergence adjustment so the user can remain at the eyepieces while making the adjustment, there will be a lot less ladder travel. How do you make an interpupillary adjustment?It should be easy for each viewer to match eyepiece separation to his or her own eye width while looking through the eyepieces. This is called interpupillary distance (IPD). The mechanism to accomplish separation change should have these properties:
ATMs have usually done this two ways:
Rotating upper ends are heavier and must be carefully constructed so as to avoid degrading collimation as the secondary rotates. The focusers must be mounted so rotation doesn't prevent minimum separation. Sliding focusers are lighter in weight, but require that the focus be adjusted each time they are moved. The sliding focuser must also carry the tertiary with it. Why should the design keep the eyepieces horizontal?The eyepieces should both be aligned horizontally with the ground. Even though collimation doesn't require it, the viewer tends to hold his or her head level so the eyes are parallel to the ground. Unfortunately, the binocular telescope is not like a pair of hand-held binoculars that the user easily aligns with his or her eyes. If the telescope isn't aligned this way, the viewer must tip his or her head to one side or another in order to align with the eyepieces. Inexperienced telescope users are bothered by this. Rotating upper cages simplify this adjustment, since the viewer will naturally keep the eyepieces in a horizontal line. Do you know your eyepiece's maximum width?Eyepiece choice is constrained by eye width considerations. Some eyepieces, like the 31mm Tele Vue Nagler, have such big barrels they can't be mounted close enough together for many people. If you're designing the binocular only for yourself, the choices are clear cut. If others, especially children, are going to use your telescope at star parties, a more narrow separation and 1¼" eyepieces are probably needed. Tele Vue publishes eyepiece barrel width information on their Web site. You also have to factor in the tube thickness of the focuser to compute the actual separation you'll get. How does the focuser restrict minimum separation?Focuser choice is restricted. Like big eyepieces, most commercially available 2" focusers can't be used without severe restrictions on minimum interpupillary separation, no matter how they are arranged. Smaller, 1½" focusers may cause vignetting, especially with faster f ratio primaries, and prevent the use of 2" eyepieces with their bigger aperture stops and fields of view. The way you design your focusers can aggravate the situation. Focusers get in the way, and typically impose an additional inch or so to minimum separation. I've found focuser design to be the most difficult aspect of building a binocular telescope. It's frustrating to realize something that seems so trivial is such a big headache. I highly recommend the reverse Crayford focuser sold by JMI, mounted without the base plate3. The only thing that gets in the way is the focuser barrel itself, which is about the best you can expect. Doesn't body heat from standing in front of the binocular ruin the view?Heat from the viewer's body rises in front of the optical paths of the binocular. Lightweight heat shields may help reduce image degradation. If you plan to spend most of your observing time with high-power eyepieces, you should try to insulate your body's heat radiation with adequate clothing. How can flexure be eliminated?My main worry at the outset of design has always been flexure, because it can so easily corrupt convergence. All telescopes probably flex to some degree — an unavoidable deflection due to gravity — but it usually goes unnoticed. With a binocular telescope, even slight flexure can knock out convergence and prevent image fusion — which is very noticeable. Flexure is a bigger problem in binoculars since each side may bend differently due to differences in weight or construction. This places unusual demands on the mount's construction: slight flexure-caused deviations from the parallel position may cause image shift and detectable loss of fusion. To my gratification, once binocular parallelism was achieved on my 12½", it remained aligned from the zenith to within 10º to 15º of the horizon. On my 22", I added an x-y linear stage at the top end that allows tuning of binocular alignment while looking through the eyepieces. This permits the use of higher powers, where misalignment of the two images is even more noticeable. I believe a binocular Newtonian without a way to adjust convergence at the eyepieces is going to be very difficult to live with. Do the secondaries have to be bigger in a binocular Newtonian?The focal planes of the side-by-side binocular telescope must be projected well beyond the edge of the incoming light cylinder. This increases the size of the diagonal, but can be mitigated by using a Barlow lens or a longer focal ratio. The Barlow, unfortunately, compromises low-power viewing. Will the ladder block the light path?The viewer must stand in front of the telescope. At the zenith, this requires a ladder that will let the viewer climb high enough to lean over the binocular. Balance is harder for some to maintain at such high altitudes and gravity-defying angles.The ladder should not block the light path. Adjustable-height ladders are best for this. Platform ladders with handrails are the safest. They are sold in portable versions and can be carried on top of a vehicle or in a trailer. I use two Little Giant ladders made by Wing Enterprises2. One is a compact step ladder, for lower elevations; the other is an extension ladder for higher positions. Is binocular collimation difficult?Getting good collimation in each half of a binocular telescope is not much more complicated than monocular collimation. The same kind of controls are required for the primary, secondary and focuser. Because the optical train of a Newtonian binocular adds a tertiary mirror, it also must be adjustable. A laser collimator helps. Fortunately, the tertiary is similar to a 45º or 90º star diagonal: it is close to the focal plane and has a less harmful effect on collimation than other errors. Collimation procedures for the individual telescopes of a binocular are no different than those used for monocular telescopes. My 12½" binocular has a Barlow lens, which must be temporarily removed before collimation can be performed with a laser or Cheshire collimator. (A Barlow lens permits a smaller secondary, but reduces low-power flexibility.) Why keep the eyepieces coplanar?Not only must the telescopes be optically parallel, but the ends of each individually focused eyepiece should fall in the same plane (coplanar). Otherwise, the distances from the eye to the eyepieces will differ and binocular fusion will be more difficult because the viewer has to rotate his or her head to compensate. To avoid this, design your binocular so that the two telescopes can be adjusted individually up or down along the binocular's structural axis. This is something that only needs to be done once and should not need repeating between different setups. What is image rotation?In a visual binocular, image rotation is the condition where the two images that are so counter-rotated they can no longer be fused. If one or both of the telescopes' optical paths do not lie in a plane, image rotation may occur. This is usually unnoticed if the tertiary is properly aligned during collimation.
Imagine what would happen to a properly aligned Newtonian binocular if you rotated one of the tertiary mirrors — and its eyepiece — around their common axis. The two fields would rotate. You can avoid this by providing a way to adjust the rotation angle of the tertiary. Image rotation can also occur if the binocular's optical train is improperly designed. For example, tertiaries normally return the light paths to the eyepieces so they are parallel to the primaries' optical paths. If, however, the tertiaries are rotated so the optical paths emerge at right angles to the primaries' optical paths, the images will be counter-rotated. |
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