I suppose it depends on how you define "low end". Without changing the design much, careful choice of materials and attention to details can make a huge difference in both load capacity and performance. I use 1" diameter aluminum tubing with a 1/8" thick wall for my aluminum parallelogram, but a square section of 1-1/8" for my maple parallelogram. The aluminum tube is weakened considerably more by drilling holes for pivot bolt bearings (nylon inserts), than the wood arms. The net result is a lower capacity in terms of weight. A 1/8" overall increase in sectional dimension doesn't sound like much, but the capacity of the maple parallelogram is about 3 times the capactiy of the aluminum one. There's a lot of wood there, and it's strong. My 3-arm parallelogram (which incidentally incorporates a horizontal offset between upper and lower arms, just like my 2-arm design) uses wood arms of a 1-3/8" section. Three arms in a triangle arraingement means that the entire arm assembly is much stiffer in every dimension, than a two-arm assembly. Hardwoods such as maple, oak, and ash are terrifically stronger under both bending and torsional loads than pine, fir, or alder, softwoods typically used by the do-it-yourselfer. My 2-arm maple design holds my 20x80mm units just fine. If I shortened the arms by about 10 inches I'd consider mounting the 100mm units, but not as-is. Most commercial parallelograms use off-the-shelf square extruded aluminum tubing of relatively thin wall. All plates and brackets are similarly made from thin gauge material for ease of fabrication and low cost. The kinematics may sometimes be clever, but the excecution is always insufficient for larger binoculars. The large bino mounts that are substantial enough for heavy loads have ergonomic problems, as Kurt pointed out. Another drawback of all the commercial units is the small diameter of the central azimuth bearing. I use a large, 7" diameter base with Teflon-on-Ebony Star as a central pivot on the 3-arm unit, a 2" diameter on the 2-arm unit. I realize that most people are locked-into having to choose a commercial product- but realize that most of the commercial parallelogram offerings are the results of a cottage industry. I haven't sold any complete parallelograms or kits of the 100mm, 3-arm design- I wanted to use it for a year or so before doing so, but perhaps I can skip the waiting period if there is enough interest. I've never built astronomical items as a business enterprise for profit- my only motivation has always been to just enable certain folks, and keep my hobby self-supporting. I have most of the parts done for a second unit, maybe I can finish it in the next few weeks if someone wants to test drive it this winter? All it needs is a set of 3 tripod legs. Bill Vorce (Telescope Warehouse) sells Meade tripod legs that should suffice. --- Canopus56 <canopus56@yahoo.com> wrote:
My experience with 20x70s and a homemade parallelogram mount is that for low-end wood parallelograms, 20x80 is near or over the limit.
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