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Tube Length And Flexion

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Posted by Daniel Johnson on November 5, 2002 02:58:53 UTC

Keep in mind that the flexibility of a tube goes up exponentially with length (at least with the square--or is it the cube or fourth power? In any case, it's waaaay more than linear. Imagine using a 3-foot-long diving board. Now imagine diving from a board of the same material but 10 feet long--a whole lot springier, no?) And even if you have a stiff tube, vibration damping time increases with length. Plus, you give the wind a much longer lever arm than with a short tube, making it hard to keep a sufficiently wind-free environment. You have a tube that's, say, four times more flexible than a 5-foot tube, being hit by three to five times the torque from wind as a 5-foot tube, and the factors are multiplied against each other to find the total effect. So when you're thinking of a 10-foot tube, keep that in mind. Ever thought of a folded refractor? You lose a little light with each reflection, but the gain in stability might be worth it. I don't think the average Joe realizes the mechanical difficulty of mounting a long refractor. Remember, at 250x, every vibration is magnified 250 times, and long refractors are made to be used at high magnifications. Length is the enemy of stability.

--Dan Johnson

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