Daniel posted about a generic "engineering" trend. Good observation, but it is, of course, totally dependant on design and materials. 99% (conservatively) of modern large-aperture amateur-made Newts are Dobs, and most utilize wooden construction for the secondary "cage". My caveat is that low-frequency vibrations can all too easily be traded for high-frequency vibrations in high-tension spider designs. Also, too much tension can easily pull a secondary cage made of wood out-of-round if the spider tension is too great, and the cage design isn't robust enough to handle the tension stress. The good news is, high-frequency vibrations aren't nearly as noticeable as low-frequency vibrations. Our brains are slow, lol. Wood is a very good material to use in structural elements, to dampen vibration. That's more good news. But... "Stiffness" in a design can contribute to higher-frequency vibration that, while not always noticeable visually, can be problematic when imaging, so be warned. Not as many folks imaging with large Dobs, so we can wipe our brows yet again. Moving on... A 3-vane spider will produce less overall diffraction than a 4-vane, while still allowing structural "stiffness" to be added to the secondary cage, all other things being equal. The tradeoff is that you end up with 6 diffraction spikes, instead of 4, as with a 4-vane spider, though each of those six is less intense than the 4-vane spikes. But allways make sure that your secondary cage can withstand the tension. I'd be willing to bet that if audited, many modern large Dobs with spiders in tension no-longer had perfectly round secondary cages. And thus, unequal tension and more secondary vibration than they were assembled with. A cardboard or composite tube will never be able to withstand long-term spider tension, without additional reinforcement not susceptible to long-term "creep". More anon. On Thu, Mar 8, 2012 at 3:38 PM, Brent Watson <brentjwatson@yahoo.com> wrote:
Funny you should mention spokes. That is what I used on the 22 inch - a bicycle rim on the inside of the tube and the secondary supported by eight spokes. That was very stiff.