Quote:
Originally Posted by OzEclipse
Locating the primary mirror cell as close to the end of the tube as possible will shorten the tube.
On its own, a big secondary mirror size doesn't change the tube length by any significant amount. However using a big secondary and moving the focus out further will shorten the tube by the same distance you move the focus out away from but this isn't a great design strategy. You risk vignetting and are moving the weight of the camera out further possibly creating balance and flexure problems. Having weight hanging a long way off the tube makes balancing difficult and that long distance can cause flexure on the tube around the base of the focusser due to the torque imparted by the long focus distance. This is only going to save you a small amount tube length at most. You may as well design it with just the minimum amount of focus distance required for current (and future) cameras and avoid other problems that may be created by artificially trying to shorten the tube.
Joe
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Thanks Joe, that's exactly what I thought might happen, I was just wondering if I had to shorten the tube with a bigger secondary or if I could leave the distance to the focuser the same. Newt Online was already giving me vignetting on the 100% ray with a taller focuser so I didn't really want to make it any longer if I could help it, but the 88mm secondary is the biggest I can go so it's not really a problem.
At the moment I'm trading tube aperture for illumination as the wider tube is making the light cone narrower at the focal plane. I'm sure I'll get my head around this eventually
