Quote:
Originally Posted by LAW
Thanks Jay and Peter, I wasn't sure how big the tube should be around the mirror, I just put in 13" because it was bigger than 12"  
The 88mm secondary is the one that comes in the GSO 12" f/4 kit, and it's the biggest they make. I think I'll happily continue playing with my DSLRs for now, a dedicated astro CCD isn't really on the horizon for me at the moment.
Would a bigger secondary move the focuser further down (shorten) the tube? |
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