I have a 8" F8 telescope that has a PVC tube and it is HEAVY ! ! I even went to the process of trying to lighten the tube a bit by cutting sections of it out and it is still a quite heavy scope.
I'd more suggest that you consider using a Hastings Irrigation Pipe aluminum tube instead if you don't like the concrete form tubes.
They have a special astro section on their website www.hastings.com (I think is the site name) so that you can order the pipe to the length you want and there's a choice of thicknesses and the tubes should be at least 1/2 lighter than the PVC.
As to a mirror size, I'd suggest something that is at least 2x the diameter of your present scope as you will then gain much more light to see the things up there. That means at least a 6" which is what you are thinking of but you may want to more consider a 10" scope for better viewing.
I'll also suggest that you try grinding the primary mirror yourself and gain the expertise in making mirrors. You already have a telescope so doing the grinding and polishing will be encouraged by the minor frustrations of not being able to really see some object. When you succeed, you will then be able to really say that you built it yourself.

• Hastings Aluminum Tubes - dburton - October 23, 2002 - 13:52 UTC
  • Tubes - bobmay - October 23, 2002 - 18:31 UTC
    • tubes - mgallagher - November 10, 2002 - 13:16 UTC