Potential vignetting Problem?

Fellow ATM's, Will there be a vignetting problem if I use a 12" ID tube with an 11" F/5.6 primary? Should I go with a 14" ID tube? I already have a 2.6 MA secondary. Thanks for your valuable opinions.

There may be problems. See if you can find on the internet a free program called Newt !! Use Yahoo and search for "Newt". You will find it handy. As far as I can tell, you could use a 2.1 inch secondary with the right tube. Be sure to enter accurate numbers in the fields for Newt.

There would be a minor vignetting (nearly 1", or 1 degree fully illuminated field, with illumination 1" off axis better ~90%), and you'll probably have more vignetting at the diagonal anyway. I would be more concerned with possible thermal problems due to the tube walls being soclose to the incoming light bundle.