Refractor objective question

I have a question about a very large beseler projector lens that I'd like to use to make a telescope. It is over 5" in diameter with only an 18" E.F. I'm concerned about the width of that very short light cone. Even with a 2" wide focuser, won't I be losing most of the light simply because my focuser and eyepieces aren't physically wide enough for the cone?
If this happens, what do you lose, the light gathering capacity of the wide objective, or just field size, or both?
I'm not too concerned about field size, but if I'm going to end up with the equivalent of a 4" objective in terms of light gathering then it's not such an attractive project.
Thanks very much for the help-
Larry Schiebel

Larry

Before you put very much work into the project, I suggest that you make a quick test set up to check performance. You might strap the lens to a board and use a small block of wood to support an eyepiece against the board about centered on the lens. With something like this, you will get a fair idea how it is going to work.

I have tried a few of these 5" Beselers and was unhappy with the results. These lenses are designed to focus on a 10"x10" surface 18" from the lens and project the image 10' to 20' away. They are not optimized for infinity focus.

Good Luck
Tom