Hi Matt,
I read yur post with interest. You are not going to be able to do what you propose with only a 2" diagonal. At a minimum you will need a 2 5/8" to get the focal plain wgere you want it. This Assumes a 12" diameter tube and the focal plane approximately 120mm out from the wall of the tube. If you redraw your your layout and take into account a focal plane diameter of 1" you will see where the trouble is. You need to draw in the light path from each edge of the focal plane. This 1" diameter focal plane is about what you would want for photography with a 35mm film format. this circle will be 100%illuminated with some falloff outside the 1" circle. Redraw and see what I mean. If you have any questions, you can e-mail me at jkoptic@juno.com

JK