This is a subjec that many overblow to a large degree.
Newt reports both the 100% illumination circle and the 75% circle. Any intrusion of the 100% circle is considered a vignetting of the light.
By putting in a slightly larger secondary, you allow the FOV to be larger for the 100% circle and this ends up causing some minor vignetting of the image off axis. The on-axis image isn't affected by this tho which is important.
You should really find out how much attenuation of the light is for a 75% illumination of the image compared to 100% - hint, it is a lot less than you think that it is!
In the end, you start with a scope that has an obstruction of 18% (in this case) of the diameter of the scope so you are never really going to be getting all of the light of the 10" aperture. As you go from the 100$ circle of illumination to the 75%, you gradually lose additional light (mostly from the edge where you are getting the worst of the coma anyway) until you hit the 75% illumination circle (which I might note is just an arbitrary value to indicate) and on from there until the light is fully blocked off.
In other words, a slight vignetting is of no real consequence as you won't notice it.

• thank you - dsellors - February 20, 2003 - 17:38 UTC