The Illuminated Image deals with the illumination level of the eyepiece at a particular location in the image plane. If the secondary (this is considering a Newt.) is small enough, there will never be 100% illumination as the full image from the primary cannot be seen by the eyepiece. At the other end, you can make the sec. large enough so that even at the edge of a 2" EP (about .9" from center) you can see all of the primary mirror. If the sec. is that large then you will have a 100% illuminated EP. What happens when there isn't full illumination is that there is less light because not all of the light from the pri. got reflected in the sec. to the EP. Now for the second subject that comes up concerning the size of a secondary is that the contrast goes down due to difraction and light block effects on the airy disc! Larger secondaries cause the contrast to lower so the background has more difracted light where it doesn't belong. Also, larger secondaries also let less light by them to the rest of the optics. An interesting scope is the Tasco 4.5" Newtonian scope. It has a very small secondary so that the image is never fully illuminated by the pri. and as a result, while you take the FL as being very short, you actually have a longer FL instrument and the spherical pri. never gives that bad an image as a result. Hope that this answers some of the questions you have concerning this.