Hi Gary from what I can tell you have probably gone a little too far with the hill at the centre and if the photo is inside radius of curvature then you have actually created a slight hole in the middle. This can be of consequence depending on the size of the hole and the size of the secondary mirror you are going to use. If the size of the hole is smaller than the secondary then don't worry about it if it is bigger then you can try 1/3 strokes centre on centre again and this may eliminate it or you can leave it and start to figure the mirror. When you start doing the strokes to figure the mirror you are actually opening the ronchi lines up a little. which is how the middle picture looks. To test whether it is a TDE or difraction hold a piece of paper just below the TDE and if it looks turned the it is diffraction. What I do is always look inside ROC and try to get 4 lines in the ronchi picture.It is a more accurate measurement than outside ROC. Here are a few diagrams of something that may help. If you can't get good images send me a pm and another email adress so that I can send them at a higher resolution. Try this website this guy is good.
http://bi-staff.beckman.uiuc.edu/~me...te/oblate.html
Regards Cristian