Be sure to get a reality check with a Ronchi screen.
The center is hard to measure with Foucault, but
easy to inspect with Ronchi.
If you really have a high center, you'll see a "Magnified" zone
there as you move the screen side-to-side.
Bill Prewitt
My webpage of telescope making and caving
http://home.neo.rr.com/imaging
> The center is high by about .018"