Thanks John
. Clear as mud Alex
.
John, I'm wondering if you're able to confirm something else for me - that the curved field of commercial SCT's is of opposite sign to Newts?
Here's my thinking: the wavefront leaves the primary mirror curved in the same direction as a Newt; the secondary has a shorter radius of curvature and therefore produces stronger curvature than the primary; since the secondary is convex it changes the wavefront to the opposite sign.