Thanks for the comments guys. I only interested in the SCT curvature as a matter of curiosity. I must remember to check which way the crayford focuser moves in order to improve the edge of field next time I'm out.
From the links John: "
For a pair of mirrors, the curvatures are combined" and
" field curvature;
of interest are Petzval field curvature, which is not affected by the corrector", which supports my earlier assumption. The corrector may not have an impact on curvature because it's curve is so shallow?
Wavytone, for simplicity I'm only looking at the standard Meade/Celestron f/10 SCT's with spherical mirrors, although it is interesting to consider what happens with shallower primaries, larger secondaries, etc.