Mike,

Try a 16" f/5 with a 3.1" diagonal 12.6" from focus (my current planetary "grab & go" scope).
Assuming a perfect primary and a 1/10 wave P-V diagonal curvature, how much astigmatism does one get?

I'm asking the question because this page claims that you can have 4 or 5 waves of curvature on the diagonal without problem.

Robert