What you're seeing is roughly equal parts coma and astigmatism and as someone else explained that is a more or less direct consequence of having a preferred axis in this system. 1.5 waves of total wavefront error off axis is actually not so bad.
Most users consider that perfectly acceptable in a visual telescope.
If you don't know how to calculate them use the examples in the table on page 65. Also try changing the primary focal ratio and secondary magnification.