Don, Thanks a great review
I know I was gobsmacked at the scattering of faint stars through the field when I had a good look with a 20" F4 and Paracor: it clearly outshone the F5 in image quality.
As astigmatism is such a dominant aberation at the edge of the field of most ( non-Nagler) eyepieces, I think it has distracted observers from the fact that Coma bloats star images in a linear fashion as soon as you look away from the centre of the field and the Paracor fixes that. I expect that my next large scope , a 24" F3.3 , will have the Paracor `super-glued ' into the focusser
Mark