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Exactly, Don.
It is very easy to forget not just personal observing styles, but also that the eye is an active element in the optical chain, and as a result WILL bring its own aberrations, not just whatever the scope and EP do.
Astigmatism is one key aberration I use in evaluating an EPs suitability. Newts are especially hard on EPs, and an EP can show astigmatism in a Newt, but not in a refractor. And as you said, astigmatism cannot be corrected for, so for EPs I use in my Newts this is the one aberration I look for. As for other aberrations, this comes down to both personal preference and how your own eyes work.
HOWEVER, some astigmatism does not need to dismiss outright an EP as being useful. If some can be seen along the say the outer 5 to 10% of the FOV, this is not a deal breaker for some people as no one does any serious observing along the edge, and this is perfectly acceptable for many people. To be a stickler for edge to edge performance is more an indicator of personal observing style. This is where much agro lies in the persistence of some sort of holy grail performance and insisting that this is the ONLY thing that matters. Not everyone may be able to afford a particular EP. Not everyone cares for "perfect", but prefer pragmatism. Damned be anyone who judges someone else's personal preferences that don't match their own.
This is one reason why Bill's reviews are so good - he describes what he sees beside certain contemporary standards, but allows the person reading his review to make up their own mind according to the scope/scopes they use.
Alex.
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