Eric has been using a type I Paracorr on his f/3.3 scope and it works just fine. Everyone assumes it is a type II, and is surprised when they find how good the view is through the old one.
If you already have the type I, at f/4 I'd just keep it and never look back.
Same thing with eyepieces. Kennedy said that one customer was angry because if the customer cocked his head all the way sideways, he could see some artifact at the very edge of the field on the 21mm Ethos at f/3.3. He wanted a whole new mirror made at f/4. Now, you have to ask yourself about the logic of throwing out an entire mirror because of a view through a single eyepiece (let alone that you need to really strain to see the "problem").
There will always be better eyepieces as time goes on. No eyepiece should be considered the be all end all.
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