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Sean
Only limited info in your post. What eyepieces have you got?
Secondly, my usual advice is use the ones you have. They may not be perfect, but they don't cost anything more and you can spend/waste a lot of $$$ chasing perfection that sometimes isn't actually obtainable.
An f4 newtonian is going to have coma. Coma is an unavoidable consequence of the newtonian design. I built a little dob using an f4 mirror and it works quite well. You certainly see distortions when using the widefield eyepieces but that is the price for the convenience of f4.
Malcolm
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