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Hi Peter,
What might the focal length of your refractor be? Reason being if the resulting focal ratio is fast (focal ratio is the focal length divided by the objective's/primary mirror's diameter), this may cause any inherant astigmatism in the eyepiece you are using to show up. The shape of the abberation you describe suggests astigmatism in the eyepiece.
Is your 50mm a GSO SuperView eyepiece? These are fantastic eyepieces for their price, but there is a limit to their performance, particularly in "fast" scopes. I have the 30mm SuperView, and while in my C8 the edge performance is ok, in my fast newtonians, f/4.5 & f/4, the astigmatism is very noticeable.
Coma it isn't as coma is a phenomenon related to reflectors only. The 'similar' abberation in fast refractors is chromatic abberation, due to the objective lens not being able to focus the whole spectrum of light to a single point. This is seen as a blue/violet halo around bright objects, particularly at high magnification.
Astigmatism can be controlled by using better quality eyepieces. The othe alternative is to understand and accept the eyepiece's shortcomings. I tolerate this in mine, and I'm happy with it.
Chromatic abberation is inherant to fast 'Achromatic refractors', and can be delt with by using violet minus filter, or by really splashing out and buying an 'Apochromatic' refractor.
Mental.
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