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Hi, Paul.
The shape of the stars (left pic) looks like a classic case of field curvature (similar to "coma" in newtonians).
The Orion ED80 has very pronounced field curvature. I was surprised by how strong it was when I bought mine.
Using a focal reducer without some sort of field flattener will only make it worse.
The problem is sensitive to focus, so without buying a field-flattener the only thing you can do to minimise the effect is to spend more time on acquiring exact focus. This won't eliminate field curvature but it will help a bit.
When I buy a focal reducer for my 80ED it will be something like the Williams Optics x0.8 that has a field flattener built in so it yields (according to those who champion it) pinpoint stars out to the edge of field.
Last edited by 2020BC; 15-10-2006 at 03:22 PM.
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