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Hi Lachlan
Whilst I am not familiar with the particular scope you have, I have two that use a flattner and have round stars right to the edge of the frame. The key is to get the back focus exact. From your image it looks close but does not look to be exact and this causes the distortion you see. I found using a set of verniers from Bunnings to be the best option to get the distance exact and used these to work out the extension spacers I needed. Remember that you need to measure from a specific point on the flattner as it may not be from the edge of the flattner. This distance and the point where it is measured from should be in the documentation with the scope. Also need to allow for the glass cover over the camera chip and the thickness of any filters used.
It can be an exercise in frustration getting the exact back focus but it is worth it.
Brian
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