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Looks like vignetting to me. Vignetting does not mean no light in the corners. It means a reduction of light in the corners. You will still see stars etc in the corners of a vignetted image if its going to be correctable with flats. Just light drop off.
If you move the filter and it the circle gets larger I think you have your answer. F5 in a Newt is relatively fast and the steep light cone seems to be at least part of the issue. Your filter may be fine on some other scope that is not so fast.
Whether the idea that the circle should get smaller if you move closer to the rear of the corrector may be incorrect. You'd need a ray trace of your scope to see what is happening with the light when you move. I am not 100% sure but I think a corrector is bending the light back to more straight lines rather than a cone.
They usually only slighty increase the F ratio if at all, so that is something to consider. So moving it forward to backwards should not make too much difference in terms of vignetting unless the filter is too small for your light path.
If you can get your filter even closer and get that circle bigger you may solve the problem.
Greg.
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