Hi Chris and thanks for this feedback.
I am probably closer to F3'ish because I am using a double reducer combination, a 0.63x attached to the SCT threads, then the visual back, then another 0.5x attached to the Infinity nosepiece. Also my scope is a C8 Edge HD. I'm sorry to make this more confusing because I was quoting an earlier post by Spacedout with a C11 at F5. This made both posts run together.
Anyway I am getting the severe vingnetting but I'll try a light pollution filter to see if that helps. Also I'll vary the spacing between the 0.5x reducer and camera body to lessen the overall focal reduction and take some pictures next time.
Thanks again for the advice.