[Stefannebula (Stefan)]

Hello everyone seeking a bit of advice if possible..

Getting used to using my new ASI1600MC camera on my Skywatcher Ed100. I'm using AstroLive USB capture software and when I stretch the image I see extremely light corners in my images..
I was thinking it was a back focus issue from the focal reducer but I'm not sure. If this just very bad vignetting?
Any tips to possibly fix/avoid this?

P.s the image is pre-stretched quite heavily, but the issue shows up in post-processing.

Thanks!