Extreme vignetting issue?

[Stefannebula]

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!

[sil]

vignetting is darkening around the outside. wondering if it could be a bit of light leakage / internal reflection, thermal noise is usually more even from a single location.

hard to know without an unprocessed source image to see as it also looks like nasty ringing from an overprocessed step

[Atmos]

Is this a calibrated or uncalibrated image?

[atalas]

Yeah,looks like a bad flat....need to subtract a bias from light frame. Or,optics fogging up...although I don't see that on the stars.

What was the adu reading on your flat?

[rustigsmed]

need to see the original stack.
but yep, either bad flats or you haven't applied them. flats add 500% improvement to the end result.