APP Vignetting Example:

Series of images/steps taken to fix the wildly vignetted image ~ this image taken using Altair 183C with Lehance dual-band
filter using APP to extract OIII algorithm ~ images are all at 30% BG,  2 sigma, o,o% base ~ with is not the strength of stretching
you'd use on the final processing of the image ~ used to enhance what's happening.