I’d recommend a “difference of gaussians” to remove the vignette, where one of the gaussians is not required.
calculate a strong lowpass (gaussian, median, box blur, anything), \sigma much more than 10. subtract that from the source image. values will vary around 0, so work with floats or signed integers.
this will let you catch those faint contrasts away from the center.
this method is basically an “adaptive threshold”.
it’s hard to make out in your picture what’s going on. there’s some seemingly bright pieces, then some reflections, …
plainly said, the picture is bad. magic (post-processing) will not save it. garbage in, garbage out. the acquisition needs to be rethought.
