Astrophotography diagnosis of SH2-188: Light pollution gradient, background chroma noise and 1 other

The optical and mechanical verdict is excellent. The PSF board shows round stars everywhere: elongation 1.02 at centre against a field floor of 1.01, values between 1.01 and 1.07 across the nine zones, and a PA dispersion of 44 degrees which precisely reflects round stars with no preferred direction. The corner/centre FWHM ratio of 1.14 and the horizontal asymmetry of 0.08 stay below field curvature thresholds: no tilt, no backfocus error, no coma, no tracking drift. On a long focal length RC10, that is a collimation and guiding result worth highlighting.

What remains is entirely about sky background and post-processing, and the two are linked. The 4% plane gradient oriented at 11 degrees, with R2 of 0.83 and a plane/radial dominance of 5.33, is the clear signature of an oriented gradient, not vignetting: your flat is doing its job, it is the gradient extraction that was not pushed far enough. This slightly tilted background in turn accentuates the perceived colour mottling in the empty areas, where OIII and SII SNR is weakest.

Finally, three or four bright stars show clipped white cores, with no major aesthetic consequence but limiting the refinement of the star rendering. None of these three points requires going back under the sky with the same gear: careful reprocessing plus extra integration on the weak channels will do.