Astrophotography diagnosis of SH2-221 + SH2-217: Light pollution gradient, backfocus error and 1 other

The PSF panel confirms healthy tracking and optics at center: elong 1.07 at center for a field floor of 1.02, FWHM of 1.46px, high PA dispersion (58 degrees) indicating essentially round stars, no drift or guiding oscillation. This is a quality acquisition on the mount side (well-tuned iOptron CEM26). The curvature/backfocus pre-verdict holds but in mild form: FWHM rises fairly symmetrically toward edges (B 1.98, R 2.72, BR 1.64) with oval stars and no clear radial coma, corner/center ratio only 1.135. The R zone is biased by a bright star, so its 2.72px should be down-weighted. This is an optimization point, not a blocking defect.

The real issue is the sky background: a very marked plane gradient (253%, R2=0.89) oriented at 228 degrees largely dominates any radial symmetry. Moon at 0%, so urban/atmospheric origin. It darkens the rendering and pulls the background toward mauve in the top-left corner, which also shifts color balance. Gradient extraction on the linear data followed by color calibration would restore a neutral background and bring out the filamentary Ha signal.

Priorities: handle the gradient, then recalibrate color, and finally adjust backfocus for future sessions.