Astrophotography diagnosis of C12: Residual vignetting and sensor tilt

On the background, the measurement is decisive: a 22% radial falloff with rotational symmetry (plane/radial dominance 0.03, anisotropy 0.02), corners filled with pure sky that the DSS confirms has no nebulosity. So this is uncorrected instrumental vignetting, not IFN nor an oriented gradient. The concrete priority is therefore flat-field calibration, otherwise a simple background extraction (GraXpert/DBE) will flatten the master.

On the stars, the PSF panel is clear: round center (elong 1.12, at the field floor 1.10), so neither tracking nor global focus issues, the EQ6R mount and guiding do their job. But the TL/BR diagonal corners climb to 1.57 elongation while the other diagonal stays near 1.22, with a corner/center ratio of 1.28: the signature of a focal-plane tilt rather than symmetric backfocus. On an f/4 Newtonian this defect is common and is fixed mechanically.

The rest is healthy: no walking noise, no FPN or banding, no obvious residual calibration patterns. The real remaining lever after tilt and flats is integration: the background is barely lifted (e_margin 2.24) and the galactic signal faint, so stacking more subs will markedly strengthen the final result.