Astrophotography diagnosis of ngc6946 + ngc6939: Tracking drift, light pollution gradient and 1 other

The PSF panel is reassuring on the optics side: homogeneous FWHM (2.4 to 2.7 px), corner/center ratio of only 1.08 and near-zero FWHM asymmetry. No significant tilt, backfocus or coma. However, elongation is consistent everywhere, including the center (elong 1.15 versus a 1.12 floor, PA grouped at 9.2° dispersion). This is the signature of a tracking/guiding drift, not a field defect: it stays low, so stars look nearly round, but it is measurable. Polar alignment and autoguiding are the levers.

On the background, a linear gradient of about 10% oriented ~244° dominates the radial profile (plane R²=0.70, x3.9). The DSS shows no extended nebulosity here, ruling out IFN: this is indeed a light-pollution artifact, likely reinforced by the 44% Moon. Gradient extraction will easily correct it.

Finally, SNR is the weak point: an e_margin of 1.51 reveals a background barely lifted off the black point, and the spiral arms remain noisy. Three hours in an L filter on a low surface brightness galaxy, under moonlight, is insufficient. More hours and/or longer subs will make the difference.