Astrophotography diagnosis of NGC 6888: Light pollution gradient, tracking drift and 1 other

On star shape, the PSF panel shows generally healthy stars: the center is slightly elliptical (elong 1.17) and crucially this elongation persists with a nearly constant direction across all nine zones (PA spread 4.7°). A deformation reaching the center with a uniform direction is not a field optics issue but a tracking signature: slow uncorrected drift or imperfect guiding. The 1.10 corner/center ratio and negligible FWHM asymmetry confirm there is no tilt, backfocus or coma to report.

The dominant issue is the sky background. The illumination map reveals a plane gradient of 71% amplitude, oriented at 55°, dominating the radial component by a factor of 4.3 (R²=0.78). Under Bortle 2 with the Moon set, this is not a lunar gradient; it is an atmospheric/skyglow residual or low haze, fully recoverable with gradient extraction. A fine oblique pattern at ~158° additionally suggests a walking-noise remnant, typical when dithering is insufficient and the tracking drift propagates it.

Overall this is a promising master: clean optics, well-framed target, solid Halpha signal. The priority is to improve tracking/guiding and tighten dithering at acquisition, then extract the gradient in processing.