Astrophotography diagnosis of NGC 6888: Tracking drift and sensor tilt

The image is a technically clean mono narrowband sub, so the diagnosis logically centers on star shape as measured by the aberration inspector. The verdict is clear: tracking drift. The elongation is consistent in direction across the whole field (PA dispersion of only 10.5deg) and, crucially, present AT THE CENTER (elong 1.30 vs a field floor of 1.29), which rules out a pure field-optics cause. Over 300s without perfect guiding, it is the mount talking. The corner/center FWHM ratio stays moderate (1.139): the optics are broadly fine, corners do not collapse. The only reservation is a slight single-axis asymmetry (BL more spread, elong 2.00) that could betray minor tilt, but this signal is weak and partly buried in the drift; it cannot be cleanly resolved until tracking is under control. On sky and calibration there is nothing to flag: dark Bortle 2 background, gradient measured at 4% with no vignetting, healthy exposure for a raw sub. The priority is therefore single and clear: fix guiding, then re-measure star shape on a master to confirm or dismiss tilt.