Astrophotography diagnosis of NGC 281W: Tracking drift

This Hα sub is of very good quality. The aberration inspector shows a homogeneous, fine PSF across the nine zones (FWHM 2.0-2.4 px, center elong 1.13 against a field floor of 1.09), with no radial degradation (corner/center ratio 1.12) and no diagonal asymmetry (H asym 0.039, V 0.027). In other words the field optics are clean: no detectable tilt, backfocus, coma or astigmatism. The only real signal is a weak but present elongation reaching the center, with very consistent position angles (15.6° spread). A deformation affecting the center as much as the edges is not optical but mechanical: it is the signature of a slight tracking drift during the 300s. The streak stays straight and uniform, ruling out periodic error (curvature) and guiding oscillation (bean/zigzag). The amplitude here is very modest and near the instrumental floor, so the visual impact is minor: the sub stacks perfectly. On the background side, the illumination map shows only a 7% radial falloff toward the edges (normal optical vignetting before flats, to be corrected by flats, not a residual defect), with no oriented gradient. The DSS confirms that all visible nebulosity is real. In short, a healthy acquisition whose only room for improvement is tighter guiding.