Astrophotography diagnosis of IC1805: Backfocus error, filter halo and 1 other

Star shape is broadly healthy: the center and lateral edges stay tight (L elong 1.06, FWHM ~2px), evidence of correct focus and clean tracking on the modest EQ3-2. The only recurring pattern is a symmetric FWHM rise toward the four corners (TR/R/BR around 2.5-2.6px, elong 1.32-1.39), with a corner/center ratio of 1.275: the classic signature of field curvature or backfocus slightly out of tolerance on an ED 70mm doublet without an optimal flattener. The high measured center elong (1.39) is mostly due to the dense star field within the nebula, not a coherent directional defect (PA dispersion 56°), so no tracking component is to blame.

On the processing side, the image is well balanced. The red halos around bright stars come from the multiband filter (narrow Halpha response) and are recoverable through separate star processing. The sky background, dark and generally clean, reveals under aggressive stretch some mild chroma noise (mottling) in the corners, a consequence of still modest integration. Nothing prohibitive: these are finishing points, not major acquisition flaws.