Astrophotography diagnosis of NGC 2024 (Flame Nebula): Astigmatism and residual hot and cold pixels

This Hα master is well executed. Tracking is flawless (no directional elongation, scattered PA indicating fundamentally round stars at center) and the Bortle 3 sky with moon below horizon yields a clean, gradient-free background, confirmed by the background map (+60% variation attributed to the nebula, non-optical profile). The Flame Nebula is well resolved with its sharp central dark dust lanes.

The only optical point is mild field astigmatism: the center is sharp (elong 1.06, FWHM 3.5 px) but stars progressively stretch toward the corners (elong 1.25 bottom-left) without strong FWHM increase, the typical signature of a slightly imperfect coma corrector spacing on an f/4. This is not tilt (no diagonal asymmetry) nor pure coma (no clear radial commas). The impact remains modest and correctable at acquisition.

On calibration, a few residual hot pixels remain in the background, a sign of perfectible darks or insufficient dithering. Nothing blocking: the master is fully usable for SHO stacking or monochrome Hα work.