Astrophotography diagnosis of SH2-119: Backfocus error, noise / underexposure and 1 other

The diagnosis comes down to two threads that do not intersect. The first is purely optical: the aberration inspector shows a clean center (elong 1.09 against a field floor of 1.04, FWHM 3.11 px) and degradation rising with radius across all four corners, corner/center ratio 1.209. That is the classic signature of uncompensated field curvature, so a backfocus outside tolerance rather than a tracking fault, which would have elongated the central stars too. The bottom-left corner stands out (elong 1.56, FWHM 4.48 px): not asymmetric enough to call it true tilt, but it suggests that on top of the spacing, the optical train has slight mechanical play on one side. The order of attack is therefore spacing first, clamping only afterwards.

The second thread is a photon budget problem. The measured background-to-black-point margin (e_margin 1.73) and the positive Bowley skew say the sky background is barely lifted off the black point, and the result shows in the background grain, particularly in the right half where the faint nebulosity sinks into noise. With under three hours on such a diffuse target, no processing will recover what was never captured: smoothing would only erase the very extensions you are trying to show.

The rest is cosmetic. A few bright star cores are clipped, a limited loss of nuance easily avoided with a set of short complementary exposures. Overall the image is already very presentable, and the two improvement levers are clear and independent of one another.