Astrophotography diagnosis of Caldwell 101: Astigmatism

Very favourable conditions: Bortle 3 and the Moon below the horizon, 126.8 degrees from the target, so no lunar pollution is possible on this sub. That is exactly the context needed for a faint galaxy like NGC 6946 in broadband luminance, where the slightest urban gradient would be crippling. The measured sky background matches that Bortle: dark, with no significant oriented gradient (plane amplitude 5%, R squared 0.02). In other words, the limiting factor here is not the sky but the amount of signal accumulated. Make the most of these moonless nights to stack many hours on the outer spiral arms and the surrounding IFN.

Coherent setup for the target: 1100 mm at f/6 gives a 0.57 degree field, and NGC 6946 (about 11 arcmin) fits easily with room for the faint arms. The 0.544 arcsec/px sampling is however clearly oversampled for typical seeing: the measured FWHM of 7.9 px at centre corresponds to about 4.3 arcsec, confirming you are seeing-limited, not optics-limited. Binning 2 at integration would give back signal-to-noise without losing real detail. On the settings side: gain 9 and a 180s luminance sub give a background/black-point margin of only 2.1 with 58% dark clipping, which is too tight. Raise the gain towards unity gain or extend to 300s to become sky-limited. Offset 100 and cooling at -19.9C are healthy.