Astrophotography diagnosis of M 27: Backfocus error

Favorable conditions. The stated Bortle 4 gives a moderate sky background and the histogram confirms it: median 0.02, dark clipping 0.01 %, no background saturation. On the Moon side, the measured context shows a 10.3 % illuminated crescent, 26.3° above the horizon and above all 111.2° away from the target: at that separation and illumination, lunar contribution is negligible, no moon gradient is expected nor observed. M 27 is a bright planetary nebula, perfectly compatible with this sky in luminance. Use nights like this for luminance and save high-Moon nights for narrowband.

Setup is sound in principle, but demanding. With 1764 mm and 3.76 µm pixels you sample at 0.44"/px: heavily oversampled, well below the comfortable 1 to 2"/px range under typical seeing. The 8.1 px central FWHM corresponds to roughly 3.6", honest in absolute terms, but it means the signal is spread over many pixels. A 2x bin in processing would gain signal to noise with no real loss of detail. The 0.76° field frames M 27 comfortably. On settings, gain 125 and offset 30 are healthy, no dark clipping, sensor temperature at -10 °C is fine. The background to black point margin of 4.17 is a little tight: in luminance you can stay at 120 s and make up for it with sub count.