Astrophotography diagnosis of N?buleuse de la Grotte: Residual vignetting, green cast and 3 others

Conditions were excellent and are not to blame. The Moon, although 89.9% gibbous, was below the horizon during the session and 122 degrees from the target, so it contributed nothing to the sky background. The stated Bortle 3 is a rare quality sky that allows broadband work (IR/UV cut) on an emission nebula like Sh2-155, with no need to switch to dual-band. In that context the green background and the peripheral falloff come from the optical train and the missing calibration, not from the sky. You can capitalize on this: under such a sky, lengthening subs to 300 s and accumulating more hours will pay off directly, with no risk of swamping the background.

The setup is well matched to the target. At 540 mm and 1.436 arcsec/px the sampling is comfortable for typical seeing, and the measured FWHM of 3.9 to 4.8 px (5.6 to 6.9 arcsec) point to average seeing or slight drift rather than a sampling problem. The 2.49 degree field frames Sh2-155 with room to spare; the composition leaves the nebula off-center toward the upper left but fully contained. Gain 125 and offset 50 are healthy, with no dark clipping (0%) and only 0.02% saturated. The -20C sensor temperature is very good. The real weakness is procedural rather than optical: the large 6252x4176 sensor behind a C11 absolutely requires flats, and the 180 s exposure leaves a low background-to-black-point margin (1.79), so it is barely sky-limited under Bortle 3.