Astrophotography diagnosis of IC1396_Askar50P: Astigmatism

Excellent conditions. Bortle 2 and the Moon below the horizon despite a 98 % phase, the 113° separation being irrelevant since it lights nothing: no lunar gradient expected, and the background map confirms it (plane amplitude of only 6 %, R² 0.05, so no significant oriented gradient). On IC 1396, an extended Ha emission target, you are in the best possible case: the sky background is dark and most of the signal comes from the nebula itself. You can therefore extend integration without fear of saturating the background, and aim for several hours to bring out the faint extensions of the complex.

Setup is coherent in framing, debatable in sampling. At 190 mm focal length, the 3.36° field comfortably encloses IC 1396 with the Elephant Trunk well placed: successful composition. On the other hand, 3.15"/px is heavily undersampled: stars sit at 1.2 to 1.5 px FWHM, below the Nyquist criterion. You lose fine detail and shape measurement becomes unstable, but in exchange you gain signal per pixel, a defensible choice on an extended nebula. On settings, gain 150 on an ASI585MM, offset 15 with no dark clipping and sensor at -13 °C: all healthy. The background-to-black-point margin of 1.69 is low but that is NORMAL in narrowband under Bortle 2 with a very dark sky, do not lengthen exposure for that.