Astrophotography diagnosis of M 31: Tracking drift

Very favorable conditions. The Bortle 2 sky is what explains the near neutral background and the lack of any significant light pollution gradient, and the Moon, although 86.6% illuminated, is below the horizon and 93 degrees from the target: its influence on this frame is nil. This is exactly the ideal context for broadband M31, a target that does not forgive a milky sky. The only measured gradient (4% amplitude, 32.6 degree orientation) is very weak and will come out easily with background extraction. In short, nothing in the conditions limits this session: the room for improvement is mechanical, not atmospheric.

A coherent setup, but short on focal length for the target. At 190mm the 3.36 degree field takes in M31 and its companions very generously: the framing leaves the galaxy fairly small in the frame with a lot of empty sky around it, comfortable but not ambitious in resolution. The 3.15 arcsec/px sampling is clearly undersampled, consistent with measured FWHM of only 1.5-1.8 px: you lose detail, and it also makes star shape measurement more sensitive to noise. On settings, gain 130 and -15C are healthy, and the absence of dark clipping shows offset 15 is sufficient. The background-to-black-point margin (e_margin 2.36) stays low: under Bortle 2 with a 300s sub that is expected, you are limited by the very dark sky rather than by a faulty setting.