Astrophotography diagnosis of M 31_Lum: Sensor tilt and clipped stars

Near ideal conditions for this target. The Bortle 2 sky gives a very dark background, which explains the measured background to black point margin of 1.97: on luminance under a sky this dark you stay close to the read-noise-limited regime rather than sky-limited, but 300 s at gain 160 remains a good compromise. The Moon, waning gibbous at 68.9 percent, is below the horizon and 139.7 degrees from the target: no lunar contribution is possible, and indeed no lunar gradient is measured. The measured plane gradient (29 percent amplitude, 155 degree orientation) is consistent with the real nebulosity of M31 and the galactic background visible in the DSS. Make the most of nights like this to pile up integration on the faint disk extensions.

A coherent setup, but clearly undersampled for M31. At 190 mm focal length and 2.9 micron pixels you sit at 3.148 arcsec/px: the measured 1.76 px FWHM at center corresponds to roughly 5.5 arcsec, so pixels are the limit, not seeing or optics. The arms and dust lanes stay readable but you lose fine detail. In exchange, the 3.36 degree field frames M31 perfectly with M110 and M32, and the composition leaves room for the disk extensions: the right choice for a wide view. On settings, gain 160, offset 15 and cooling at -20C are healthy, with no measured dark clipping (0 percent). If you want detail in the core, a longer focal length in a second session is the way.