Astrophotography diagnosis of NGC 253: Sensor tilt and clipped stars

Very favourable conditions: a waning crescent at 3% illumination, so no lunar contamination of the sky background, which the background map confirms (gradient amplitude of only 1%, zero azimuthal anisotropy). For a broadband galaxy like NGC 253 this is exactly the window you want, since that kind of target does not tolerate moonlight the way a filtered emission nebula does. The measured background stays close to the black point (e_margin 2.51), a sign of a dark sky rather than a problematic underexposure with stacked 60s subs. Make the most of these moonless nights to push total integration on the faint outer halo.

The setup matches the target. With a 0.55° wide field and a galaxy filling roughly a third of the frame, NGC 253 is comfortably contained with margin for the outer halo, although a slightly wider field would let the extensions breathe. The central FWHM of 5.4 px is a little generous: it mixes seeing and sampling, and suggests a somewhat oversampled setup at this focal length, which is not penalising but limits how much sharpness you can gain. The 60s subs are short for a broadband galaxy: the background to black point margin of 2.51 shows you are not firmly sky-limited, so longer subs or a higher gain would improve the signal to read noise ratio.