Astrophotography diagnosis of M33: Noise / underexposure, sensor tilt and 1 other

M33 is centered and usable, but the diagnosis converges on a dominant issue: lack of signal. The histogram is clear (e_margin=1, dark clipping 10.5%, Bowley skew 0.67), the 4016s integration is too short for this low-surface-brightness galaxy. The result is a grainy background where the spiral arms and HII regions, clearly present in the DSS reference, remain buried. This is the number one lever: more cumulative hours will transform this image.

Optically, the PSF panel confirms a residual tilt: center C is sharp and round (FWHM 4.6 px, elong 1.05) while the diagonally opposed TL and BR corners degrade (elong 1.23 and 1.20), corner/center ratio of 1.41. On a factory-bonded-sensor Unistellar Odyssey, this defect is not correctable, you must work with it or crop.

Finally, a thin satellite trail crosses the upper part of the field, just above the bulge; it will be automatically rejected at integration once the number of subs is sufficient. Tracking is generally good, with no uniform directional elongation signaling drift.