Astrophotography diagnosis of Pléiades: Light pollution gradient, clipped stars and 1 other

The diagnosis comes down to two independent points, one in acquisition, one in processing. On acquisition, the sky background carries a clear linear gradient, measured at 63% amplitude oriented at 274 degrees, with a plane fitting at R2=0.96 and dominating any radial component by a factor of 39. So this is not vignetting: it is a light ramp of external origin, and with the Moon at 3% ruled out, it points to ground light pollution or a target shot too low. This gradient was clearly not removed before stretching, which bakes it into the final image and also pulls the background color toward brown-red along the left edge.

On processing, the stretch was pushed hard enough to flatten the cores of the main Pleiades into white plateaus, which costs the blue signature of those giants. The same excess contributes to bloating the star discs, an effect amplified by mediocre seeing. Finally, with a background-to-black-point margin of only 1.55, the image is not truly sky-limited and chroma noise remains visible in the dark areas.

Nothing structural: stars are round out to the corners, the Seestar calibration holds, and the file is entirely recoverable by reprocessing from the master.