Astrophotography diagnosis of N?buleuse du Requin sombre: Sensor tilt and satellite trail

Very favorable conditions. Bortle 1 is the best possible context for a dark nebula like the Shark, a target that lives only on faint reflected light and contrast against the background: every magnitude of sky darkness translates directly into nebulosity detection. The Moon at 30% illumination, only 6.8 deg above the horizon and 115 deg from the target, contributes almost no light pollution: its flux is largely absorbed by the atmospheric path at that altitude and its angular distance is comfortable. The stated 2.5 arcsec seeing remains the real limiting factor, consistent with the 4.7 px FWHM (about 6.7 arcsec) measured at center, which mostly reflects the optical defect rather than the atmosphere.

Powerful but demanding setup. A C11 in Hyperstar configuration at 540mm f/1.9 with a 6252x4176 sensor yields 2.48 deg of field, which frames the Shark Nebula, a wide diffuse object, nicely. The 1.436 arcsec/px sampling suits 2.5 arcsec seeing well, neither over nor undersampled. The downside is that f/1.9 imposes backfocus and flatness tolerances of a few tens of microns: the measured tilt is a direct consequence of that demand, not of a gross error. On settings, gain 125 and offset 50 are healthy, no dark clipping, sensor at -10C is fine. The background-to-black-point margin of 2 is low, but at f/1.9 under Bortle 1 with 60s that is expected: the sky is simply very dark.