Astrophotography diagnosis of N?buleuse de la Grotte: Tracking drift

Near ideal conditions: Bortle 1 and a gibbous Moon below the horizon, 122 degrees from the target, so no lunar contribution is possible. The sky background is indeed very low (median 0.02, background to black point margin e_margin 1.93) and no instrumental gradient is measurable. On a faint emission nebula like the Cave (Sh2-155), this dark sky is exactly what you need: you can work in broadband RGB without a light pollution filter and recover the diffuse H-alpha without fighting a parasitic background. The low e_margin simply reflects a very dark sky combined with a 180s exposure at f/1.9, not a problematic underexposure.

A coherent combination: 540mm at f/1.9 gives a 2.49 degree field, plenty to frame Sh2-155 and its surroundings, and the nebula sits well inside the frame. Sampling at 1.44 arcsec/px is comfortable for typical seeing, and the measured FWHM (4.2 px, about 6 arcsec) shows that tracking and the atmosphere are the limiting factors, not the optics or the sensor. Gain 125 on the Poseidon-C PRO and offset 50 are healthy: no dark clipping (0%) and only 0.03% saturated pixels, so dynamic range is preserved. Cooling at -20C is perfectly suited. With such a focal ratio, 120s exposures would be enough to stay sky-limited under this sky while reducing sensitivity to drift.