Astrophotography diagnosis of NGC 7635uhc: Light pollution gradient, backfocus error and 2 others

Bortle 3 site: sky background conditions are excellent on paper, yet the measured gradient reaches 42%. The declared lunar context is contradictory (gibbous 97% on one side, 3.6% crescent on the other): if the Moon really was at 3.6% illumination it contributes nothing, and the gradient then comes from a ground source toward 250 degrees or from a target low on the horizon. If instead the Moon was gibbous at 33.7 degrees altitude and 69 degrees from the target, it alone explains much of the background. In that case a dual-band H-alpha/OIII filter on NGC 7635, an emission nebula, would have protected contrast far better than a plain IRUV.

The setup is sound in principle but mis-sized on two points. At 1482 mm the 0.90 degree field frames NGC 7635 with room to spare, so framing calls for no criticism. Sampling at 0.523 arcsec/px is heavily oversampled though: under typical 2 to 3 arcsec seeing, the central FWHM of 4.75 px corresponds to about 2.5 arcsec, so the sensor is mostly resolving noise. Software 2x2 binning at stacking would return SNR with no real loss of detail. On settings, gain 120 and -10 C are healthy and offset 50 does not clip, but the 60s exposure gives e_margin=1.96: you are not sky-limited, move to 180-300s since the mount guides. On an emission target, a dual-band filter would be more relevant than the IRUV.