Astrophotography diagnosis of M16: Sensor tilt, clipped stars and 1 other

Comfortable conditions for this target. Bortle 4 with a 30 percent moon in 3 nm narrowband is almost a non-issue: 3 nm filters reject most lunar scatter and residual light pollution, which the background map confirms since no instrumental gradient was detected (the measured background variation matches the nebula itself). M16 is a strong emission target, exactly the case where moonlight costs almost nothing. The only notable effect is a background barely lifted off the black point (margin 1.01), normal in narrowband at 600 s and not a sign of underexposure. You can keep shooting this target even with a rising moon, prioritizing the OIII and SII channels which remain the limiting ones.

Coherent, high end setup for M16. The 12.5 inch CDK with the C4 gives a 0.83 degree field, framing the whole nebula with room for the faint extensions: the right compromise, neither too tight on the Pillars nor lost in empty sky. Measured FWHM around 6.5 px at center suggests fairly fine sampling relative to the seeing, typical of this kind of long focal length rig; 2x2 binning on the SII and OIII channels would buy SNR without losing real detail. The 3 nm filters are the right call here. The point to watch is mechanical: with a heavy imaging train on a CDK, sensor flatness needs regular checking, and that is exactly what the measurement is flagging.