Astrophotography diagnosis of SH2-112: Light pollution gradient and backfocus error

Moon at 90% on the shoot night: this is the main source of stray background and largely explains the measured gradient amplitude (42%). With a multiband filter on an emission target like SH2-112 this choice is defensible since narrow bands reject part of the lunar continuum, which let you bank 136x300s of usable data. On a broadband RGB target this full Moon would have been a dealbreaker. To improve further, favor moonless nights or frame the target far from the lunar disk to reduce the background ramp to model.

The Quattro 200P f/4 with the Ares-C Pro gives a 1.80 deg field, well suited to SH2-112, which sits in the central region without being lost: the framing centers it correctly with margin for the background. The Newton f/4 sampling stays comfortable for this target. The only setup point to fix is the coma corrector backfocus: the PSF panel shows FWHM rising symmetrically in the corners (ratio 1.21), typical of a slightly out of tolerance sensor distance on this demanding f/4. Fine adjustment in 0.5 mm steps will make corners match the center.