Astrophotography diagnosis of M 42: Clipped stars and astigmatism

Average but workable conditions for this target. Bortle 7 mainly penalises the sky background, and it shows: chromatic mottling and a need for gradient removal. The Moon is marginal here, 37.6% illumination only 2 degrees above the horizon and 43 degrees from the target: at that altitude it is heavily dimmed by atmospheric extinction, so its impact is negligible. Fortunately M42 is an extremely bright emission nebula, exactly the kind of target that tolerates a suburban sky. The real limiting factor is not the sky but the total duration: about 45 minutes across all exposures, which is short for cleanly pulling the faint extensions in Bortle 7.

The setup matches the target well. At 260mm focal length, the 1.33 degree wide field frames M42 comfortably with the Running Man in the same frame, with room to spare: the vertical composition is well handled. Sampling at 2.30 arcsec/px is slightly undersampled for fine detail, but that is the normal smartscope compromise and the 3.4 px FWHM at centre shows the optics keep up. Two settings deserve a review: gain 200 is high and costs dynamic range on a core as bright as the Trapezium, and the 28.9C sensor temperature is distinctly high since the S50 is not actively cooled, which feeds the thermal noise visible in the background. The histogram is otherwise healthy, with the background well clear of the black point (e_margin 19.6) and no problematic dark clipping.