Quality deep-sky astrophotographs are achieved by digitally combining the signal present in multiple subexposures (subs). In this manner, inherent noise and other artifacts are attenuated, so that even the weak signal of dim regions becomes strong enough to be displayed and even enhanced. To that end, preprocessing, which can include image calibration, cosmetic correction, debayering, alignment, and image stacking, must be performed accurately to ensure best results. In a perfect world, you could forge ahead with these tasks. In the real world, however, it’s good practice to evaluate the quality of your exposures before proceeding. Although some imperfect subs may contribute to a good result, those with particularly poor attributes, such as bad focus or tracking, weak contrast, or excessive artifacts such as airplane or satellite trails, may best be discarded rather than permitted to adversely affect the stacked master file. PixInsight (PI) offers several resources for image evaluation.